[PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr Tyshchenko]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Introduce Xen specific binding for the virtualized device (e.g. virtio)
to be used by Xen grant DMA-mapping layer in the subsequent commit.

This binding indicates that Xen grant mappings scheme needs to be
enabled for the device which DT node contains that property and specifies
the ID of Xen domain where the corresponding backend resides. The ID
(domid) is used as an argument to the grant mapping APIs.

This is needed for the option to restrict memory access using Xen grant
mappings to work which primary goal is to enable using virtio devices
in Xen guests.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
---
Changes RFC -> V1:
   - update commit subject/description and text in description
   - move to devicetree/bindings/arm/

Changes V1 -> V2:
   - update text in description
   - change the maintainer of the binding
   - fix validation issue
   - reference xen,dev-domid.yaml schema from virtio/mmio.yaml
---
 .../devicetree/bindings/arm/xen,dev-domid.yaml     | 37 ++++++++++++++++++++++
 Documentation/devicetree/bindings/virtio/mmio.yaml |  7 ++++
 2 files changed, 44 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/arm/xen,dev-domid.yaml

diff --git a/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
new file mode 100644
index 00000000..750e89e
--- /dev/null
+++ b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
@@ -0,0 +1,37 @@
+# SPDX-License-Identifier: (GPL-2.0-only or BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/xen,dev-domid.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Xen specific binding for virtualized devices (e.g. virtio)
+
+maintainers:
+  - Stefano Stabellini <sstabellini@kernel.org>
+
+select: true
+
+description:
+  This binding indicates that Xen grant mappings need to be enabled for
+  the device, and it specifies the ID of the domain where the corresponding
+  device (backend) resides. The property is required to restrict memory
+  access using Xen grant mappings.
+
+properties:
+  xen,dev-domid:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      The domid (domain ID) of the domain where the device (backend) is running.
+
+additionalProperties: true
+
+examples:
+  - |
+    virtio@3000 {
+            compatible = "virtio,mmio";
+            reg = <0x3000 0x100>;
+            interrupts = <41>;
+
+            /* The device is located in Xen domain with ID 1 */
+            xen,dev-domid = <1>;
+    };
diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
index 10c22b5..29a0932 100644
--- a/Documentation/devicetree/bindings/virtio/mmio.yaml
+++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
@@ -13,6 +13,9 @@ description:
   See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
   more details.
 
+allOf:
+  - $ref: /schemas/arm/xen,dev-domid.yaml#
+
 properties:
   compatible:
     const: virtio,mmio
@@ -33,6 +36,10 @@ properties:
     description: Required for devices making accesses thru an IOMMU.
     maxItems: 1
 
+  xen,dev-domid:
+    description: Required when Xen grant mappings need to be enabled for device.
+    $ref: /schemas/types.yaml#/definitions/uint32
+
 required:
   - compatible
   - reg
-- 
2.7.4

Re: [PATCH V2 5/7] dt-bindings: Add xen, dev-domid property description for xen-grant DMA ops

[Stefano Stabellini]

On Sat, 7 May 2022, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> 
> Introduce Xen specific binding for the virtualized device (e.g. virtio)
> to be used by Xen grant DMA-mapping layer in the subsequent commit.
> 
> This binding indicates that Xen grant mappings scheme needs to be
> enabled for the device which DT node contains that property and specifies
> the ID of Xen domain where the corresponding backend resides. The ID
> (domid) is used as an argument to the grant mapping APIs.
> 
> This is needed for the option to restrict memory access using Xen grant
> mappings to work which primary goal is to enable using virtio devices
> in Xen guests.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

The binding is OK and the wording is OK too.

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

I am not an expert on the details of writing a good schema, I'll defer
to Rob if he has any comments on that.


> ---
> Changes RFC -> V1:
>    - update commit subject/description and text in description
>    - move to devicetree/bindings/arm/
> 
> Changes V1 -> V2:
>    - update text in description
>    - change the maintainer of the binding
>    - fix validation issue
>    - reference xen,dev-domid.yaml schema from virtio/mmio.yaml
> ---
>  .../devicetree/bindings/arm/xen,dev-domid.yaml     | 37 ++++++++++++++++++++++
>  Documentation/devicetree/bindings/virtio/mmio.yaml |  7 ++++
>  2 files changed, 44 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
> 
> diff --git a/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
> new file mode 100644
> index 00000000..750e89e
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
> @@ -0,0 +1,37 @@
> +# SPDX-License-Identifier: (GPL-2.0-only or BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/arm/xen,dev-domid.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Xen specific binding for virtualized devices (e.g. virtio)
> +
> +maintainers:
> +  - Stefano Stabellini <sstabellini@kernel.org>
> +
> +select: true
> +
> +description:
> +  This binding indicates that Xen grant mappings need to be enabled for
> +  the device, and it specifies the ID of the domain where the corresponding
> +  device (backend) resides. The property is required to restrict memory
> +  access using Xen grant mappings.
> +
> +properties:
> +  xen,dev-domid:
> +    $ref: /schemas/types.yaml#/definitions/uint32
> +    description:
> +      The domid (domain ID) of the domain where the device (backend) is running.
> +
> +additionalProperties: true
> +
> +examples:
> +  - |
> +    virtio@3000 {
> +            compatible = "virtio,mmio";
> +            reg = <0x3000 0x100>;
> +            interrupts = <41>;
> +
> +            /* The device is located in Xen domain with ID 1 */
> +            xen,dev-domid = <1>;
> +    };
> diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
> index 10c22b5..29a0932 100644
> --- a/Documentation/devicetree/bindings/virtio/mmio.yaml
> +++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
> @@ -13,6 +13,9 @@ description:
>    See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
>    more details.
>  
> +allOf:
> +  - $ref: /schemas/arm/xen,dev-domid.yaml#
> +
>  properties:
>    compatible:
>      const: virtio,mmio
> @@ -33,6 +36,10 @@ properties:
>      description: Required for devices making accesses thru an IOMMU.
>      maxItems: 1
>  
> +  xen,dev-domid:
> +    description: Required when Xen grant mappings need to be enabled for device.
> +    $ref: /schemas/types.yaml#/definitions/uint32
> +
>  required:
>    - compatible
>    - reg
> -- 
> 2.7.4
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Rob Herring]

On Sat, May 07, 2022 at 09:19:06PM +0300, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> 
> Introduce Xen specific binding for the virtualized device (e.g. virtio)
> to be used by Xen grant DMA-mapping layer in the subsequent commit.
> 
> This binding indicates that Xen grant mappings scheme needs to be
> enabled for the device which DT node contains that property and specifies
> the ID of Xen domain where the corresponding backend resides. The ID
> (domid) is used as an argument to the grant mapping APIs.
> 
> This is needed for the option to restrict memory access using Xen grant
> mappings to work which primary goal is to enable using virtio devices
> in Xen guests.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> ---
> Changes RFC -> V1:
>    - update commit subject/description and text in description
>    - move to devicetree/bindings/arm/
> 
> Changes V1 -> V2:
>    - update text in description
>    - change the maintainer of the binding
>    - fix validation issue
>    - reference xen,dev-domid.yaml schema from virtio/mmio.yaml
> ---
>  .../devicetree/bindings/arm/xen,dev-domid.yaml     | 37 ++++++++++++++++++++++
>  Documentation/devicetree/bindings/virtio/mmio.yaml |  7 ++++
>  2 files changed, 44 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
> 
> diff --git a/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
> new file mode 100644
> index 00000000..750e89e
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
> @@ -0,0 +1,37 @@
> +# SPDX-License-Identifier: (GPL-2.0-only or BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/arm/xen,dev-domid.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Xen specific binding for virtualized devices (e.g. virtio)
> +
> +maintainers:
> +  - Stefano Stabellini <sstabellini@kernel.org>
> +
> +select: true

Omit. No need to apply this on every single node.

> +
> +description:
> +  This binding indicates that Xen grant mappings need to be enabled for
> +  the device, and it specifies the ID of the domain where the corresponding
> +  device (backend) resides. The property is required to restrict memory
> +  access using Xen grant mappings.
> +
> +properties:
> +  xen,dev-domid:

I kind of think 'dev' is redundant. Is there another kind of domid 
possible? Maybe xen,backend-domid or just xen,domid? I don't know Xen 
too well, so ultimately up to you all.

> +    $ref: /schemas/types.yaml#/definitions/uint32
> +    description:
> +      The domid (domain ID) of the domain where the device (backend) is running.
> +
> +additionalProperties: true
> +
> +examples:
> +  - |
> +    virtio@3000 {
> +            compatible = "virtio,mmio";
> +            reg = <0x3000 0x100>;
> +            interrupts = <41>;
> +
> +            /* The device is located in Xen domain with ID 1 */
> +            xen,dev-domid = <1>;
> +    };
> diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
> index 10c22b5..29a0932 100644
> --- a/Documentation/devicetree/bindings/virtio/mmio.yaml
> +++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
> @@ -13,6 +13,9 @@ description:
>    See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
>    more details.
>  
> +allOf:
> +  - $ref: /schemas/arm/xen,dev-domid.yaml#
> +
>  properties:
>    compatible:
>      const: virtio,mmio
> @@ -33,6 +36,10 @@ properties:
>      description: Required for devices making accesses thru an IOMMU.
>      maxItems: 1
>  
> +  xen,dev-domid:
> +    description: Required when Xen grant mappings need to be enabled for device.
> +    $ref: /schemas/types.yaml#/definitions/uint32

No need to define the type again nor describe it again.

Instead, just change additionalProperties to unevaluateProperties in 
this doc. The diff is the latter takes $ref's into account.

> +
>  required:
>    - compatible
>    - reg
> -- 
> 2.7.4
> 
> 

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 17.05.22 03:27, Rob Herring wrote:

Hello Rob, all


> On Sat, May 07, 2022 at 09:19:06PM +0300, Oleksandr Tyshchenko wrote:
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>
>> Introduce Xen specific binding for the virtualized device (e.g. virtio)
>> to be used by Xen grant DMA-mapping layer in the subsequent commit.
>>
>> This binding indicates that Xen grant mappings scheme needs to be
>> enabled for the device which DT node contains that property and specifies
>> the ID of Xen domain where the corresponding backend resides. The ID
>> (domid) is used as an argument to the grant mapping APIs.
>>
>> This is needed for the option to restrict memory access using Xen grant
>> mappings to work which primary goal is to enable using virtio devices
>> in Xen guests.
>>
>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>> ---
>> Changes RFC -> V1:
>>     - update commit subject/description and text in description
>>     - move to devicetree/bindings/arm/
>>
>> Changes V1 -> V2:
>>     - update text in description
>>     - change the maintainer of the binding
>>     - fix validation issue
>>     - reference xen,dev-domid.yaml schema from virtio/mmio.yaml
>> ---
>>   .../devicetree/bindings/arm/xen,dev-domid.yaml     | 37 ++++++++++++++++++++++
>>   Documentation/devicetree/bindings/virtio/mmio.yaml |  7 ++++
>>   2 files changed, 44 insertions(+)
>>   create mode 100644 Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
>>
>> diff --git a/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
>> new file mode 100644
>> index 00000000..750e89e
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/arm/xen,dev-domid.yaml
>> @@ -0,0 +1,37 @@
>> +# SPDX-License-Identifier: (GPL-2.0-only or BSD-2-Clause)
>> +%YAML 1.2
>> +---
>> +$id: http://devicetree.org/schemas/arm/xen,dev-domid.yaml#
>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>> +
>> +title: Xen specific binding for virtualized devices (e.g. virtio)
>> +
>> +maintainers:
>> +  - Stefano Stabellini <sstabellini@kernel.org>
>> +
>> +select: true
> Omit. No need to apply this on every single node.

ok, will do


>
>> +
>> +description:
>> +  This binding indicates that Xen grant mappings need to be enabled for
>> +  the device, and it specifies the ID of the domain where the corresponding
>> +  device (backend) resides. The property is required to restrict memory
>> +  access using Xen grant mappings.
>> +
>> +properties:
>> +  xen,dev-domid:
> I kind of think 'dev' is redundant. Is there another kind of domid
> possible?


In general, yes. It is driver(frontend) domid. But, at least for now, I 
don't see why we will need an additional property for that.


>   Maybe xen,backend-domid or just xen,domid? I don't know Xen
> too well, so ultimately up to you all.

xen,domid sounds ambiguous to me.

xen,backend-domid sounds perfectly fine to me, I even think it fits better.



Stefano, Juergen, would you be happy with new xen,backend-domid name?

If yes, Stefano could you please clarify, would you be OK if I retained 
your R-b tags (for all patches in current series which touch that 
property) after doing such renaming?




>
>> +    $ref: /schemas/types.yaml#/definitions/uint32
>> +    description:
>> +      The domid (domain ID) of the domain where the device (backend) is running.
>> +
>> +additionalProperties: true
>> +
>> +examples:
>> +  - |
>> +    virtio@3000 {
>> +            compatible = "virtio,mmio";
>> +            reg = <0x3000 0x100>;
>> +            interrupts = <41>;
>> +
>> +            /* The device is located in Xen domain with ID 1 */
>> +            xen,dev-domid = <1>;
>> +    };
>> diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
>> index 10c22b5..29a0932 100644
>> --- a/Documentation/devicetree/bindings/virtio/mmio.yaml
>> +++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
>> @@ -13,6 +13,9 @@ description:
>>     See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
>>     more details.
>>   
>> +allOf:
>> +  - $ref: /schemas/arm/xen,dev-domid.yaml#
>> +
>>   properties:
>>     compatible:
>>       const: virtio,mmio
>> @@ -33,6 +36,10 @@ properties:
>>       description: Required for devices making accesses thru an IOMMU.
>>       maxItems: 1
>>   
>> +  xen,dev-domid:
>> +    description: Required when Xen grant mappings need to be enabled for device.
>> +    $ref: /schemas/types.yaml#/definitions/uint32
> No need to define the type again nor describe it again.
>
> Instead, just change additionalProperties to unevaluateProperties in
> this doc. The diff is the latter takes $ref's into account.

ok, will do. Could you please clarify, shall I use?

unevaluatedProperties: false

or

unevaluatedProperties:

type: object


I am not too familiar with this stuff. Both variants seem to pass 
validation.


Thank you.


>
>> +
>>   required:
>>     - compatible
>>     - reg
>> -- 
>> 2.7.4
>>
>>
-- 
Regards,

Oleksandr Tyshchenko

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Arnd Bergmann]

On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@gmail.com> wrote:
>
> diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
> index 10c22b5..29a0932 100644
> --- a/Documentation/devicetree/bindings/virtio/mmio.yaml
> +++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
> @@ -13,6 +13,9 @@ description:
>    See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
>    more details.
>
> +allOf:
> +  - $ref: /schemas/arm/xen,dev-domid.yaml#
> +
>  properties:
>    compatible:
>      const: virtio,mmio
> @@ -33,6 +36,10 @@ properties:
>      description: Required for devices making accesses thru an IOMMU.
>      maxItems: 1
>
> +  xen,dev-domid:
> +    description: Required when Xen grant mappings need to be enabled for device.
> +    $ref: /schemas/types.yaml#/definitions/uint32
> +
>  required:
>    - compatible
>    - reg

Sorry for joining the discussion late. Have you considered using the
generic iommu
binding here instead of a custom property? This would mean having a device
node for the grant-table mechanism that can be referred to using the 'iommus'
phandle property, with the domid as an additional argument.

It does not quite fit the model that Linux currently uses for iommus,
as that has an allocator for dma_addr_t space, but it would think it's
conceptually close enough that it makes sense for the binding.

         Arnd

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 18.05.22 17:32, Arnd Bergmann wrote:


Hello Arnd


> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@gmail.com> wrote:
>> diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
>> index 10c22b5..29a0932 100644
>> --- a/Documentation/devicetree/bindings/virtio/mmio.yaml
>> +++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
>> @@ -13,6 +13,9 @@ description:
>>     See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
>>     more details.
>>
>> +allOf:
>> +  - $ref: /schemas/arm/xen,dev-domid.yaml#
>> +
>>   properties:
>>     compatible:
>>       const: virtio,mmio
>> @@ -33,6 +36,10 @@ properties:
>>       description: Required for devices making accesses thru an IOMMU.
>>       maxItems: 1
>>
>> +  xen,dev-domid:
>> +    description: Required when Xen grant mappings need to be enabled for device.
>> +    $ref: /schemas/types.yaml#/definitions/uint32
>> +
>>   required:
>>     - compatible
>>     - reg
> Sorry for joining the discussion late. Have you considered using the
> generic iommu
> binding here instead of a custom property?

I have to admit - no, I haven't. I was thinking that Xen specific 
feature should be communicated using Xen specific DT property.


>   This would mean having a device
> node for the grant-table mechanism that can be referred to using the 'iommus'
> phandle property, with the domid as an additional argument.

I assume, you are speaking about something like the following?


xen_dummy_iommu {
    compatible = "xen,dummy-iommu";
    #iommu-cells = <1>;
};

virtio@3000 {
    compatible = "virtio,mmio";
    reg = <0x3000 0x100>;
    interrupts = <41>;

    /* The device is located in Xen domain with ID 1 */
    iommus = <&xen_dummy_iommu 1>;
};


>
> It does not quite fit the model that Linux currently uses for iommus,
> as that has an allocator for dma_addr_t space

yes (# 3/7 adds grant-table based allocator)


> , but it would think it's
> conceptually close enough that it makes sense for the binding.

Interesting idea. I am wondering, do we need an extra actions for this 
to work in Linux guest (dummy IOMMU driver, etc)?


>
>           Arnd

-- 
Regards,

Oleksandr Tyshchenko

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Arnd Bergmann]

On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
> On 18.05.22 17:32, Arnd Bergmann wrote:
> > On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@gmail.com> wrote:
>
> >   This would mean having a device
> > node for the grant-table mechanism that can be referred to using the 'iommus'
> > phandle property, with the domid as an additional argument.
>
> I assume, you are speaking about something like the following?
>
>
> xen_dummy_iommu {
>     compatible = "xen,dummy-iommu";
>     #iommu-cells = <1>;
> };
>
> virtio@3000 {
>     compatible = "virtio,mmio";
>     reg = <0x3000 0x100>;
>     interrupts = <41>;
>
>     /* The device is located in Xen domain with ID 1 */
>     iommus = <&xen_dummy_iommu 1>;
> };

Right, that's that's the idea, except I would not call it a 'dummy'.
From the perspective of the DT, this behaves just like an IOMMU,
even if the exact mechanism is different from most hardware IOMMU
implementations.

> > It does not quite fit the model that Linux currently uses for iommus,
> > as that has an allocator for dma_addr_t space
>
> yes (# 3/7 adds grant-table based allocator)
>
>
> > , but it would think it's
> > conceptually close enough that it makes sense for the binding.
>
> Interesting idea. I am wondering, do we need an extra actions for this
> to work in Linux guest (dummy IOMMU driver, etc)?

It depends on how closely the guest implementation can be made to
resemble a normal iommu. If you do allocate dma_addr_t addresses,
it may actually be close enough that you can just turn the grant-table
code into a normal iommu driver and change nothing else.

        Arnd

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 18.05.22 19:39, Arnd Bergmann wrote:


Hello Arnd


> On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
>> On 18.05.22 17:32, Arnd Bergmann wrote:
>>> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@gmail.com> wrote:
>>>    This would mean having a device
>>> node for the grant-table mechanism that can be referred to using the 'iommus'
>>> phandle property, with the domid as an additional argument.
>> I assume, you are speaking about something like the following?
>>
>>
>> xen_dummy_iommu {
>>      compatible = "xen,dummy-iommu";
>>      #iommu-cells = <1>;
>> };
>>
>> virtio@3000 {
>>      compatible = "virtio,mmio";
>>      reg = <0x3000 0x100>;
>>      interrupts = <41>;
>>
>>      /* The device is located in Xen domain with ID 1 */
>>      iommus = <&xen_dummy_iommu 1>;
>> };
> Right, that's that's the idea,

thank you for the confirmation



>   except I would not call it a 'dummy'.
>  From the perspective of the DT, this behaves just like an IOMMU,
> even if the exact mechanism is different from most hardware IOMMU
> implementations.

well, agree


>
>>> It does not quite fit the model that Linux currently uses for iommus,
>>> as that has an allocator for dma_addr_t space
>> yes (# 3/7 adds grant-table based allocator)
>>
>>
>>> , but it would think it's
>>> conceptually close enough that it makes sense for the binding.
>> Interesting idea. I am wondering, do we need an extra actions for this
>> to work in Linux guest (dummy IOMMU driver, etc)?
> It depends on how closely the guest implementation can be made to
> resemble a normal iommu. If you do allocate dma_addr_t addresses,
> it may actually be close enough that you can just turn the grant-table
> code into a normal iommu driver and change nothing else.

Unfortunately, I failed to find a way how use grant references at the 
iommu_ops level (I mean to fully pretend that we are an IOMMU driver). I 
am not too familiar with that, so what is written below might be wrong 
or at least not precise.

The normal IOMMU driver in Linux doesn’t allocate DMA addresses by 
itself, it just maps (IOVA-PA) what was requested to be mapped by the 
upper layer. The DMA address allocation is done by the upper layer 
(DMA-IOMMU which is the glue layer between DMA API and IOMMU API 
allocates IOVA for PA?). But, all what we need here is just to allocate 
our specific grant-table based DMA addresses (DMA address = grant 
reference + offset in the page), so let’s say we need an entity to take 
a physical address as parameter and return a DMA address (what actually 
commit #3/7 is doing), and that’s all. So working at the dma_ops layer 
we get exactly what we need, with the minimal changes to guest 
infrastructure. In our case the Xen itself acts as an IOMMU.

Assuming that we want to reuse the IOMMU infrastructure somehow for our 
needs. I think, in that case we will likely need to introduce a new 
specific IOVA allocator (alongside with a generic one) to be hooked up 
by the DMA-IOMMU layer if we run on top of Xen. But, even having the 
specific IOVA allocator to return what we indeed need (DMA address = 
grant reference + offset in the page) we will still need the specific 
minimal required IOMMU driver to be present in the system anyway in 
order to track the mappings(?) and do nothing with them, returning a 
success (this specific IOMMU driver should have all mandatory callbacks 
implemented).

I completely agree, it would be really nice to reuse generic IOMMU 
bindings rather than introducing Xen specific property if what we are 
trying to implement in current patch series fits in the usage of 
"iommus" in Linux more-less. But, if we will have to add more 
complexity/more components to the code for the sake of reusing device 
tree binding, this raises a question whether that’s worthwhile.

Or I really missed something?


>
>          Arnd

-- 
Regards,

Oleksandr Tyshchenko

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Stefano Stabellini]

[-- Attachment #1: Type: text/plain, Size: 4419 bytes --]

On Thu, 19 May 2022, Oleksandr wrote:
> > On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
> > > On 18.05.22 17:32, Arnd Bergmann wrote:
> > > > On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
> > > > <olekstysh@gmail.com> wrote:
> > > >    This would mean having a device
> > > > node for the grant-table mechanism that can be referred to using the
> > > > 'iommus'
> > > > phandle property, with the domid as an additional argument.
> > > I assume, you are speaking about something like the following?
> > > 
> > > 
> > > xen_dummy_iommu {
> > >      compatible = "xen,dummy-iommu";
> > >      #iommu-cells = <1>;
> > > };
> > > 
> > > virtio@3000 {
> > >      compatible = "virtio,mmio";
> > >      reg = <0x3000 0x100>;
> > >      interrupts = <41>;
> > > 
> > >      /* The device is located in Xen domain with ID 1 */
> > >      iommus = <&xen_dummy_iommu 1>;
> > > };
> > Right, that's that's the idea,
> 
> thank you for the confirmation
> 
> 
> 
> >   except I would not call it a 'dummy'.
> >  From the perspective of the DT, this behaves just like an IOMMU,
> > even if the exact mechanism is different from most hardware IOMMU
> > implementations.
> 
> well, agree
> 
> 
> > 
> > > > It does not quite fit the model that Linux currently uses for iommus,
> > > > as that has an allocator for dma_addr_t space
> > > yes (# 3/7 adds grant-table based allocator)
> > > 
> > > 
> > > > , but it would think it's
> > > > conceptually close enough that it makes sense for the binding.
> > > Interesting idea. I am wondering, do we need an extra actions for this
> > > to work in Linux guest (dummy IOMMU driver, etc)?
> > It depends on how closely the guest implementation can be made to
> > resemble a normal iommu. If you do allocate dma_addr_t addresses,
> > it may actually be close enough that you can just turn the grant-table
> > code into a normal iommu driver and change nothing else.
> 
> Unfortunately, I failed to find a way how use grant references at the
> iommu_ops level (I mean to fully pretend that we are an IOMMU driver). I am
> not too familiar with that, so what is written below might be wrong or at
> least not precise.
> 
> The normal IOMMU driver in Linux doesn’t allocate DMA addresses by itself, it
> just maps (IOVA-PA) what was requested to be mapped by the upper layer. The
> DMA address allocation is done by the upper layer (DMA-IOMMU which is the glue
> layer between DMA API and IOMMU API allocates IOVA for PA?). But, all what we
> need here is just to allocate our specific grant-table based DMA addresses
> (DMA address = grant reference + offset in the page), so let’s say we need an
> entity to take a physical address as parameter and return a DMA address (what
> actually commit #3/7 is doing), and that’s all. So working at the dma_ops
> layer we get exactly what we need, with the minimal changes to guest
> infrastructure. In our case the Xen itself acts as an IOMMU.
> 
> Assuming that we want to reuse the IOMMU infrastructure somehow for our needs.
> I think, in that case we will likely need to introduce a new specific IOVA
> allocator (alongside with a generic one) to be hooked up by the DMA-IOMMU
> layer if we run on top of Xen. But, even having the specific IOVA allocator to
> return what we indeed need (DMA address = grant reference + offset in the
> page) we will still need the specific minimal required IOMMU driver to be
> present in the system anyway in order to track the mappings(?) and do nothing
> with them, returning a success (this specific IOMMU driver should have all
> mandatory callbacks implemented).
> 
> I completely agree, it would be really nice to reuse generic IOMMU bindings
> rather than introducing Xen specific property if what we are trying to
> implement in current patch series fits in the usage of "iommus" in Linux
> more-less. But, if we will have to add more complexity/more components to the
> code for the sake of reusing device tree binding, this raises a question
> whether that’s worthwhile.
> 
> Or I really missed something?

I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
bindings, not necessarily the IOMMU drivers framework in Linux (although
that would be an added bonus.)

I know from previous discussions with you that making the grant table
fit in the existing IOMMU drivers model is difficult, but just reusing
the Device Tree bindings seems feasible?

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 19.05.22 04:06, Stefano Stabellini wrote:


Hello Stefano

> On Thu, 19 May 2022, Oleksandr wrote:
>>> On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
>>>> On 18.05.22 17:32, Arnd Bergmann wrote:
>>>>> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
>>>>> <olekstysh@gmail.com> wrote:
>>>>>     This would mean having a device
>>>>> node for the grant-table mechanism that can be referred to using the
>>>>> 'iommus'
>>>>> phandle property, with the domid as an additional argument.
>>>> I assume, you are speaking about something like the following?
>>>>
>>>>
>>>> xen_dummy_iommu {
>>>>       compatible = "xen,dummy-iommu";
>>>>       #iommu-cells = <1>;
>>>> };
>>>>
>>>> virtio@3000 {
>>>>       compatible = "virtio,mmio";
>>>>       reg = <0x3000 0x100>;
>>>>       interrupts = <41>;
>>>>
>>>>       /* The device is located in Xen domain with ID 1 */
>>>>       iommus = <&xen_dummy_iommu 1>;
>>>> };
>>> Right, that's that's the idea,
>> thank you for the confirmation
>>
>>
>>
>>>    except I would not call it a 'dummy'.
>>>   From the perspective of the DT, this behaves just like an IOMMU,
>>> even if the exact mechanism is different from most hardware IOMMU
>>> implementations.
>> well, agree
>>
>>
>>>>> It does not quite fit the model that Linux currently uses for iommus,
>>>>> as that has an allocator for dma_addr_t space
>>>> yes (# 3/7 adds grant-table based allocator)
>>>>
>>>>
>>>>> , but it would think it's
>>>>> conceptually close enough that it makes sense for the binding.
>>>> Interesting idea. I am wondering, do we need an extra actions for this
>>>> to work in Linux guest (dummy IOMMU driver, etc)?
>>> It depends on how closely the guest implementation can be made to
>>> resemble a normal iommu. If you do allocate dma_addr_t addresses,
>>> it may actually be close enough that you can just turn the grant-table
>>> code into a normal iommu driver and change nothing else.
>> Unfortunately, I failed to find a way how use grant references at the
>> iommu_ops level (I mean to fully pretend that we are an IOMMU driver). I am
>> not too familiar with that, so what is written below might be wrong or at
>> least not precise.
>>
>> The normal IOMMU driver in Linux doesn’t allocate DMA addresses by itself, it
>> just maps (IOVA-PA) what was requested to be mapped by the upper layer. The
>> DMA address allocation is done by the upper layer (DMA-IOMMU which is the glue
>> layer between DMA API and IOMMU API allocates IOVA for PA?). But, all what we
>> need here is just to allocate our specific grant-table based DMA addresses
>> (DMA address = grant reference + offset in the page), so let’s say we need an
>> entity to take a physical address as parameter and return a DMA address (what
>> actually commit #3/7 is doing), and that’s all. So working at the dma_ops
>> layer we get exactly what we need, with the minimal changes to guest
>> infrastructure. In our case the Xen itself acts as an IOMMU.
>>
>> Assuming that we want to reuse the IOMMU infrastructure somehow for our needs.
>> I think, in that case we will likely need to introduce a new specific IOVA
>> allocator (alongside with a generic one) to be hooked up by the DMA-IOMMU
>> layer if we run on top of Xen. But, even having the specific IOVA allocator to
>> return what we indeed need (DMA address = grant reference + offset in the
>> page) we will still need the specific minimal required IOMMU driver to be
>> present in the system anyway in order to track the mappings(?) and do nothing
>> with them, returning a success (this specific IOMMU driver should have all
>> mandatory callbacks implemented).
>>
>> I completely agree, it would be really nice to reuse generic IOMMU bindings
>> rather than introducing Xen specific property if what we are trying to
>> implement in current patch series fits in the usage of "iommus" in Linux
>> more-less. But, if we will have to add more complexity/more components to the
>> code for the sake of reusing device tree binding, this raises a question
>> whether that’s worthwhile.
>>
>> Or I really missed something?
> I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
> bindings, not necessarily the IOMMU drivers framework in Linux (although
> that would be an added bonus.)
>
> I know from previous discussions with you that making the grant table
> fit in the existing IOMMU drivers model is difficult, but just reusing
> the Device Tree bindings seems feasible?

I started experimenting with that. As wrote in a separate email, I got a 
deferred probe timeout,

after inserting required nodes into guest device tree, which seems to be 
a consequence of the unavailability of IOMMU, I will continue to 
investigate this question.



-- 
Regards,

Oleksandr Tyshchenko

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 19.05.22 09:03, Oleksandr wrote:

Hello Stefano, all


>
> On 19.05.22 04:06, Stefano Stabellini wrote:
>
>
> Hello Stefano, all
>
>> On Thu, 19 May 2022, Oleksandr wrote:
>>>> On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
>>>>> On 18.05.22 17:32, Arnd Bergmann wrote:
>>>>>> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
>>>>>> <olekstysh@gmail.com> wrote:
>>>>>>     This would mean having a device
>>>>>> node for the grant-table mechanism that can be referred to using the
>>>>>> 'iommus'
>>>>>> phandle property, with the domid as an additional argument.
>>>>> I assume, you are speaking about something like the following?
>>>>>
>>>>>
>>>>> xen_dummy_iommu {
>>>>>       compatible = "xen,dummy-iommu";
>>>>>       #iommu-cells = <1>;
>>>>> };
>>>>>
>>>>> virtio@3000 {
>>>>>       compatible = "virtio,mmio";
>>>>>       reg = <0x3000 0x100>;
>>>>>       interrupts = <41>;
>>>>>
>>>>>       /* The device is located in Xen domain with ID 1 */
>>>>>       iommus = <&xen_dummy_iommu 1>;
>>>>> };
>>>> Right, that's that's the idea,
>>> thank you for the confirmation
>>>
>>>
>>>
>>>>    except I would not call it a 'dummy'.
>>>>   From the perspective of the DT, this behaves just like an IOMMU,
>>>> even if the exact mechanism is different from most hardware IOMMU
>>>> implementations.
>>> well, agree
>>>
>>>
>>>>>> It does not quite fit the model that Linux currently uses for 
>>>>>> iommus,
>>>>>> as that has an allocator for dma_addr_t space
>>>>> yes (# 3/7 adds grant-table based allocator)
>>>>>
>>>>>
>>>>>> , but it would think it's
>>>>>> conceptually close enough that it makes sense for the binding.
>>>>> Interesting idea. I am wondering, do we need an extra actions for 
>>>>> this
>>>>> to work in Linux guest (dummy IOMMU driver, etc)?
>>>> It depends on how closely the guest implementation can be made to
>>>> resemble a normal iommu. If you do allocate dma_addr_t addresses,
>>>> it may actually be close enough that you can just turn the grant-table
>>>> code into a normal iommu driver and change nothing else.
>>> Unfortunately, I failed to find a way how use grant references at the
>>> iommu_ops level (I mean to fully pretend that we are an IOMMU 
>>> driver). I am
>>> not too familiar with that, so what is written below might be wrong 
>>> or at
>>> least not precise.
>>>
>>> The normal IOMMU driver in Linux doesn’t allocate DMA addresses by 
>>> itself, it
>>> just maps (IOVA-PA) what was requested to be mapped by the upper 
>>> layer. The
>>> DMA address allocation is done by the upper layer (DMA-IOMMU which 
>>> is the glue
>>> layer between DMA API and IOMMU API allocates IOVA for PA?). But, 
>>> all what we
>>> need here is just to allocate our specific grant-table based DMA 
>>> addresses
>>> (DMA address = grant reference + offset in the page), so let’s say 
>>> we need an
>>> entity to take a physical address as parameter and return a DMA 
>>> address (what
>>> actually commit #3/7 is doing), and that’s all. So working at the 
>>> dma_ops
>>> layer we get exactly what we need, with the minimal changes to guest
>>> infrastructure. In our case the Xen itself acts as an IOMMU.
>>>
>>> Assuming that we want to reuse the IOMMU infrastructure somehow for 
>>> our needs.
>>> I think, in that case we will likely need to introduce a new 
>>> specific IOVA
>>> allocator (alongside with a generic one) to be hooked up by the 
>>> DMA-IOMMU
>>> layer if we run on top of Xen. But, even having the specific IOVA 
>>> allocator to
>>> return what we indeed need (DMA address = grant reference + offset 
>>> in the
>>> page) we will still need the specific minimal required IOMMU driver 
>>> to be
>>> present in the system anyway in order to track the mappings(?) and 
>>> do nothing
>>> with them, returning a success (this specific IOMMU driver should 
>>> have all
>>> mandatory callbacks implemented).
>>>
>>> I completely agree, it would be really nice to reuse generic IOMMU 
>>> bindings
>>> rather than introducing Xen specific property if what we are trying to
>>> implement in current patch series fits in the usage of "iommus" in 
>>> Linux
>>> more-less. But, if we will have to add more complexity/more 
>>> components to the
>>> code for the sake of reusing device tree binding, this raises a 
>>> question
>>> whether that’s worthwhile.
>>>
>>> Or I really missed something?
>> I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
>> bindings, not necessarily the IOMMU drivers framework in Linux (although
>> that would be an added bonus.)
>>
>> I know from previous discussions with you that making the grant table
>> fit in the existing IOMMU drivers model is difficult, but just reusing
>> the Device Tree bindings seems feasible?
>
> I started experimenting with that. As wrote in a separate email, I got 
> a deferred probe timeout,
>
> after inserting required nodes into guest device tree, which seems to 
> be a consequence of the unavailability of IOMMU, I will continue to 
> investigate this question.


I have experimented with that. Yes, just reusing the Device Tree 
bindings is technically feasible (and we are able to do this by only 
touching grant-dma-ops.c), although deferred probe timeout still stands 
(as there is no IOMMU driver being present actually).

[    0.583771] virtio-mmio 2000000.virtio: deferred probe timeout, 
ignoring dependency
[    0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks 
(2.10 GB/1.95 GiB)


Below the working diff (on top of current series):

diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
index da9c7ff..6586152 100644
--- a/drivers/xen/grant-dma-ops.c
+++ b/drivers/xen/grant-dma-ops.c
@@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops = {

  bool xen_is_grant_dma_device(struct device *dev)
  {
+       struct device_node *iommu_np;
+       bool has_iommu;
+
         /* XXX Handle only DT devices for now */
         if (!dev->of_node)
                 return false;

-       return of_property_read_bool(dev->of_node, "xen,backend-domid");
+       iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
+       has_iommu = iommu_np && of_device_is_compatible(iommu_np, 
"xen,grant-dma");
+       of_node_put(iommu_np);
+
+       return has_iommu;
  }

  void xen_grant_setup_dma_ops(struct device *dev)
  {
         struct xen_grant_dma_data *data;
-       uint32_t domid;
+       struct of_phandle_args iommu_spec;

         data = find_xen_grant_dma_data(dev);
         if (data) {
@@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
         if (!dev->of_node)
                 goto err;

-       if (of_property_read_u32(dev->of_node, "xen,backend-domid", 
&domid)) {
-               dev_err(dev, "xen,backend-domid property is not present\n");
+       if (of_parse_phandle_with_args(dev->of_node, "iommus", 
"#iommu-cells",
+                       0, &iommu_spec)) {
+               dev_err(dev, "Cannot parse iommus property\n");
+               goto err;
+       }
+
+       if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
+                       iommu_spec.args_count != 1) {
+               dev_err(dev, "Incompatible IOMMU node\n");
+               of_node_put(iommu_spec.np);
                 goto err;
         }

+       of_node_put(iommu_spec.np);
+
         data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
         if (!data)
                 goto err;

-       data->backend_domid = domid;
+       /*
+        * The endpoint ID here means the ID of the domain where the 
corresponding
+        * backend is running
+        */
+       data->backend_domid = iommu_spec.args[0];

         if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, 
data,
                         GFP_KERNEL))) {
(END)



Below, the nodes generated by Xen toolstack:

         xen_grant_dma {
                 compatible = "xen,grant-dma";
                 #iommu-cells = <0x01>;
                 phandle = <0xfde9>;
         };

         virtio@2000000 {
                 compatible = "virtio,mmio";
                 reg = <0x00 0x2000000 0x00 0x200>;
                 interrupts = <0x00 0x01 0xf01>;
                 interrupt-parent = <0xfde8>;
                 dma-coherent;
                 iommus = <0xfde9 0x01>;
         };



I am wondering, would be the proper solution to eliminate deferred probe 
timeout issue in our particular case (without introducing an extra IOMMU 
driver)?




>
>
>
>
-- 
Regards,

Oleksandr Tyshchenko

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Stefano Stabellini]

[-- Attachment #1: Type: text/plain, Size: 9846 bytes --]

On Mon, 23 May 2022, Oleksandr wrote:
> > > On Thu, 19 May 2022, Oleksandr wrote:
> > > > > On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
> > > > > > On 18.05.22 17:32, Arnd Bergmann wrote:
> > > > > > > On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
> > > > > > > <olekstysh@gmail.com> wrote:
> > > > > > >     This would mean having a device
> > > > > > > node for the grant-table mechanism that can be referred to using
> > > > > > > the
> > > > > > > 'iommus'
> > > > > > > phandle property, with the domid as an additional argument.
> > > > > > I assume, you are speaking about something like the following?
> > > > > > 
> > > > > > 
> > > > > > xen_dummy_iommu {
> > > > > >       compatible = "xen,dummy-iommu";
> > > > > >       #iommu-cells = <1>;
> > > > > > };
> > > > > > 
> > > > > > virtio@3000 {
> > > > > >       compatible = "virtio,mmio";
> > > > > >       reg = <0x3000 0x100>;
> > > > > >       interrupts = <41>;
> > > > > > 
> > > > > >       /* The device is located in Xen domain with ID 1 */
> > > > > >       iommus = <&xen_dummy_iommu 1>;
> > > > > > };
> > > > > Right, that's that's the idea,
> > > > thank you for the confirmation
> > > > 
> > > > 
> > > > 
> > > > >    except I would not call it a 'dummy'.
> > > > >   From the perspective of the DT, this behaves just like an IOMMU,
> > > > > even if the exact mechanism is different from most hardware IOMMU
> > > > > implementations.
> > > > well, agree
> > > > 
> > > > 
> > > > > > > It does not quite fit the model that Linux currently uses for
> > > > > > > iommus,
> > > > > > > as that has an allocator for dma_addr_t space
> > > > > > yes (# 3/7 adds grant-table based allocator)
> > > > > > 
> > > > > > 
> > > > > > > , but it would think it's
> > > > > > > conceptually close enough that it makes sense for the binding.
> > > > > > Interesting idea. I am wondering, do we need an extra actions for
> > > > > > this
> > > > > > to work in Linux guest (dummy IOMMU driver, etc)?
> > > > > It depends on how closely the guest implementation can be made to
> > > > > resemble a normal iommu. If you do allocate dma_addr_t addresses,
> > > > > it may actually be close enough that you can just turn the grant-table
> > > > > code into a normal iommu driver and change nothing else.
> > > > Unfortunately, I failed to find a way how use grant references at the
> > > > iommu_ops level (I mean to fully pretend that we are an IOMMU driver). I
> > > > am
> > > > not too familiar with that, so what is written below might be wrong or
> > > > at
> > > > least not precise.
> > > > 
> > > > The normal IOMMU driver in Linux doesn’t allocate DMA addresses by
> > > > itself, it
> > > > just maps (IOVA-PA) what was requested to be mapped by the upper layer.
> > > > The
> > > > DMA address allocation is done by the upper layer (DMA-IOMMU which is
> > > > the glue
> > > > layer between DMA API and IOMMU API allocates IOVA for PA?). But, all
> > > > what we
> > > > need here is just to allocate our specific grant-table based DMA
> > > > addresses
> > > > (DMA address = grant reference + offset in the page), so let’s say we
> > > > need an
> > > > entity to take a physical address as parameter and return a DMA address
> > > > (what
> > > > actually commit #3/7 is doing), and that’s all. So working at the
> > > > dma_ops
> > > > layer we get exactly what we need, with the minimal changes to guest
> > > > infrastructure. In our case the Xen itself acts as an IOMMU.
> > > > 
> > > > Assuming that we want to reuse the IOMMU infrastructure somehow for our
> > > > needs.
> > > > I think, in that case we will likely need to introduce a new specific
> > > > IOVA
> > > > allocator (alongside with a generic one) to be hooked up by the
> > > > DMA-IOMMU
> > > > layer if we run on top of Xen. But, even having the specific IOVA
> > > > allocator to
> > > > return what we indeed need (DMA address = grant reference + offset in
> > > > the
> > > > page) we will still need the specific minimal required IOMMU driver to
> > > > be
> > > > present in the system anyway in order to track the mappings(?) and do
> > > > nothing
> > > > with them, returning a success (this specific IOMMU driver should have
> > > > all
> > > > mandatory callbacks implemented).
> > > > 
> > > > I completely agree, it would be really nice to reuse generic IOMMU
> > > > bindings
> > > > rather than introducing Xen specific property if what we are trying to
> > > > implement in current patch series fits in the usage of "iommus" in Linux
> > > > more-less. But, if we will have to add more complexity/more components
> > > > to the
> > > > code for the sake of reusing device tree binding, this raises a question
> > > > whether that’s worthwhile.
> > > > 
> > > > Or I really missed something?
> > > I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
> > > bindings, not necessarily the IOMMU drivers framework in Linux (although
> > > that would be an added bonus.)
> > > 
> > > I know from previous discussions with you that making the grant table
> > > fit in the existing IOMMU drivers model is difficult, but just reusing
> > > the Device Tree bindings seems feasible?
> > 
> > I started experimenting with that. As wrote in a separate email, I got a
> > deferred probe timeout,
> > 
> > after inserting required nodes into guest device tree, which seems to be a
> > consequence of the unavailability of IOMMU, I will continue to investigate
> > this question.
> 
> 
> I have experimented with that. Yes, just reusing the Device Tree bindings is
> technically feasible (and we are able to do this by only touching
> grant-dma-ops.c), although deferred probe timeout still stands (as there is no
> IOMMU driver being present actually).
> 
> [    0.583771] virtio-mmio 2000000.virtio: deferred probe timeout, ignoring
> dependency
> [    0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks (2.10
> GB/1.95 GiB)
> 
> 
> Below the working diff (on top of current series):
> 
> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
> index da9c7ff..6586152 100644
> --- a/drivers/xen/grant-dma-ops.c
> +++ b/drivers/xen/grant-dma-ops.c
> @@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops = {
> 
>  bool xen_is_grant_dma_device(struct device *dev)
>  {
> +       struct device_node *iommu_np;
> +       bool has_iommu;
> +
>         /* XXX Handle only DT devices for now */
>         if (!dev->of_node)
>                 return false;
> 
> -       return of_property_read_bool(dev->of_node, "xen,backend-domid");
> +       iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
> +       has_iommu = iommu_np && of_device_is_compatible(iommu_np,
> "xen,grant-dma");
> +       of_node_put(iommu_np);
> +
> +       return has_iommu;
>  }
> 
>  void xen_grant_setup_dma_ops(struct device *dev)
>  {
>         struct xen_grant_dma_data *data;
> -       uint32_t domid;
> +       struct of_phandle_args iommu_spec;
> 
>         data = find_xen_grant_dma_data(dev);
>         if (data) {
> @@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
>         if (!dev->of_node)
>                 goto err;
> 
> -       if (of_property_read_u32(dev->of_node, "xen,backend-domid", &domid)) {
> -               dev_err(dev, "xen,backend-domid property is not present\n");
> +       if (of_parse_phandle_with_args(dev->of_node, "iommus", "#iommu-cells",
> +                       0, &iommu_spec)) {
> +               dev_err(dev, "Cannot parse iommus property\n");
> +               goto err;
> +       }
> +
> +       if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
> +                       iommu_spec.args_count != 1) {
> +               dev_err(dev, "Incompatible IOMMU node\n");
> +               of_node_put(iommu_spec.np);
>                 goto err;
>         }
> 
> +       of_node_put(iommu_spec.np);
> +
>         data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
>         if (!data)
>                 goto err;
> 
> -       data->backend_domid = domid;
> +       /*
> +        * The endpoint ID here means the ID of the domain where the
> corresponding
> +        * backend is running
> +        */
> +       data->backend_domid = iommu_spec.args[0];
> 
>         if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
>                         GFP_KERNEL))) {
> (END)
> 
> 
> 
> Below, the nodes generated by Xen toolstack:
> 
>         xen_grant_dma {
>                 compatible = "xen,grant-dma";
>                 #iommu-cells = <0x01>;
>                 phandle = <0xfde9>;
>         };
> 
>         virtio@2000000 {
>                 compatible = "virtio,mmio";
>                 reg = <0x00 0x2000000 0x00 0x200>;
>                 interrupts = <0x00 0x01 0xf01>;
>                 interrupt-parent = <0xfde8>;
>                 dma-coherent;
>                 iommus = <0xfde9 0x01>;
>         };
 
Not bad! I like it.
 
 
> I am wondering, would be the proper solution to eliminate deferred probe
> timeout issue in our particular case (without introducing an extra IOMMU
> driver)?

In reality I don't think there is a way to do that. I would create an
empty skelethon IOMMU driver for xen,grant-dma.

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Rob Herring]

+Saravana

On Mon, May 23, 2022 at 06:58:13PM -0700, Stefano Stabellini wrote:
> On Mon, 23 May 2022, Oleksandr wrote:
> > > > On Thu, 19 May 2022, Oleksandr wrote:
> > > > > > On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
> > > > > > > On 18.05.22 17:32, Arnd Bergmann wrote:
> > > > > > > > On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
> > > > > > > > <olekstysh@gmail.com> wrote:
> > > > > > > >     This would mean having a device
> > > > > > > > node for the grant-table mechanism that can be referred to using
> > > > > > > > the
> > > > > > > > 'iommus'
> > > > > > > > phandle property, with the domid as an additional argument.
> > > > > > > I assume, you are speaking about something like the following?
> > > > > > > 
> > > > > > > 
> > > > > > > xen_dummy_iommu {
> > > > > > >       compatible = "xen,dummy-iommu";
> > > > > > >       #iommu-cells = <1>;
> > > > > > > };
> > > > > > > 
> > > > > > > virtio@3000 {
> > > > > > >       compatible = "virtio,mmio";
> > > > > > >       reg = <0x3000 0x100>;
> > > > > > >       interrupts = <41>;
> > > > > > > 
> > > > > > >       /* The device is located in Xen domain with ID 1 */
> > > > > > >       iommus = <&xen_dummy_iommu 1>;
> > > > > > > };
> > > > > > Right, that's that's the idea,
> > > > > thank you for the confirmation
> > > > > 
> > > > > 
> > > > > 
> > > > > >    except I would not call it a 'dummy'.
> > > > > >   From the perspective of the DT, this behaves just like an IOMMU,
> > > > > > even if the exact mechanism is different from most hardware IOMMU
> > > > > > implementations.
> > > > > well, agree
> > > > > 
> > > > > 
> > > > > > > > It does not quite fit the model that Linux currently uses for
> > > > > > > > iommus,
> > > > > > > > as that has an allocator for dma_addr_t space
> > > > > > > yes (# 3/7 adds grant-table based allocator)
> > > > > > > 
> > > > > > > 
> > > > > > > > , but it would think it's
> > > > > > > > conceptually close enough that it makes sense for the binding.
> > > > > > > Interesting idea. I am wondering, do we need an extra actions for
> > > > > > > this
> > > > > > > to work in Linux guest (dummy IOMMU driver, etc)?
> > > > > > It depends on how closely the guest implementation can be made to
> > > > > > resemble a normal iommu. If you do allocate dma_addr_t addresses,
> > > > > > it may actually be close enough that you can just turn the grant-table
> > > > > > code into a normal iommu driver and change nothing else.
> > > > > Unfortunately, I failed to find a way how use grant references at the
> > > > > iommu_ops level (I mean to fully pretend that we are an IOMMU driver). I
> > > > > am
> > > > > not too familiar with that, so what is written below might be wrong or
> > > > > at
> > > > > least not precise.
> > > > > 
> > > > > The normal IOMMU driver in Linux doesn’t allocate DMA addresses by
> > > > > itself, it
> > > > > just maps (IOVA-PA) what was requested to be mapped by the upper layer.
> > > > > The
> > > > > DMA address allocation is done by the upper layer (DMA-IOMMU which is
> > > > > the glue
> > > > > layer between DMA API and IOMMU API allocates IOVA for PA?). But, all
> > > > > what we
> > > > > need here is just to allocate our specific grant-table based DMA
> > > > > addresses
> > > > > (DMA address = grant reference + offset in the page), so let’s say we
> > > > > need an
> > > > > entity to take a physical address as parameter and return a DMA address
> > > > > (what
> > > > > actually commit #3/7 is doing), and that’s all. So working at the
> > > > > dma_ops
> > > > > layer we get exactly what we need, with the minimal changes to guest
> > > > > infrastructure. In our case the Xen itself acts as an IOMMU.
> > > > > 
> > > > > Assuming that we want to reuse the IOMMU infrastructure somehow for our
> > > > > needs.
> > > > > I think, in that case we will likely need to introduce a new specific
> > > > > IOVA
> > > > > allocator (alongside with a generic one) to be hooked up by the
> > > > > DMA-IOMMU
> > > > > layer if we run on top of Xen. But, even having the specific IOVA
> > > > > allocator to
> > > > > return what we indeed need (DMA address = grant reference + offset in
> > > > > the
> > > > > page) we will still need the specific minimal required IOMMU driver to
> > > > > be
> > > > > present in the system anyway in order to track the mappings(?) and do
> > > > > nothing
> > > > > with them, returning a success (this specific IOMMU driver should have
> > > > > all
> > > > > mandatory callbacks implemented).
> > > > > 
> > > > > I completely agree, it would be really nice to reuse generic IOMMU
> > > > > bindings
> > > > > rather than introducing Xen specific property if what we are trying to
> > > > > implement in current patch series fits in the usage of "iommus" in Linux
> > > > > more-less. But, if we will have to add more complexity/more components
> > > > > to the
> > > > > code for the sake of reusing device tree binding, this raises a question
> > > > > whether that’s worthwhile.
> > > > > 
> > > > > Or I really missed something?
> > > > I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
> > > > bindings, not necessarily the IOMMU drivers framework in Linux (although
> > > > that would be an added bonus.)
> > > > 
> > > > I know from previous discussions with you that making the grant table
> > > > fit in the existing IOMMU drivers model is difficult, but just reusing
> > > > the Device Tree bindings seems feasible?
> > > 
> > > I started experimenting with that. As wrote in a separate email, I got a
> > > deferred probe timeout,
> > > 
> > > after inserting required nodes into guest device tree, which seems to be a
> > > consequence of the unavailability of IOMMU, I will continue to investigate
> > > this question.
> > 
> > 
> > I have experimented with that. Yes, just reusing the Device Tree bindings is
> > technically feasible (and we are able to do this by only touching
> > grant-dma-ops.c), although deferred probe timeout still stands (as there is no
> > IOMMU driver being present actually).
> > 
> > [    0.583771] virtio-mmio 2000000.virtio: deferred probe timeout, ignoring
> > dependency
> > [    0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks (2.10
> > GB/1.95 GiB)
> > 
> > 
> > Below the working diff (on top of current series):
> > 
> > diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
> > index da9c7ff..6586152 100644
> > --- a/drivers/xen/grant-dma-ops.c
> > +++ b/drivers/xen/grant-dma-ops.c
> > @@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops = {
> > 
> >  bool xen_is_grant_dma_device(struct device *dev)
> >  {
> > +       struct device_node *iommu_np;
> > +       bool has_iommu;
> > +
> >         /* XXX Handle only DT devices for now */
> >         if (!dev->of_node)
> >                 return false;
> > 
> > -       return of_property_read_bool(dev->of_node, "xen,backend-domid");
> > +       iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
> > +       has_iommu = iommu_np && of_device_is_compatible(iommu_np,
> > "xen,grant-dma");
> > +       of_node_put(iommu_np);
> > +
> > +       return has_iommu;
> >  }
> > 
> >  void xen_grant_setup_dma_ops(struct device *dev)
> >  {
> >         struct xen_grant_dma_data *data;
> > -       uint32_t domid;
> > +       struct of_phandle_args iommu_spec;
> > 
> >         data = find_xen_grant_dma_data(dev);
> >         if (data) {
> > @@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
> >         if (!dev->of_node)
> >                 goto err;
> > 
> > -       if (of_property_read_u32(dev->of_node, "xen,backend-domid", &domid)) {
> > -               dev_err(dev, "xen,backend-domid property is not present\n");
> > +       if (of_parse_phandle_with_args(dev->of_node, "iommus", "#iommu-cells",
> > +                       0, &iommu_spec)) {
> > +               dev_err(dev, "Cannot parse iommus property\n");
> > +               goto err;
> > +       }
> > +
> > +       if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
> > +                       iommu_spec.args_count != 1) {
> > +               dev_err(dev, "Incompatible IOMMU node\n");
> > +               of_node_put(iommu_spec.np);
> >                 goto err;
> >         }
> > 
> > +       of_node_put(iommu_spec.np);
> > +
> >         data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
> >         if (!data)
> >                 goto err;
> > 
> > -       data->backend_domid = domid;
> > +       /*
> > +        * The endpoint ID here means the ID of the domain where the
> > corresponding
> > +        * backend is running
> > +        */
> > +       data->backend_domid = iommu_spec.args[0];
> > 
> >         if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
> >                         GFP_KERNEL))) {
> > (END)
> > 
> > 
> > 
> > Below, the nodes generated by Xen toolstack:
> > 
> >         xen_grant_dma {

Nit: iommu {

> >                 compatible = "xen,grant-dma";
> >                 #iommu-cells = <0x01>;
> >                 phandle = <0xfde9>;
> >         };
> > 
> >         virtio@2000000 {
> >                 compatible = "virtio,mmio";
> >                 reg = <0x00 0x2000000 0x00 0x200>;
> >                 interrupts = <0x00 0x01 0xf01>;
> >                 interrupt-parent = <0xfde8>;
> >                 dma-coherent;
> >                 iommus = <0xfde9 0x01>;
> >         };
>  
> Not bad! I like it.
>  
>  
> > I am wondering, would be the proper solution to eliminate deferred probe
> > timeout issue in our particular case (without introducing an extra IOMMU
> > driver)?
> 
> In reality I don't think there is a way to do that. I would create an
> empty skelethon IOMMU driver for xen,grant-dma.

Does it have to be an empty driver? Originally, IOMMU 'drivers' were not 
drivers, but they've been getting converted. Can that be done here?

Short of that, I think we could have some sort of skip probe list for 
deferred probe. Not sure if that would be easiest as IOMMU specific or 
global.

Rob

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Saravana Kannan]

"

On Tue, May 24, 2022 at 9:01 AM Rob Herring <robh@kernel.org> wrote:
>
> +Saravana
>
> On Mon, May 23, 2022 at 06:58:13PM -0700, Stefano Stabellini wrote:
> > On Mon, 23 May 2022, Oleksandr wrote:
> > > > > On Thu, 19 May 2022, Oleksandr wrote:
> > > > > > > On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
> > > > > > > > On 18.05.22 17:32, Arnd Bergmann wrote:
> > > > > > > > > On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
> > > > > > > > > <olekstysh@gmail.com> wrote:
> > > > > > > > >     This would mean having a device
> > > > > > > > > node for the grant-table mechanism that can be referred to using
> > > > > > > > > the
> > > > > > > > > 'iommus'
> > > > > > > > > phandle property, with the domid as an additional argument.
> > > > > > > > I assume, you are speaking about something like the following?
> > > > > > > >
> > > > > > > >
> > > > > > > > xen_dummy_iommu {
> > > > > > > >       compatible = "xen,dummy-iommu";
> > > > > > > >       #iommu-cells = <1>;
> > > > > > > > };
> > > > > > > >
> > > > > > > > virtio@3000 {
> > > > > > > >       compatible = "virtio,mmio";
> > > > > > > >       reg = <0x3000 0x100>;
> > > > > > > >       interrupts = <41>;
> > > > > > > >
> > > > > > > >       /* The device is located in Xen domain with ID 1 */
> > > > > > > >       iommus = <&xen_dummy_iommu 1>;
> > > > > > > > };
> > > > > > > Right, that's that's the idea,
> > > > > > thank you for the confirmation
> > > > > >
> > > > > >
> > > > > >
> > > > > > >    except I would not call it a 'dummy'.
> > > > > > >   From the perspective of the DT, this behaves just like an IOMMU,
> > > > > > > even if the exact mechanism is different from most hardware IOMMU
> > > > > > > implementations.
> > > > > > well, agree
> > > > > >
> > > > > >
> > > > > > > > > It does not quite fit the model that Linux currently uses for
> > > > > > > > > iommus,
> > > > > > > > > as that has an allocator for dma_addr_t space
> > > > > > > > yes (# 3/7 adds grant-table based allocator)
> > > > > > > >
> > > > > > > >
> > > > > > > > > , but it would think it's
> > > > > > > > > conceptually close enough that it makes sense for the binding.
> > > > > > > > Interesting idea. I am wondering, do we need an extra actions for
> > > > > > > > this
> > > > > > > > to work in Linux guest (dummy IOMMU driver, etc)?
> > > > > > > It depends on how closely the guest implementation can be made to
> > > > > > > resemble a normal iommu. If you do allocate dma_addr_t addresses,
> > > > > > > it may actually be close enough that you can just turn the grant-table
> > > > > > > code into a normal iommu driver and change nothing else.
> > > > > > Unfortunately, I failed to find a way how use grant references at the
> > > > > > iommu_ops level (I mean to fully pretend that we are an IOMMU driver). I
> > > > > > am
> > > > > > not too familiar with that, so what is written below might be wrong or
> > > > > > at
> > > > > > least not precise.
> > > > > >
> > > > > > The normal IOMMU driver in Linux doesn’t allocate DMA addresses by
> > > > > > itself, it
> > > > > > just maps (IOVA-PA) what was requested to be mapped by the upper layer.
> > > > > > The
> > > > > > DMA address allocation is done by the upper layer (DMA-IOMMU which is
> > > > > > the glue
> > > > > > layer between DMA API and IOMMU API allocates IOVA for PA?). But, all
> > > > > > what we
> > > > > > need here is just to allocate our specific grant-table based DMA
> > > > > > addresses
> > > > > > (DMA address = grant reference + offset in the page), so let’s say we
> > > > > > need an
> > > > > > entity to take a physical address as parameter and return a DMA address
> > > > > > (what
> > > > > > actually commit #3/7 is doing), and that’s all. So working at the
> > > > > > dma_ops
> > > > > > layer we get exactly what we need, with the minimal changes to guest
> > > > > > infrastructure. In our case the Xen itself acts as an IOMMU.
> > > > > >
> > > > > > Assuming that we want to reuse the IOMMU infrastructure somehow for our
> > > > > > needs.
> > > > > > I think, in that case we will likely need to introduce a new specific
> > > > > > IOVA
> > > > > > allocator (alongside with a generic one) to be hooked up by the
> > > > > > DMA-IOMMU
> > > > > > layer if we run on top of Xen. But, even having the specific IOVA
> > > > > > allocator to
> > > > > > return what we indeed need (DMA address = grant reference + offset in
> > > > > > the
> > > > > > page) we will still need the specific minimal required IOMMU driver to
> > > > > > be
> > > > > > present in the system anyway in order to track the mappings(?) and do
> > > > > > nothing
> > > > > > with them, returning a success (this specific IOMMU driver should have
> > > > > > all
> > > > > > mandatory callbacks implemented).
> > > > > >
> > > > > > I completely agree, it would be really nice to reuse generic IOMMU
> > > > > > bindings
> > > > > > rather than introducing Xen specific property if what we are trying to
> > > > > > implement in current patch series fits in the usage of "iommus" in Linux
> > > > > > more-less. But, if we will have to add more complexity/more components
> > > > > > to the
> > > > > > code for the sake of reusing device tree binding, this raises a question
> > > > > > whether that’s worthwhile.
> > > > > >
> > > > > > Or I really missed something?
> > > > > I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
> > > > > bindings, not necessarily the IOMMU drivers framework in Linux (although
> > > > > that would be an added bonus.)
> > > > >
> > > > > I know from previous discussions with you that making the grant table
> > > > > fit in the existing IOMMU drivers model is difficult, but just reusing
> > > > > the Device Tree bindings seems feasible?
> > > >
> > > > I started experimenting with that. As wrote in a separate email, I got a
> > > > deferred probe timeout,
> > > >
> > > > after inserting required nodes into guest device tree, which seems to be a
> > > > consequence of the unavailability of IOMMU, I will continue to investigate
> > > > this question.
> > >
> > >
> > > I have experimented with that. Yes, just reusing the Device Tree bindings is
> > > technically feasible (and we are able to do this by only touching
> > > grant-dma-ops.c), although deferred probe timeout still stands (as there is no
> > > IOMMU driver being present actually).
> > >
> > > [    0.583771] virtio-mmio 2000000.virtio: deferred probe timeout, ignoring
> > > dependency
> > > [    0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks (2.10
> > > GB/1.95 GiB)
> > >
> > >
> > > Below the working diff (on top of current series):
> > >
> > > diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
> > > index da9c7ff..6586152 100644
> > > --- a/drivers/xen/grant-dma-ops.c
> > > +++ b/drivers/xen/grant-dma-ops.c
> > > @@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops = {
> > >
> > >  bool xen_is_grant_dma_device(struct device *dev)
> > >  {
> > > +       struct device_node *iommu_np;
> > > +       bool has_iommu;
> > > +
> > >         /* XXX Handle only DT devices for now */
> > >         if (!dev->of_node)
> > >                 return false;
> > >
> > > -       return of_property_read_bool(dev->of_node, "xen,backend-domid");
> > > +       iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
> > > +       has_iommu = iommu_np && of_device_is_compatible(iommu_np,
> > > "xen,grant-dma");
> > > +       of_node_put(iommu_np);
> > > +
> > > +       return has_iommu;
> > >  }
> > >
> > >  void xen_grant_setup_dma_ops(struct device *dev)
> > >  {
> > >         struct xen_grant_dma_data *data;
> > > -       uint32_t domid;
> > > +       struct of_phandle_args iommu_spec;
> > >
> > >         data = find_xen_grant_dma_data(dev);
> > >         if (data) {
> > > @@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
> > >         if (!dev->of_node)
> > >                 goto err;
> > >
> > > -       if (of_property_read_u32(dev->of_node, "xen,backend-domid", &domid)) {
> > > -               dev_err(dev, "xen,backend-domid property is not present\n");
> > > +       if (of_parse_phandle_with_args(dev->of_node, "iommus", "#iommu-cells",
> > > +                       0, &iommu_spec)) {
> > > +               dev_err(dev, "Cannot parse iommus property\n");
> > > +               goto err;
> > > +       }
> > > +
> > > +       if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
> > > +                       iommu_spec.args_count != 1) {
> > > +               dev_err(dev, "Incompatible IOMMU node\n");
> > > +               of_node_put(iommu_spec.np);
> > >                 goto err;
> > >         }
> > >
> > > +       of_node_put(iommu_spec.np);
> > > +
> > >         data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
> > >         if (!data)
> > >                 goto err;
> > >
> > > -       data->backend_domid = domid;
> > > +       /*
> > > +        * The endpoint ID here means the ID of the domain where the
> > > corresponding
> > > +        * backend is running
> > > +        */
> > > +       data->backend_domid = iommu_spec.args[0];
> > >
> > >         if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
> > >                         GFP_KERNEL))) {
> > > (END)
> > >
> > >
> > >
> > > Below, the nodes generated by Xen toolstack:
> > >
> > >         xen_grant_dma {
>
> Nit: iommu {
>
> > >                 compatible = "xen,grant-dma";
> > >                 #iommu-cells = <0x01>;
> > >                 phandle = <0xfde9>;
> > >         };
> > >
> > >         virtio@2000000 {
> > >                 compatible = "virtio,mmio";
> > >                 reg = <0x00 0x2000000 0x00 0x200>;
> > >                 interrupts = <0x00 0x01 0xf01>;
> > >                 interrupt-parent = <0xfde8>;
> > >                 dma-coherent;
> > >                 iommus = <0xfde9 0x01>;
> > >         };
> >
> > Not bad! I like it.
> >
> >
> > > I am wondering, would be the proper solution to eliminate deferred probe
> > > timeout issue in our particular case (without introducing an extra IOMMU
> > > driver)?
> >
> > In reality I don't think there is a way to do that. I would create an
> > empty skelethon IOMMU driver for xen,grant-dma.
>
> Does it have to be an empty driver? Originally, IOMMU 'drivers' were not
> drivers, but they've been getting converted. Can that be done here?
>
> Short of that, I think we could have some sort of skip probe list for
> deferred probe. Not sure if that would be easiest as IOMMU specific or
> global.

Hi Oleksandr,

If you do fw_devlink.strict=1, you'll notice that the consumers of
this "iommu" won't probe at all or will delay the boot by some number
of seconds. The eventual goal is to go towards fw_devlink.strict=1
being the default.

From a fw_devlik perspective, please implement a driver. Ideally a
real one, but at least an empty one. The empty one doesn't need to be
an IOMMU driver, but at least just do a return 0 in the probe
function. Also, if it's not a device, why even have a "compatible"
property (removing it won't necessarily remove the deferred probe
timeout issue you see)? Will any code be using "xen,grant-dma" to look
up the node? If so, that driver could be the one that probes this
device. At least from a fw_devlink perspective, it just needs to have
a driver that binds to this device.

Also, if we aren't going to implement a driver and have the supplier
("xen,grant-dma") behave like a device (as in, have a driver that
probes), I'd rather that the iommu binding not be used at all as this
would be an exception to how every other iommu device behaves.

-Saravana

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 24.05.22 21:34, Saravana Kannan wrote:

Hello all

> "
>
> On Tue, May 24, 2022 at 9:01 AM Rob Herring <robh@kernel.org> wrote:
>> +Saravana
>>
>> On Mon, May 23, 2022 at 06:58:13PM -0700, Stefano Stabellini wrote:
>>> On Mon, 23 May 2022, Oleksandr wrote:
>>>>>> On Thu, 19 May 2022, Oleksandr wrote:
>>>>>>>> On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
>>>>>>>>> On 18.05.22 17:32, Arnd Bergmann wrote:
>>>>>>>>>> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
>>>>>>>>>> <olekstysh@gmail.com> wrote:
>>>>>>>>>>      This would mean having a device
>>>>>>>>>> node for the grant-table mechanism that can be referred to using
>>>>>>>>>> the
>>>>>>>>>> 'iommus'
>>>>>>>>>> phandle property, with the domid as an additional argument.
>>>>>>>>> I assume, you are speaking about something like the following?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> xen_dummy_iommu {
>>>>>>>>>        compatible = "xen,dummy-iommu";
>>>>>>>>>        #iommu-cells = <1>;
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> virtio@3000 {
>>>>>>>>>        compatible = "virtio,mmio";
>>>>>>>>>        reg = <0x3000 0x100>;
>>>>>>>>>        interrupts = <41>;
>>>>>>>>>
>>>>>>>>>        /* The device is located in Xen domain with ID 1 */
>>>>>>>>>        iommus = <&xen_dummy_iommu 1>;
>>>>>>>>> };
>>>>>>>> Right, that's that's the idea,
>>>>>>> thank you for the confirmation
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>     except I would not call it a 'dummy'.
>>>>>>>>    From the perspective of the DT, this behaves just like an IOMMU,
>>>>>>>> even if the exact mechanism is different from most hardware IOMMU
>>>>>>>> implementations.
>>>>>>> well, agree
>>>>>>>
>>>>>>>
>>>>>>>>>> It does not quite fit the model that Linux currently uses for
>>>>>>>>>> iommus,
>>>>>>>>>> as that has an allocator for dma_addr_t space
>>>>>>>>> yes (# 3/7 adds grant-table based allocator)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> , but it would think it's
>>>>>>>>>> conceptually close enough that it makes sense for the binding.
>>>>>>>>> Interesting idea. I am wondering, do we need an extra actions for
>>>>>>>>> this
>>>>>>>>> to work in Linux guest (dummy IOMMU driver, etc)?
>>>>>>>> It depends on how closely the guest implementation can be made to
>>>>>>>> resemble a normal iommu. If you do allocate dma_addr_t addresses,
>>>>>>>> it may actually be close enough that you can just turn the grant-table
>>>>>>>> code into a normal iommu driver and change nothing else.
>>>>>>> Unfortunately, I failed to find a way how use grant references at the
>>>>>>> iommu_ops level (I mean to fully pretend that we are an IOMMU driver). I
>>>>>>> am
>>>>>>> not too familiar with that, so what is written below might be wrong or
>>>>>>> at
>>>>>>> least not precise.
>>>>>>>
>>>>>>> The normal IOMMU driver in Linux doesn’t allocate DMA addresses by
>>>>>>> itself, it
>>>>>>> just maps (IOVA-PA) what was requested to be mapped by the upper layer.
>>>>>>> The
>>>>>>> DMA address allocation is done by the upper layer (DMA-IOMMU which is
>>>>>>> the glue
>>>>>>> layer between DMA API and IOMMU API allocates IOVA for PA?). But, all
>>>>>>> what we
>>>>>>> need here is just to allocate our specific grant-table based DMA
>>>>>>> addresses
>>>>>>> (DMA address = grant reference + offset in the page), so let’s say we
>>>>>>> need an
>>>>>>> entity to take a physical address as parameter and return a DMA address
>>>>>>> (what
>>>>>>> actually commit #3/7 is doing), and that’s all. So working at the
>>>>>>> dma_ops
>>>>>>> layer we get exactly what we need, with the minimal changes to guest
>>>>>>> infrastructure. In our case the Xen itself acts as an IOMMU.
>>>>>>>
>>>>>>> Assuming that we want to reuse the IOMMU infrastructure somehow for our
>>>>>>> needs.
>>>>>>> I think, in that case we will likely need to introduce a new specific
>>>>>>> IOVA
>>>>>>> allocator (alongside with a generic one) to be hooked up by the
>>>>>>> DMA-IOMMU
>>>>>>> layer if we run on top of Xen. But, even having the specific IOVA
>>>>>>> allocator to
>>>>>>> return what we indeed need (DMA address = grant reference + offset in
>>>>>>> the
>>>>>>> page) we will still need the specific minimal required IOMMU driver to
>>>>>>> be
>>>>>>> present in the system anyway in order to track the mappings(?) and do
>>>>>>> nothing
>>>>>>> with them, returning a success (this specific IOMMU driver should have
>>>>>>> all
>>>>>>> mandatory callbacks implemented).
>>>>>>>
>>>>>>> I completely agree, it would be really nice to reuse generic IOMMU
>>>>>>> bindings
>>>>>>> rather than introducing Xen specific property if what we are trying to
>>>>>>> implement in current patch series fits in the usage of "iommus" in Linux
>>>>>>> more-less. But, if we will have to add more complexity/more components
>>>>>>> to the
>>>>>>> code for the sake of reusing device tree binding, this raises a question
>>>>>>> whether that’s worthwhile.
>>>>>>>
>>>>>>> Or I really missed something?
>>>>>> I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
>>>>>> bindings, not necessarily the IOMMU drivers framework in Linux (although
>>>>>> that would be an added bonus.)
>>>>>>
>>>>>> I know from previous discussions with you that making the grant table
>>>>>> fit in the existing IOMMU drivers model is difficult, but just reusing
>>>>>> the Device Tree bindings seems feasible?
>>>>> I started experimenting with that. As wrote in a separate email, I got a
>>>>> deferred probe timeout,
>>>>>
>>>>> after inserting required nodes into guest device tree, which seems to be a
>>>>> consequence of the unavailability of IOMMU, I will continue to investigate
>>>>> this question.
>>>>
>>>> I have experimented with that. Yes, just reusing the Device Tree bindings is
>>>> technically feasible (and we are able to do this by only touching
>>>> grant-dma-ops.c), although deferred probe timeout still stands (as there is no
>>>> IOMMU driver being present actually).
>>>>
>>>> [    0.583771] virtio-mmio 2000000.virtio: deferred probe timeout, ignoring
>>>> dependency
>>>> [    0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks (2.10
>>>> GB/1.95 GiB)
>>>>
>>>>
>>>> Below the working diff (on top of current series):
>>>>
>>>> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>>>> index da9c7ff..6586152 100644
>>>> --- a/drivers/xen/grant-dma-ops.c
>>>> +++ b/drivers/xen/grant-dma-ops.c
>>>> @@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops = {
>>>>
>>>>   bool xen_is_grant_dma_device(struct device *dev)
>>>>   {
>>>> +       struct device_node *iommu_np;
>>>> +       bool has_iommu;
>>>> +
>>>>          /* XXX Handle only DT devices for now */
>>>>          if (!dev->of_node)
>>>>                  return false;
>>>>
>>>> -       return of_property_read_bool(dev->of_node, "xen,backend-domid");
>>>> +       iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
>>>> +       has_iommu = iommu_np && of_device_is_compatible(iommu_np,
>>>> "xen,grant-dma");
>>>> +       of_node_put(iommu_np);
>>>> +
>>>> +       return has_iommu;
>>>>   }
>>>>
>>>>   void xen_grant_setup_dma_ops(struct device *dev)
>>>>   {
>>>>          struct xen_grant_dma_data *data;
>>>> -       uint32_t domid;
>>>> +       struct of_phandle_args iommu_spec;
>>>>
>>>>          data = find_xen_grant_dma_data(dev);
>>>>          if (data) {
>>>> @@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
>>>>          if (!dev->of_node)
>>>>                  goto err;
>>>>
>>>> -       if (of_property_read_u32(dev->of_node, "xen,backend-domid", &domid)) {
>>>> -               dev_err(dev, "xen,backend-domid property is not present\n");
>>>> +       if (of_parse_phandle_with_args(dev->of_node, "iommus", "#iommu-cells",
>>>> +                       0, &iommu_spec)) {
>>>> +               dev_err(dev, "Cannot parse iommus property\n");
>>>> +               goto err;
>>>> +       }
>>>> +
>>>> +       if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
>>>> +                       iommu_spec.args_count != 1) {
>>>> +               dev_err(dev, "Incompatible IOMMU node\n");
>>>> +               of_node_put(iommu_spec.np);
>>>>                  goto err;
>>>>          }
>>>>
>>>> +       of_node_put(iommu_spec.np);
>>>> +
>>>>          data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
>>>>          if (!data)
>>>>                  goto err;
>>>>
>>>> -       data->backend_domid = domid;
>>>> +       /*
>>>> +        * The endpoint ID here means the ID of the domain where the
>>>> corresponding
>>>> +        * backend is running
>>>> +        */
>>>> +       data->backend_domid = iommu_spec.args[0];
>>>>
>>>>          if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
>>>>                          GFP_KERNEL))) {
>>>> (END)
>>>>
>>>>
>>>>
>>>> Below, the nodes generated by Xen toolstack:
>>>>
>>>>          xen_grant_dma {
>> Nit: iommu {
>>
>>>>                  compatible = "xen,grant-dma";
>>>>                  #iommu-cells = <0x01>;
>>>>                  phandle = <0xfde9>;
>>>>          };
>>>>
>>>>          virtio@2000000 {
>>>>                  compatible = "virtio,mmio";
>>>>                  reg = <0x00 0x2000000 0x00 0x200>;
>>>>                  interrupts = <0x00 0x01 0xf01>;
>>>>                  interrupt-parent = <0xfde8>;
>>>>                  dma-coherent;
>>>>                  iommus = <0xfde9 0x01>;
>>>>          };
>>> Not bad! I like it.
>>>
>>>
>>>> I am wondering, would be the proper solution to eliminate deferred probe
>>>> timeout issue in our particular case (without introducing an extra IOMMU
>>>> driver)?
>>> In reality I don't think there is a way to do that. I would create an
>>> empty skelethon IOMMU driver for xen,grant-dma.
>> Does it have to be an empty driver? Originally, IOMMU 'drivers' were not
>> drivers, but they've been getting converted. Can that be done here?
>>
>> Short of that, I think we could have some sort of skip probe list for
>> deferred probe. Not sure if that would be easiest as IOMMU specific or
>> global.
> Hi Oleksandr,
>
> If you do fw_devlink.strict=1, you'll notice that the consumers of
> this "iommu" won't probe at all or will delay the boot by some number
> of seconds. The eventual goal is to go towards fw_devlink.strict=1
> being the default.

ok, I got it.

Let's me please explain our particular case in details, sorry I may 
repeat some information which I have already mentioned elsewhere, but it 
maybe better to keep the whole context here.

We have Xen grant DMA-mapping layer added by previous commit [1]. For it 
to operate properly we need a way to communicate some per-device 
information using device-tree,
and this information is Xen specific. This is what the current commit is 
doing by introducing new binding to describe that. The next commit [2] 
will use that new binding to retrieve required information. There was a 
suggestion to consider reusing generic device-tree IOMMU bindings to 
communicate this specific information instead of introducing a custom 
property.

Although it requires more effort for the Xen toolstack (instead of 
adding a code to insert a single "xen,backend-domid" property, we need 
to generate fake IOMMU node, reserve phandle for it, etc), from the 
device tree PoV it looks indeed good (we reuse endpoint ID to pass the 
ID of the domain where the corresponding backend is running), and 
resulting code to retrieve this information in our DMA-mapping layer 
also looks simple enough [3].

Using generic device-tree IOMMU bindings:

          iommu {
                  compatible = "xen,grant-dma";
                  #iommu-cells = <0x01>;
                  phandle = <0xfde9>;
          };
          virtio@2000000 {
                  compatible = "virtio,mmio";
                  reg = <0x00 0x2000000 0x00 0x200>;
                  interrupts = <0x00 0x01 0xf01>;
                  interrupt-parent = <0xfde8>;
                  dma-coherent;
                  iommus = <0xfde9 0x01>;
          };

Using Xen specific property:

          virtio@2000000 {
                  compatible = "virtio,mmio";
                  reg = <0x00 0x2000000 0x00 0x200>;
                  interrupts = <0x00 0x01 0xf01>;
                  interrupt-parent = <0xfde8>;
                  dma-coherent;
                  xen,backend-domid = <0x01>;
          };


The main problem is that idea doesn't quite fit into how Linux currently 
behaves for the "iommus" property. Of course, just reusing IOMMU 
bindings (without having a corresponding driver) leads to the deferred 
probe timeout issue afterwards, because the IOMMU device never becomes 
available. From my understanding, our DMA-mapping layer we are consider 
to reuse IOMMU bindings for, *cannot* be converted into the proper IOMMU 
driver.

Sure, we will need to find a way how to deal with it, if we really want 
to reuse the IOMMU bindings. And yes, one idea was to just implement 
stub IOMMU driver for that purpose, I have rechecked, it works fine with 
that stub driver [4].

>
>  From a fw_devlik perspective, please implement a driver. Ideally a
> real one, but at least an empty one. The empty one doesn't need to be
> an IOMMU driver, but at least just do a return 0 in the probe
> function.


If I got things right, I am afraid, for the "of_iommu" case the empty 
driver is not enough. The driver should at least register iommu_ops, but 
the "of_xlate" callback should be *not* implemented.
In that case, we will get NO_IOMMU (>0 : there is no IOMMU, or one was 
unavailable for non-fatal reasons) which is also a success condition, so 
-EPROBE_DEFER won't be returned.

https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L32

Otherwise, of_iommu_xlate() will call driver_deferred_probe_check_state().

https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L43

>   Also, if it's not a device, why even have a "compatible"
> property (removing it won't necessarily remove the deferred probe
> timeout issue you see)? Will any code be using "xen,grant-dma" to look
> up the node?

Yes


>   If so, that driver could be the one that probes this
> device. At least from a fw_devlink perspective, it just needs to have
> a driver that binds to this device.

Agree


>
> Also, if we aren't going to implement a driver and have the supplier
> ("xen,grant-dma") behave like a device (as in, have a driver that
> probes), I'd rather that the iommu binding not be used at all as this
> would be an exception to how every other iommu device behaves.

Agree


Saravana, thank you for the explanation.


To summarize, as I understand, we have three options (the first two are 
clear enough, the third is unclear yet):

1. Do not try to reuse IOMMU bindings for current xen-virtio enabling 
work, use "xen,backend-domid" property.
2. Reuse IOMMU bindings, for that purpose introduce stub IOMMU driver. 
It is a standalone entity in my example, but it can be a part of 
grant-dma-ops.c which
actually uses "xen,grant-dma" compatible to look up a node.
3. Try to find other options how to reuse IOMMU bindings but *without* 
introducing stub IOMMU driver, such as skip list for deferred probe, etc.


What do the maintainers think regarding the option to go forward?


[1] 
https://lore.kernel.org/xen-devel/1651947548-4055-4-git-send-email-olekstysh@gmail.com/

[2] 
https://lore.kernel.org/xen-devel/1651947548-4055-7-git-send-email-olekstysh@gmail.com/

[3] 
https://lore.kernel.org/xen-devel/390ba7bb-ee9e-b7b7-5f08-71a7245fa4ec@gmail.com/

[4] 
https://lore.kernel.org/xen-devel/606dfdcc-ec10-0c4a-04e9-72cd73ee6676@gmail.com/

>
> -Saravana

-- 
Regards,

Oleksandr Tyshchenko

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 24.05.22 04:58, Stefano Stabellini wrote:

Hello Stefano, all

> On Mon, 23 May 2022, Oleksandr wrote:
>>>> On Thu, 19 May 2022, Oleksandr wrote:
>>>>>> On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com> wrote:
>>>>>>> On 18.05.22 17:32, Arnd Bergmann wrote:
>>>>>>>> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
>>>>>>>> <olekstysh@gmail.com> wrote:
>>>>>>>>      This would mean having a device
>>>>>>>> node for the grant-table mechanism that can be referred to using
>>>>>>>> the
>>>>>>>> 'iommus'
>>>>>>>> phandle property, with the domid as an additional argument.
>>>>>>> I assume, you are speaking about something like the following?
>>>>>>>
>>>>>>>
>>>>>>> xen_dummy_iommu {
>>>>>>>        compatible = "xen,dummy-iommu";
>>>>>>>        #iommu-cells = <1>;
>>>>>>> };
>>>>>>>
>>>>>>> virtio@3000 {
>>>>>>>        compatible = "virtio,mmio";
>>>>>>>        reg = <0x3000 0x100>;
>>>>>>>        interrupts = <41>;
>>>>>>>
>>>>>>>        /* The device is located in Xen domain with ID 1 */
>>>>>>>        iommus = <&xen_dummy_iommu 1>;
>>>>>>> };
>>>>>> Right, that's that's the idea,
>>>>> thank you for the confirmation
>>>>>
>>>>>
>>>>>
>>>>>>     except I would not call it a 'dummy'.
>>>>>>    From the perspective of the DT, this behaves just like an IOMMU,
>>>>>> even if the exact mechanism is different from most hardware IOMMU
>>>>>> implementations.
>>>>> well, agree
>>>>>
>>>>>
>>>>>>>> It does not quite fit the model that Linux currently uses for
>>>>>>>> iommus,
>>>>>>>> as that has an allocator for dma_addr_t space
>>>>>>> yes (# 3/7 adds grant-table based allocator)
>>>>>>>
>>>>>>>
>>>>>>>> , but it would think it's
>>>>>>>> conceptually close enough that it makes sense for the binding.
>>>>>>> Interesting idea. I am wondering, do we need an extra actions for
>>>>>>> this
>>>>>>> to work in Linux guest (dummy IOMMU driver, etc)?
>>>>>> It depends on how closely the guest implementation can be made to
>>>>>> resemble a normal iommu. If you do allocate dma_addr_t addresses,
>>>>>> it may actually be close enough that you can just turn the grant-table
>>>>>> code into a normal iommu driver and change nothing else.
>>>>> Unfortunately, I failed to find a way how use grant references at the
>>>>> iommu_ops level (I mean to fully pretend that we are an IOMMU driver). I
>>>>> am
>>>>> not too familiar with that, so what is written below might be wrong or
>>>>> at
>>>>> least not precise.
>>>>>
>>>>> The normal IOMMU driver in Linux doesn’t allocate DMA addresses by
>>>>> itself, it
>>>>> just maps (IOVA-PA) what was requested to be mapped by the upper layer.
>>>>> The
>>>>> DMA address allocation is done by the upper layer (DMA-IOMMU which is
>>>>> the glue
>>>>> layer between DMA API and IOMMU API allocates IOVA for PA?). But, all
>>>>> what we
>>>>> need here is just to allocate our specific grant-table based DMA
>>>>> addresses
>>>>> (DMA address = grant reference + offset in the page), so let’s say we
>>>>> need an
>>>>> entity to take a physical address as parameter and return a DMA address
>>>>> (what
>>>>> actually commit #3/7 is doing), and that’s all. So working at the
>>>>> dma_ops
>>>>> layer we get exactly what we need, with the minimal changes to guest
>>>>> infrastructure. In our case the Xen itself acts as an IOMMU.
>>>>>
>>>>> Assuming that we want to reuse the IOMMU infrastructure somehow for our
>>>>> needs.
>>>>> I think, in that case we will likely need to introduce a new specific
>>>>> IOVA
>>>>> allocator (alongside with a generic one) to be hooked up by the
>>>>> DMA-IOMMU
>>>>> layer if we run on top of Xen. But, even having the specific IOVA
>>>>> allocator to
>>>>> return what we indeed need (DMA address = grant reference + offset in
>>>>> the
>>>>> page) we will still need the specific minimal required IOMMU driver to
>>>>> be
>>>>> present in the system anyway in order to track the mappings(?) and do
>>>>> nothing
>>>>> with them, returning a success (this specific IOMMU driver should have
>>>>> all
>>>>> mandatory callbacks implemented).
>>>>>
>>>>> I completely agree, it would be really nice to reuse generic IOMMU
>>>>> bindings
>>>>> rather than introducing Xen specific property if what we are trying to
>>>>> implement in current patch series fits in the usage of "iommus" in Linux
>>>>> more-less. But, if we will have to add more complexity/more components
>>>>> to the
>>>>> code for the sake of reusing device tree binding, this raises a question
>>>>> whether that’s worthwhile.
>>>>>
>>>>> Or I really missed something?
>>>> I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
>>>> bindings, not necessarily the IOMMU drivers framework in Linux (although
>>>> that would be an added bonus.)
>>>>
>>>> I know from previous discussions with you that making the grant table
>>>> fit in the existing IOMMU drivers model is difficult, but just reusing
>>>> the Device Tree bindings seems feasible?
>>> I started experimenting with that. As wrote in a separate email, I got a
>>> deferred probe timeout,
>>>
>>> after inserting required nodes into guest device tree, which seems to be a
>>> consequence of the unavailability of IOMMU, I will continue to investigate
>>> this question.
>>
>> I have experimented with that. Yes, just reusing the Device Tree bindings is
>> technically feasible (and we are able to do this by only touching
>> grant-dma-ops.c), although deferred probe timeout still stands (as there is no
>> IOMMU driver being present actually).
>>
>> [    0.583771] virtio-mmio 2000000.virtio: deferred probe timeout, ignoring
>> dependency
>> [    0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks (2.10
>> GB/1.95 GiB)
>>
>>
>> Below the working diff (on top of current series):
>>
>> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>> index da9c7ff..6586152 100644
>> --- a/drivers/xen/grant-dma-ops.c
>> +++ b/drivers/xen/grant-dma-ops.c
>> @@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops = {
>>
>>   bool xen_is_grant_dma_device(struct device *dev)
>>   {
>> +       struct device_node *iommu_np;
>> +       bool has_iommu;
>> +
>>          /* XXX Handle only DT devices for now */
>>          if (!dev->of_node)
>>                  return false;
>>
>> -       return of_property_read_bool(dev->of_node, "xen,backend-domid");
>> +       iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
>> +       has_iommu = iommu_np && of_device_is_compatible(iommu_np,
>> "xen,grant-dma");
>> +       of_node_put(iommu_np);
>> +
>> +       return has_iommu;
>>   }
>>
>>   void xen_grant_setup_dma_ops(struct device *dev)
>>   {
>>          struct xen_grant_dma_data *data;
>> -       uint32_t domid;
>> +       struct of_phandle_args iommu_spec;
>>
>>          data = find_xen_grant_dma_data(dev);
>>          if (data) {
>> @@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
>>          if (!dev->of_node)
>>                  goto err;
>>
>> -       if (of_property_read_u32(dev->of_node, "xen,backend-domid", &domid)) {
>> -               dev_err(dev, "xen,backend-domid property is not present\n");
>> +       if (of_parse_phandle_with_args(dev->of_node, "iommus", "#iommu-cells",
>> +                       0, &iommu_spec)) {
>> +               dev_err(dev, "Cannot parse iommus property\n");
>> +               goto err;
>> +       }
>> +
>> +       if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
>> +                       iommu_spec.args_count != 1) {
>> +               dev_err(dev, "Incompatible IOMMU node\n");
>> +               of_node_put(iommu_spec.np);
>>                  goto err;
>>          }
>>
>> +       of_node_put(iommu_spec.np);
>> +
>>          data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
>>          if (!data)
>>                  goto err;
>>
>> -       data->backend_domid = domid;
>> +       /*
>> +        * The endpoint ID here means the ID of the domain where the
>> corresponding
>> +        * backend is running
>> +        */
>> +       data->backend_domid = iommu_spec.args[0];
>>
>>          if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev, data,
>>                          GFP_KERNEL))) {
>> (END)
>>
>>
>>
>> Below, the nodes generated by Xen toolstack:
>>
>>          xen_grant_dma {
>>                  compatible = "xen,grant-dma";
>>                  #iommu-cells = <0x01>;
>>                  phandle = <0xfde9>;
>>          };
>>
>>          virtio@2000000 {
>>                  compatible = "virtio,mmio";
>>                  reg = <0x00 0x2000000 0x00 0x200>;
>>                  interrupts = <0x00 0x01 0xf01>;
>>                  interrupt-parent = <0xfde8>;
>>                  dma-coherent;
>>                  iommus = <0xfde9 0x01>;
>>          };
>   
> Not bad! I like it.


Good.



>   
>   
>> I am wondering, would be the proper solution to eliminate deferred probe
>> timeout issue in our particular case (without introducing an extra IOMMU
>> driver)?
> In reality I don't think there is a way to do that. I would create an
> empty skelethon IOMMU driver for xen,grant-dma.

Ok, I found yet another option how we can avoid deferred probe timeout 
issue. I am not sure whether it will be welcome. But it doesn't really 
require introducing stub IOMMU driver or other changes in the guest. The 
idea is to make IOMMU device unavailable (status = "disabled"), this way 
of_iommu_configure() will treat that as success condition also.

https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L31
https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L149

         xen_grant_dma {
                 compatible = "xen,grant-dma";
                 #iommu-cells = <0x01>;
                 phandle = <0xfde9>;
                 status = "disabled";
         };
         virtio@2000000 {
                 compatible = "virtio,mmio";
                 reg = <0x00 0x2000000 0x00 0x200>;
                 interrupts = <0x00 0x01 0xf01>;
                 interrupt-parent = <0xfde8>;
                 dma-coherent;
                 iommus = <0xfde9 0x01>;
         };

I have checked, this "fixes" deferred probe timeout issue.


Or we indeed need to introduce stub IOMMU driver (I placed it to 
driver/xen instead of driver/iommu, also we can even squash it with 
grant-dma-ops.c?).
This stub driver also results in NO_IOMMU condition (as "of_xlate" 
callback is not implemented).

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index a7bd8ce..35b91b9 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -335,6 +335,10 @@ config XEN_UNPOPULATED_ALLOC
           having to balloon out RAM regions in order to obtain physical 
memory
           space to create such mappings.

+config XEN_GRANT_DMA_IOMMU
+       bool
+       select IOMMU_API
+
  config XEN_GRANT_DMA_OPS
         bool
         select DMA_OPS
@@ -343,6 +347,7 @@ config XEN_VIRTIO
         bool "Xen virtio support"
         depends on VIRTIO
         select XEN_GRANT_DMA_OPS
+       select XEN_GRANT_DMA_IOMMU
         help
           Enable virtio support for running as Xen guest. Depending on the
           guest type this will require special support on the backend side
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 1a23cb0..c0503f1 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -40,3 +40,4 @@ xen-privcmd-y                         := privcmd.o 
privcmd-buf.o
  obj-$(CONFIG_XEN_FRONT_PGDIR_SHBUF)    += xen-front-pgdir-shbuf.o
  obj-$(CONFIG_XEN_UNPOPULATED_ALLOC)    += unpopulated-alloc.o
  obj-$(CONFIG_XEN_GRANT_DMA_OPS)                += grant-dma-ops.o
+obj-$(CONFIG_XEN_GRANT_DMA_IOMMU)      += grant-dma-iommu.o
diff --git a/drivers/xen/grant-dma-iommu.c b/drivers/xen/grant-dma-iommu.c
new file mode 100644
index 00000000..b8aad8a
--- /dev/null
+++ b/drivers/xen/grant-dma-iommu.c
@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Stub IOMMU driver which does nothing.
+ * The main purpose of it being present is to reuse generic device-tree 
IOMMU
+ * bindings by Xen grant DMA-mapping layer.
+ */
+
+#include <linux/iommu.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+
+struct grant_dma_iommu_device {
+       struct device *dev;
+       struct iommu_device iommu;
+};
+
+/* Nothing is really needed here */
+static const struct iommu_ops grant_dma_iommu_ops;
+
+static const struct of_device_id grant_dma_iommu_of_match[] = {
+       { .compatible = "xen,grant-dma" },
+       { },
+};
+
+static int grant_dma_iommu_probe(struct platform_device *pdev)
+{
+       struct grant_dma_iommu_device *mmu;
+       int ret;
+
+       mmu = devm_kzalloc(&pdev->dev, sizeof(*mmu), GFP_KERNEL);
+       if (!mmu)
+               return -ENOMEM;
+
+       mmu->dev = &pdev->dev;
+
+       ret = iommu_device_register(&mmu->iommu, &grant_dma_iommu_ops, 
&pdev->dev);
+       if (ret)
+               return ret;
+
+       platform_set_drvdata(pdev, mmu);
+
+       return 0;
+}
+
+static int grant_dma_iommu_remove(struct platform_device *pdev)
+{
+       struct grant_dma_iommu_device *mmu = platform_get_drvdata(pdev);
+
+       platform_set_drvdata(pdev, NULL);
+       iommu_device_unregister(&mmu->iommu);
+
+       return 0;
+}
+
+static struct platform_driver grant_dma_iommu_driver = {
+       .driver = {
+               .name = "grant-dma-iommu",
+               .of_match_table = grant_dma_iommu_of_match,
+       },
+       .probe = grant_dma_iommu_probe,
+       .remove = grant_dma_iommu_remove,
+};
+
+static int __init grant_dma_iommu_init(void)
+{
+       struct device_node *iommu_np;
+
+       iommu_np = of_find_matching_node(NULL, grant_dma_iommu_of_match);
+       if (!iommu_np)
+               return 0;
+
+       of_node_put(iommu_np);
+
+       return platform_driver_register(&grant_dma_iommu_driver);
+}
+subsys_initcall(grant_dma_iommu_init);

I have checked, this also "fixes" deferred probe timeout issue.

Personally I would prefer the first option, but I would be also happy to 
use second option in order to unblock the series.

What do the maintainers think?


-- 
Regards,

Oleksandr Tyshchenko

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Stefano Stabellini]

[-- Attachment #1: Type: text/plain, Size: 17380 bytes --]

On Tue, 24 May 2022, Oleksandr wrote:
> > On Mon, 23 May 2022, Oleksandr wrote:
> > > > > On Thu, 19 May 2022, Oleksandr wrote:
> > > > > > > On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com>
> > > > > > > wrote:
> > > > > > > > On 18.05.22 17:32, Arnd Bergmann wrote:
> > > > > > > > > On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
> > > > > > > > > <olekstysh@gmail.com> wrote:
> > > > > > > > >      This would mean having a device
> > > > > > > > > node for the grant-table mechanism that can be referred to
> > > > > > > > > using
> > > > > > > > > the
> > > > > > > > > 'iommus'
> > > > > > > > > phandle property, with the domid as an additional argument.
> > > > > > > > I assume, you are speaking about something like the following?
> > > > > > > > 
> > > > > > > > 
> > > > > > > > xen_dummy_iommu {
> > > > > > > >        compatible = "xen,dummy-iommu";
> > > > > > > >        #iommu-cells = <1>;
> > > > > > > > };
> > > > > > > > 
> > > > > > > > virtio@3000 {
> > > > > > > >        compatible = "virtio,mmio";
> > > > > > > >        reg = <0x3000 0x100>;
> > > > > > > >        interrupts = <41>;
> > > > > > > > 
> > > > > > > >        /* The device is located in Xen domain with ID 1 */
> > > > > > > >        iommus = <&xen_dummy_iommu 1>;
> > > > > > > > };
> > > > > > > Right, that's that's the idea,
> > > > > > thank you for the confirmation
> > > > > > 
> > > > > > 
> > > > > > 
> > > > > > >     except I would not call it a 'dummy'.
> > > > > > >    From the perspective of the DT, this behaves just like an
> > > > > > > IOMMU,
> > > > > > > even if the exact mechanism is different from most hardware IOMMU
> > > > > > > implementations.
> > > > > > well, agree
> > > > > > 
> > > > > > 
> > > > > > > > > It does not quite fit the model that Linux currently uses for
> > > > > > > > > iommus,
> > > > > > > > > as that has an allocator for dma_addr_t space
> > > > > > > > yes (# 3/7 adds grant-table based allocator)
> > > > > > > > 
> > > > > > > > 
> > > > > > > > > , but it would think it's
> > > > > > > > > conceptually close enough that it makes sense for the binding.
> > > > > > > > Interesting idea. I am wondering, do we need an extra actions
> > > > > > > > for
> > > > > > > > this
> > > > > > > > to work in Linux guest (dummy IOMMU driver, etc)?
> > > > > > > It depends on how closely the guest implementation can be made to
> > > > > > > resemble a normal iommu. If you do allocate dma_addr_t addresses,
> > > > > > > it may actually be close enough that you can just turn the
> > > > > > > grant-table
> > > > > > > code into a normal iommu driver and change nothing else.
> > > > > > Unfortunately, I failed to find a way how use grant references at
> > > > > > the
> > > > > > iommu_ops level (I mean to fully pretend that we are an IOMMU
> > > > > > driver). I
> > > > > > am
> > > > > > not too familiar with that, so what is written below might be wrong
> > > > > > or
> > > > > > at
> > > > > > least not precise.
> > > > > > 
> > > > > > The normal IOMMU driver in Linux doesn’t allocate DMA addresses by
> > > > > > itself, it
> > > > > > just maps (IOVA-PA) what was requested to be mapped by the upper
> > > > > > layer.
> > > > > > The
> > > > > > DMA address allocation is done by the upper layer (DMA-IOMMU which
> > > > > > is
> > > > > > the glue
> > > > > > layer between DMA API and IOMMU API allocates IOVA for PA?). But,
> > > > > > all
> > > > > > what we
> > > > > > need here is just to allocate our specific grant-table based DMA
> > > > > > addresses
> > > > > > (DMA address = grant reference + offset in the page), so let’s say
> > > > > > we
> > > > > > need an
> > > > > > entity to take a physical address as parameter and return a DMA
> > > > > > address
> > > > > > (what
> > > > > > actually commit #3/7 is doing), and that’s all. So working at the
> > > > > > dma_ops
> > > > > > layer we get exactly what we need, with the minimal changes to guest
> > > > > > infrastructure. In our case the Xen itself acts as an IOMMU.
> > > > > > 
> > > > > > Assuming that we want to reuse the IOMMU infrastructure somehow for
> > > > > > our
> > > > > > needs.
> > > > > > I think, in that case we will likely need to introduce a new
> > > > > > specific
> > > > > > IOVA
> > > > > > allocator (alongside with a generic one) to be hooked up by the
> > > > > > DMA-IOMMU
> > > > > > layer if we run on top of Xen. But, even having the specific IOVA
> > > > > > allocator to
> > > > > > return what we indeed need (DMA address = grant reference + offset
> > > > > > in
> > > > > > the
> > > > > > page) we will still need the specific minimal required IOMMU driver
> > > > > > to
> > > > > > be
> > > > > > present in the system anyway in order to track the mappings(?) and
> > > > > > do
> > > > > > nothing
> > > > > > with them, returning a success (this specific IOMMU driver should
> > > > > > have
> > > > > > all
> > > > > > mandatory callbacks implemented).
> > > > > > 
> > > > > > I completely agree, it would be really nice to reuse generic IOMMU
> > > > > > bindings
> > > > > > rather than introducing Xen specific property if what we are trying
> > > > > > to
> > > > > > implement in current patch series fits in the usage of "iommus" in
> > > > > > Linux
> > > > > > more-less. But, if we will have to add more complexity/more
> > > > > > components
> > > > > > to the
> > > > > > code for the sake of reusing device tree binding, this raises a
> > > > > > question
> > > > > > whether that’s worthwhile.
> > > > > > 
> > > > > > Or I really missed something?
> > > > > I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
> > > > > bindings, not necessarily the IOMMU drivers framework in Linux
> > > > > (although
> > > > > that would be an added bonus.)
> > > > > 
> > > > > I know from previous discussions with you that making the grant table
> > > > > fit in the existing IOMMU drivers model is difficult, but just reusing
> > > > > the Device Tree bindings seems feasible?
> > > > I started experimenting with that. As wrote in a separate email, I got a
> > > > deferred probe timeout,
> > > > 
> > > > after inserting required nodes into guest device tree, which seems to be
> > > > a
> > > > consequence of the unavailability of IOMMU, I will continue to
> > > > investigate
> > > > this question.
> > > 
> > > I have experimented with that. Yes, just reusing the Device Tree bindings
> > > is
> > > technically feasible (and we are able to do this by only touching
> > > grant-dma-ops.c), although deferred probe timeout still stands (as there
> > > is no
> > > IOMMU driver being present actually).
> > > 
> > > [    0.583771] virtio-mmio 2000000.virtio: deferred probe timeout,
> > > ignoring
> > > dependency
> > > [    0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks
> > > (2.10
> > > GB/1.95 GiB)
> > > 
> > > 
> > > Below the working diff (on top of current series):
> > > 
> > > diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
> > > index da9c7ff..6586152 100644
> > > --- a/drivers/xen/grant-dma-ops.c
> > > +++ b/drivers/xen/grant-dma-ops.c
> > > @@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops =
> > > {
> > > 
> > >   bool xen_is_grant_dma_device(struct device *dev)
> > >   {
> > > +       struct device_node *iommu_np;
> > > +       bool has_iommu;
> > > +
> > >          /* XXX Handle only DT devices for now */
> > >          if (!dev->of_node)
> > >                  return false;
> > > 
> > > -       return of_property_read_bool(dev->of_node, "xen,backend-domid");
> > > +       iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
> > > +       has_iommu = iommu_np && of_device_is_compatible(iommu_np,
> > > "xen,grant-dma");
> > > +       of_node_put(iommu_np);
> > > +
> > > +       return has_iommu;
> > >   }
> > > 
> > >   void xen_grant_setup_dma_ops(struct device *dev)
> > >   {
> > >          struct xen_grant_dma_data *data;
> > > -       uint32_t domid;
> > > +       struct of_phandle_args iommu_spec;
> > > 
> > >          data = find_xen_grant_dma_data(dev);
> > >          if (data) {
> > > @@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
> > >          if (!dev->of_node)
> > >                  goto err;
> > > 
> > > -       if (of_property_read_u32(dev->of_node, "xen,backend-domid",
> > > &domid)) {
> > > -               dev_err(dev, "xen,backend-domid property is not
> > > present\n");
> > > +       if (of_parse_phandle_with_args(dev->of_node, "iommus",
> > > "#iommu-cells",
> > > +                       0, &iommu_spec)) {
> > > +               dev_err(dev, "Cannot parse iommus property\n");
> > > +               goto err;
> > > +       }
> > > +
> > > +       if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
> > > +                       iommu_spec.args_count != 1) {
> > > +               dev_err(dev, "Incompatible IOMMU node\n");
> > > +               of_node_put(iommu_spec.np);
> > >                  goto err;
> > >          }
> > > 
> > > +       of_node_put(iommu_spec.np);
> > > +
> > >          data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
> > >          if (!data)
> > >                  goto err;
> > > 
> > > -       data->backend_domid = domid;
> > > +       /*
> > > +        * The endpoint ID here means the ID of the domain where the
> > > corresponding
> > > +        * backend is running
> > > +        */
> > > +       data->backend_domid = iommu_spec.args[0];
> > > 
> > >          if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev,
> > > data,
> > >                          GFP_KERNEL))) {
> > > (END)
> > > 
> > > 
> > > 
> > > Below, the nodes generated by Xen toolstack:
> > > 
> > >          xen_grant_dma {
> > >                  compatible = "xen,grant-dma";
> > >                  #iommu-cells = <0x01>;
> > >                  phandle = <0xfde9>;
> > >          };
> > > 
> > >          virtio@2000000 {
> > >                  compatible = "virtio,mmio";
> > >                  reg = <0x00 0x2000000 0x00 0x200>;
> > >                  interrupts = <0x00 0x01 0xf01>;
> > >                  interrupt-parent = <0xfde8>;
> > >                  dma-coherent;
> > >                  iommus = <0xfde9 0x01>;
> > >          };
> >   Not bad! I like it.
> 
> 
> Good.
> 
> 
> 
> >     
> > > I am wondering, would be the proper solution to eliminate deferred probe
> > > timeout issue in our particular case (without introducing an extra IOMMU
> > > driver)?
> > In reality I don't think there is a way to do that. I would create an
> > empty skelethon IOMMU driver for xen,grant-dma.
> 
> Ok, I found yet another option how we can avoid deferred probe timeout issue.
> I am not sure whether it will be welcome. But it doesn't really require
> introducing stub IOMMU driver or other changes in the guest. The idea is to
> make IOMMU device unavailable (status = "disabled"), this way
> of_iommu_configure() will treat that as success condition also.
> 
> https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L31
> https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L149
> 
>         xen_grant_dma {
>                 compatible = "xen,grant-dma";
>                 #iommu-cells = <0x01>;
>                 phandle = <0xfde9>;
>                 status = "disabled";
>         };
>         virtio@2000000 {
>                 compatible = "virtio,mmio";
>                 reg = <0x00 0x2000000 0x00 0x200>;
>                 interrupts = <0x00 0x01 0xf01>;
>                 interrupt-parent = <0xfde8>;
>                 dma-coherent;
>                 iommus = <0xfde9 0x01>;
>         };
> 
> I have checked, this "fixes" deferred probe timeout issue.
> 
> 
> Or we indeed need to introduce stub IOMMU driver (I placed it to driver/xen
> instead of driver/iommu, also we can even squash it with grant-dma-ops.c?).
> This stub driver also results in NO_IOMMU condition (as "of_xlate" callback is
> not implemented).
> 
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index a7bd8ce..35b91b9 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -335,6 +335,10 @@ config XEN_UNPOPULATED_ALLOC
>           having to balloon out RAM regions in order to obtain physical memory
>           space to create such mappings.
> 
> +config XEN_GRANT_DMA_IOMMU
> +       bool
> +       select IOMMU_API
> +
>  config XEN_GRANT_DMA_OPS
>         bool
>         select DMA_OPS
> @@ -343,6 +347,7 @@ config XEN_VIRTIO
>         bool "Xen virtio support"
>         depends on VIRTIO
>         select XEN_GRANT_DMA_OPS
> +       select XEN_GRANT_DMA_IOMMU
>         help
>           Enable virtio support for running as Xen guest. Depending on the
>           guest type this will require special support on the backend side
> diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
> index 1a23cb0..c0503f1 100644
> --- a/drivers/xen/Makefile
> +++ b/drivers/xen/Makefile
> @@ -40,3 +40,4 @@ xen-privcmd-y                         := privcmd.o
> privcmd-buf.o
>  obj-$(CONFIG_XEN_FRONT_PGDIR_SHBUF)    += xen-front-pgdir-shbuf.o
>  obj-$(CONFIG_XEN_UNPOPULATED_ALLOC)    += unpopulated-alloc.o
>  obj-$(CONFIG_XEN_GRANT_DMA_OPS)                += grant-dma-ops.o
> +obj-$(CONFIG_XEN_GRANT_DMA_IOMMU)      += grant-dma-iommu.o
> diff --git a/drivers/xen/grant-dma-iommu.c b/drivers/xen/grant-dma-iommu.c
> new file mode 100644
> index 00000000..b8aad8a
> --- /dev/null
> +++ b/drivers/xen/grant-dma-iommu.c
> @@ -0,0 +1,76 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Stub IOMMU driver which does nothing.
> + * The main purpose of it being present is to reuse generic device-tree IOMMU
> + * bindings by Xen grant DMA-mapping layer.
> + */
> +
> +#include <linux/iommu.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +
> +struct grant_dma_iommu_device {
> +       struct device *dev;
> +       struct iommu_device iommu;
> +};
> +
> +/* Nothing is really needed here */
> +static const struct iommu_ops grant_dma_iommu_ops;
> +
> +static const struct of_device_id grant_dma_iommu_of_match[] = {
> +       { .compatible = "xen,grant-dma" },
> +       { },
> +};
> +
> +static int grant_dma_iommu_probe(struct platform_device *pdev)
> +{
> +       struct grant_dma_iommu_device *mmu;
> +       int ret;
> +
> +       mmu = devm_kzalloc(&pdev->dev, sizeof(*mmu), GFP_KERNEL);
> +       if (!mmu)
> +               return -ENOMEM;
> +
> +       mmu->dev = &pdev->dev;
> +
> +       ret = iommu_device_register(&mmu->iommu, &grant_dma_iommu_ops,
> &pdev->dev);
> +       if (ret)
> +               return ret;
> +
> +       platform_set_drvdata(pdev, mmu);
> +
> +       return 0;
> +}
> +
> +static int grant_dma_iommu_remove(struct platform_device *pdev)
> +{
> +       struct grant_dma_iommu_device *mmu = platform_get_drvdata(pdev);
> +
> +       platform_set_drvdata(pdev, NULL);
> +       iommu_device_unregister(&mmu->iommu);
> +
> +       return 0;
> +}
> +
> +static struct platform_driver grant_dma_iommu_driver = {
> +       .driver = {
> +               .name = "grant-dma-iommu",
> +               .of_match_table = grant_dma_iommu_of_match,
> +       },
> +       .probe = grant_dma_iommu_probe,
> +       .remove = grant_dma_iommu_remove,
> +};
> +
> +static int __init grant_dma_iommu_init(void)
> +{
> +       struct device_node *iommu_np;
> +
> +       iommu_np = of_find_matching_node(NULL, grant_dma_iommu_of_match);
> +       if (!iommu_np)
> +               return 0;
> +
> +       of_node_put(iommu_np);
> +
> +       return platform_driver_register(&grant_dma_iommu_driver);
> +}
> +subsys_initcall(grant_dma_iommu_init);
> 
> I have checked, this also "fixes" deferred probe timeout issue.
> 
> Personally I would prefer the first option, but I would be also happy to use
> second option in order to unblock the series.
> 
> What do the maintainers think?
 

I don't think it is a good idea to mark the fake IOMMU as disabled
because it implies that there is no need to use it (no need to use
dma_ops) which is a problem.

If we don't want the skelethon driver then Rob's suggestion of having a
skip list for deferred probe is better.

I think the skelethon driver also is totally fine.

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 24.05.22 20:59, Stefano Stabellini wrote:

Hello Stefano

> On Tue, 24 May 2022, Oleksandr wrote:
>>> On Mon, 23 May 2022, Oleksandr wrote:
>>>>>> On Thu, 19 May 2022, Oleksandr wrote:
>>>>>>>> On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@gmail.com>
>>>>>>>> wrote:
>>>>>>>>> On 18.05.22 17:32, Arnd Bergmann wrote:
>>>>>>>>>> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko
>>>>>>>>>> <olekstysh@gmail.com> wrote:
>>>>>>>>>>       This would mean having a device
>>>>>>>>>> node for the grant-table mechanism that can be referred to
>>>>>>>>>> using
>>>>>>>>>> the
>>>>>>>>>> 'iommus'
>>>>>>>>>> phandle property, with the domid as an additional argument.
>>>>>>>>> I assume, you are speaking about something like the following?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> xen_dummy_iommu {
>>>>>>>>>         compatible = "xen,dummy-iommu";
>>>>>>>>>         #iommu-cells = <1>;
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> virtio@3000 {
>>>>>>>>>         compatible = "virtio,mmio";
>>>>>>>>>         reg = <0x3000 0x100>;
>>>>>>>>>         interrupts = <41>;
>>>>>>>>>
>>>>>>>>>         /* The device is located in Xen domain with ID 1 */
>>>>>>>>>         iommus = <&xen_dummy_iommu 1>;
>>>>>>>>> };
>>>>>>>> Right, that's that's the idea,
>>>>>>> thank you for the confirmation
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>      except I would not call it a 'dummy'.
>>>>>>>>     From the perspective of the DT, this behaves just like an
>>>>>>>> IOMMU,
>>>>>>>> even if the exact mechanism is different from most hardware IOMMU
>>>>>>>> implementations.
>>>>>>> well, agree
>>>>>>>
>>>>>>>
>>>>>>>>>> It does not quite fit the model that Linux currently uses for
>>>>>>>>>> iommus,
>>>>>>>>>> as that has an allocator for dma_addr_t space
>>>>>>>>> yes (# 3/7 adds grant-table based allocator)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> , but it would think it's
>>>>>>>>>> conceptually close enough that it makes sense for the binding.
>>>>>>>>> Interesting idea. I am wondering, do we need an extra actions
>>>>>>>>> for
>>>>>>>>> this
>>>>>>>>> to work in Linux guest (dummy IOMMU driver, etc)?
>>>>>>>> It depends on how closely the guest implementation can be made to
>>>>>>>> resemble a normal iommu. If you do allocate dma_addr_t addresses,
>>>>>>>> it may actually be close enough that you can just turn the
>>>>>>>> grant-table
>>>>>>>> code into a normal iommu driver and change nothing else.
>>>>>>> Unfortunately, I failed to find a way how use grant references at
>>>>>>> the
>>>>>>> iommu_ops level (I mean to fully pretend that we are an IOMMU
>>>>>>> driver). I
>>>>>>> am
>>>>>>> not too familiar with that, so what is written below might be wrong
>>>>>>> or
>>>>>>> at
>>>>>>> least not precise.
>>>>>>>
>>>>>>> The normal IOMMU driver in Linux doesn’t allocate DMA addresses by
>>>>>>> itself, it
>>>>>>> just maps (IOVA-PA) what was requested to be mapped by the upper
>>>>>>> layer.
>>>>>>> The
>>>>>>> DMA address allocation is done by the upper layer (DMA-IOMMU which
>>>>>>> is
>>>>>>> the glue
>>>>>>> layer between DMA API and IOMMU API allocates IOVA for PA?). But,
>>>>>>> all
>>>>>>> what we
>>>>>>> need here is just to allocate our specific grant-table based DMA
>>>>>>> addresses
>>>>>>> (DMA address = grant reference + offset in the page), so let’s say
>>>>>>> we
>>>>>>> need an
>>>>>>> entity to take a physical address as parameter and return a DMA
>>>>>>> address
>>>>>>> (what
>>>>>>> actually commit #3/7 is doing), and that’s all. So working at the
>>>>>>> dma_ops
>>>>>>> layer we get exactly what we need, with the minimal changes to guest
>>>>>>> infrastructure. In our case the Xen itself acts as an IOMMU.
>>>>>>>
>>>>>>> Assuming that we want to reuse the IOMMU infrastructure somehow for
>>>>>>> our
>>>>>>> needs.
>>>>>>> I think, in that case we will likely need to introduce a new
>>>>>>> specific
>>>>>>> IOVA
>>>>>>> allocator (alongside with a generic one) to be hooked up by the
>>>>>>> DMA-IOMMU
>>>>>>> layer if we run on top of Xen. But, even having the specific IOVA
>>>>>>> allocator to
>>>>>>> return what we indeed need (DMA address = grant reference + offset
>>>>>>> in
>>>>>>> the
>>>>>>> page) we will still need the specific minimal required IOMMU driver
>>>>>>> to
>>>>>>> be
>>>>>>> present in the system anyway in order to track the mappings(?) and
>>>>>>> do
>>>>>>> nothing
>>>>>>> with them, returning a success (this specific IOMMU driver should
>>>>>>> have
>>>>>>> all
>>>>>>> mandatory callbacks implemented).
>>>>>>>
>>>>>>> I completely agree, it would be really nice to reuse generic IOMMU
>>>>>>> bindings
>>>>>>> rather than introducing Xen specific property if what we are trying
>>>>>>> to
>>>>>>> implement in current patch series fits in the usage of "iommus" in
>>>>>>> Linux
>>>>>>> more-less. But, if we will have to add more complexity/more
>>>>>>> components
>>>>>>> to the
>>>>>>> code for the sake of reusing device tree binding, this raises a
>>>>>>> question
>>>>>>> whether that’s worthwhile.
>>>>>>>
>>>>>>> Or I really missed something?
>>>>>> I think Arnd was primarily suggesting to reuse the IOMMU Device Tree
>>>>>> bindings, not necessarily the IOMMU drivers framework in Linux
>>>>>> (although
>>>>>> that would be an added bonus.)
>>>>>>
>>>>>> I know from previous discussions with you that making the grant table
>>>>>> fit in the existing IOMMU drivers model is difficult, but just reusing
>>>>>> the Device Tree bindings seems feasible?
>>>>> I started experimenting with that. As wrote in a separate email, I got a
>>>>> deferred probe timeout,
>>>>>
>>>>> after inserting required nodes into guest device tree, which seems to be
>>>>> a
>>>>> consequence of the unavailability of IOMMU, I will continue to
>>>>> investigate
>>>>> this question.
>>>> I have experimented with that. Yes, just reusing the Device Tree bindings
>>>> is
>>>> technically feasible (and we are able to do this by only touching
>>>> grant-dma-ops.c), although deferred probe timeout still stands (as there
>>>> is no
>>>> IOMMU driver being present actually).
>>>>
>>>> [    0.583771] virtio-mmio 2000000.virtio: deferred probe timeout,
>>>> ignoring
>>>> dependency
>>>> [    0.615556] virtio_blk virtio0: [vda] 4096000 512-byte logical blocks
>>>> (2.10
>>>> GB/1.95 GiB)
>>>>
>>>>
>>>> Below the working diff (on top of current series):
>>>>
>>>> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>>>> index da9c7ff..6586152 100644
>>>> --- a/drivers/xen/grant-dma-ops.c
>>>> +++ b/drivers/xen/grant-dma-ops.c
>>>> @@ -272,17 +272,24 @@ static const struct dma_map_ops xen_grant_dma_ops =
>>>> {
>>>>
>>>>    bool xen_is_grant_dma_device(struct device *dev)
>>>>    {
>>>> +       struct device_node *iommu_np;
>>>> +       bool has_iommu;
>>>> +
>>>>           /* XXX Handle only DT devices for now */
>>>>           if (!dev->of_node)
>>>>                   return false;
>>>>
>>>> -       return of_property_read_bool(dev->of_node, "xen,backend-domid");
>>>> +       iommu_np = of_parse_phandle(dev->of_node, "iommus", 0);
>>>> +       has_iommu = iommu_np && of_device_is_compatible(iommu_np,
>>>> "xen,grant-dma");
>>>> +       of_node_put(iommu_np);
>>>> +
>>>> +       return has_iommu;
>>>>    }
>>>>
>>>>    void xen_grant_setup_dma_ops(struct device *dev)
>>>>    {
>>>>           struct xen_grant_dma_data *data;
>>>> -       uint32_t domid;
>>>> +       struct of_phandle_args iommu_spec;
>>>>
>>>>           data = find_xen_grant_dma_data(dev);
>>>>           if (data) {
>>>> @@ -294,16 +301,30 @@ void xen_grant_setup_dma_ops(struct device *dev)
>>>>           if (!dev->of_node)
>>>>                   goto err;
>>>>
>>>> -       if (of_property_read_u32(dev->of_node, "xen,backend-domid",
>>>> &domid)) {
>>>> -               dev_err(dev, "xen,backend-domid property is not
>>>> present\n");
>>>> +       if (of_parse_phandle_with_args(dev->of_node, "iommus",
>>>> "#iommu-cells",
>>>> +                       0, &iommu_spec)) {
>>>> +               dev_err(dev, "Cannot parse iommus property\n");
>>>> +               goto err;
>>>> +       }
>>>> +
>>>> +       if (!of_device_is_compatible(iommu_spec.np, "xen,grant-dma") ||
>>>> +                       iommu_spec.args_count != 1) {
>>>> +               dev_err(dev, "Incompatible IOMMU node\n");
>>>> +               of_node_put(iommu_spec.np);
>>>>                   goto err;
>>>>           }
>>>>
>>>> +       of_node_put(iommu_spec.np);
>>>> +
>>>>           data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
>>>>           if (!data)
>>>>                   goto err;
>>>>
>>>> -       data->backend_domid = domid;
>>>> +       /*
>>>> +        * The endpoint ID here means the ID of the domain where the
>>>> corresponding
>>>> +        * backend is running
>>>> +        */
>>>> +       data->backend_domid = iommu_spec.args[0];
>>>>
>>>>           if (xa_err(xa_store(&xen_grant_dma_devices, (unsigned long)dev,
>>>> data,
>>>>                           GFP_KERNEL))) {
>>>> (END)
>>>>
>>>>
>>>>
>>>> Below, the nodes generated by Xen toolstack:
>>>>
>>>>           xen_grant_dma {
>>>>                   compatible = "xen,grant-dma";
>>>>                   #iommu-cells = <0x01>;
>>>>                   phandle = <0xfde9>;
>>>>           };
>>>>
>>>>           virtio@2000000 {
>>>>                   compatible = "virtio,mmio";
>>>>                   reg = <0x00 0x2000000 0x00 0x200>;
>>>>                   interrupts = <0x00 0x01 0xf01>;
>>>>                   interrupt-parent = <0xfde8>;
>>>>                   dma-coherent;
>>>>                   iommus = <0xfde9 0x01>;
>>>>           };
>>>    Not bad! I like it.
>>
>> Good.
>>
>>
>>
>>>      
>>>> I am wondering, would be the proper solution to eliminate deferred probe
>>>> timeout issue in our particular case (without introducing an extra IOMMU
>>>> driver)?
>>> In reality I don't think there is a way to do that. I would create an
>>> empty skelethon IOMMU driver for xen,grant-dma.
>> Ok, I found yet another option how we can avoid deferred probe timeout issue.
>> I am not sure whether it will be welcome. But it doesn't really require
>> introducing stub IOMMU driver or other changes in the guest. The idea is to
>> make IOMMU device unavailable (status = "disabled"), this way
>> of_iommu_configure() will treat that as success condition also.
>>
>> https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L31
>> https://elixir.bootlin.com/linux/v5.18/source/drivers/iommu/of_iommu.c#L149
>>
>>          xen_grant_dma {
>>                  compatible = "xen,grant-dma";
>>                  #iommu-cells = <0x01>;
>>                  phandle = <0xfde9>;
>>                  status = "disabled";
>>          };
>>          virtio@2000000 {
>>                  compatible = "virtio,mmio";
>>                  reg = <0x00 0x2000000 0x00 0x200>;
>>                  interrupts = <0x00 0x01 0xf01>;
>>                  interrupt-parent = <0xfde8>;
>>                  dma-coherent;
>>                  iommus = <0xfde9 0x01>;
>>          };
>>
>> I have checked, this "fixes" deferred probe timeout issue.
>>
>>
>> Or we indeed need to introduce stub IOMMU driver (I placed it to driver/xen
>> instead of driver/iommu, also we can even squash it with grant-dma-ops.c?).
>> This stub driver also results in NO_IOMMU condition (as "of_xlate" callback is
>> not implemented).
>>
>> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
>> index a7bd8ce..35b91b9 100644
>> --- a/drivers/xen/Kconfig
>> +++ b/drivers/xen/Kconfig
>> @@ -335,6 +335,10 @@ config XEN_UNPOPULATED_ALLOC
>>            having to balloon out RAM regions in order to obtain physical memory
>>            space to create such mappings.
>>
>> +config XEN_GRANT_DMA_IOMMU
>> +       bool
>> +       select IOMMU_API
>> +
>>   config XEN_GRANT_DMA_OPS
>>          bool
>>          select DMA_OPS
>> @@ -343,6 +347,7 @@ config XEN_VIRTIO
>>          bool "Xen virtio support"
>>          depends on VIRTIO
>>          select XEN_GRANT_DMA_OPS
>> +       select XEN_GRANT_DMA_IOMMU
>>          help
>>            Enable virtio support for running as Xen guest. Depending on the
>>            guest type this will require special support on the backend side
>> diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
>> index 1a23cb0..c0503f1 100644
>> --- a/drivers/xen/Makefile
>> +++ b/drivers/xen/Makefile
>> @@ -40,3 +40,4 @@ xen-privcmd-y                         := privcmd.o
>> privcmd-buf.o
>>   obj-$(CONFIG_XEN_FRONT_PGDIR_SHBUF)    += xen-front-pgdir-shbuf.o
>>   obj-$(CONFIG_XEN_UNPOPULATED_ALLOC)    += unpopulated-alloc.o
>>   obj-$(CONFIG_XEN_GRANT_DMA_OPS)                += grant-dma-ops.o
>> +obj-$(CONFIG_XEN_GRANT_DMA_IOMMU)      += grant-dma-iommu.o
>> diff --git a/drivers/xen/grant-dma-iommu.c b/drivers/xen/grant-dma-iommu.c
>> new file mode 100644
>> index 00000000..b8aad8a
>> --- /dev/null
>> +++ b/drivers/xen/grant-dma-iommu.c
>> @@ -0,0 +1,76 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Stub IOMMU driver which does nothing.
>> + * The main purpose of it being present is to reuse generic device-tree IOMMU
>> + * bindings by Xen grant DMA-mapping layer.
>> + */
>> +
>> +#include <linux/iommu.h>
>> +#include <linux/of.h>
>> +#include <linux/platform_device.h>
>> +
>> +struct grant_dma_iommu_device {
>> +       struct device *dev;
>> +       struct iommu_device iommu;
>> +};
>> +
>> +/* Nothing is really needed here */
>> +static const struct iommu_ops grant_dma_iommu_ops;
>> +
>> +static const struct of_device_id grant_dma_iommu_of_match[] = {
>> +       { .compatible = "xen,grant-dma" },
>> +       { },
>> +};
>> +
>> +static int grant_dma_iommu_probe(struct platform_device *pdev)
>> +{
>> +       struct grant_dma_iommu_device *mmu;
>> +       int ret;
>> +
>> +       mmu = devm_kzalloc(&pdev->dev, sizeof(*mmu), GFP_KERNEL);
>> +       if (!mmu)
>> +               return -ENOMEM;
>> +
>> +       mmu->dev = &pdev->dev;
>> +
>> +       ret = iommu_device_register(&mmu->iommu, &grant_dma_iommu_ops,
>> &pdev->dev);
>> +       if (ret)
>> +               return ret;
>> +
>> +       platform_set_drvdata(pdev, mmu);
>> +
>> +       return 0;
>> +}
>> +
>> +static int grant_dma_iommu_remove(struct platform_device *pdev)
>> +{
>> +       struct grant_dma_iommu_device *mmu = platform_get_drvdata(pdev);
>> +
>> +       platform_set_drvdata(pdev, NULL);
>> +       iommu_device_unregister(&mmu->iommu);
>> +
>> +       return 0;
>> +}
>> +
>> +static struct platform_driver grant_dma_iommu_driver = {
>> +       .driver = {
>> +               .name = "grant-dma-iommu",
>> +               .of_match_table = grant_dma_iommu_of_match,
>> +       },
>> +       .probe = grant_dma_iommu_probe,
>> +       .remove = grant_dma_iommu_remove,
>> +};
>> +
>> +static int __init grant_dma_iommu_init(void)
>> +{
>> +       struct device_node *iommu_np;
>> +
>> +       iommu_np = of_find_matching_node(NULL, grant_dma_iommu_of_match);
>> +       if (!iommu_np)
>> +               return 0;
>> +
>> +       of_node_put(iommu_np);
>> +
>> +       return platform_driver_register(&grant_dma_iommu_driver);
>> +}
>> +subsys_initcall(grant_dma_iommu_init);
>>
>> I have checked, this also "fixes" deferred probe timeout issue.
>>
>> Personally I would prefer the first option, but I would be also happy to use
>> second option in order to unblock the series.
>>
>> What do the maintainers think?
>   
>
> I don't think it is a good idea to mark the fake IOMMU as disabled
> because it implies that there is no need to use it (no need to use
> dma_ops) which is a problem.

I got your point. You are right, this indeed sounds weird. I expected 
this simple solution wouldn't be welcome.


>
> If we don't want the skelethon driver then Rob's suggestion of having a
> skip list for deferred probe is better.


I am not sure I understand the idea completely.

Does it mean that we will need new command line option (?) to pass some 
string (I assume, the "compatible" for IOMMU device) to not defer probe 
if corresponding driver is missing,

but just return NO_IOMMU right away?


>
> I think the skelethon driver also is totally fine.

ok, thank you for the feedback.


-- 
Regards,

Oleksandr Tyshchenko

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Rob Herring]

On Wed, May 18, 2022 at 03:32:27PM +0100, Arnd Bergmann wrote:
> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@gmail.com> wrote:
> >
> > diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
> > index 10c22b5..29a0932 100644
> > --- a/Documentation/devicetree/bindings/virtio/mmio.yaml
> > +++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
> > @@ -13,6 +13,9 @@ description:
> >    See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
> >    more details.
> >
> > +allOf:
> > +  - $ref: /schemas/arm/xen,dev-domid.yaml#
> > +
> >  properties:
> >    compatible:
> >      const: virtio,mmio
> > @@ -33,6 +36,10 @@ properties:
> >      description: Required for devices making accesses thru an IOMMU.
> >      maxItems: 1
> >
> > +  xen,dev-domid:
> > +    description: Required when Xen grant mappings need to be enabled for device.
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +
> >  required:
> >    - compatible
> >    - reg
> 
> Sorry for joining the discussion late. Have you considered using the
> generic iommu
> binding here instead of a custom property? This would mean having a device
> node for the grant-table mechanism that can be referred to using the 'iommus'
> phandle property, with the domid as an additional argument.
> 
> It does not quite fit the model that Linux currently uses for iommus,
> as that has an allocator for dma_addr_t space, but it would think it's
> conceptually close enough that it makes sense for the binding.

Something common is almost always better.

That may also have the issue that fw_devlink will make the 'iommu' 
driver a dependency to probe.

Rob

Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 18.05.22 21:59, Rob Herring wrote:

Hello Rob, Arnd

> On Wed, May 18, 2022 at 03:32:27PM +0100, Arnd Bergmann wrote:
>> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@gmail.com> wrote:
>>> diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
>>> index 10c22b5..29a0932 100644
>>> --- a/Documentation/devicetree/bindings/virtio/mmio.yaml
>>> +++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
>>> @@ -13,6 +13,9 @@ description:
>>>     See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
>>>     more details.
>>>
>>> +allOf:
>>> +  - $ref: /schemas/arm/xen,dev-domid.yaml#
>>> +
>>>   properties:
>>>     compatible:
>>>       const: virtio,mmio
>>> @@ -33,6 +36,10 @@ properties:
>>>       description: Required for devices making accesses thru an IOMMU.
>>>       maxItems: 1
>>>
>>> +  xen,dev-domid:
>>> +    description: Required when Xen grant mappings need to be enabled for device.
>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>> +
>>>   required:
>>>     - compatible
>>>     - reg
>> Sorry for joining the discussion late. Have you considered using the
>> generic iommu
>> binding here instead of a custom property? This would mean having a device
>> node for the grant-table mechanism that can be referred to using the 'iommus'
>> phandle property, with the domid as an additional argument.
>>
>> It does not quite fit the model that Linux currently uses for iommus,
>> as that has an allocator for dma_addr_t space, but it would think it's
>> conceptually close enough that it makes sense for the binding.
> Something common is almost always better.

agree


>
> That may also have the issue that fw_devlink will make the 'iommu'
> driver a dependency to probe.

Looks like I ran into it while experimenting. I generated the following 
nodes in guest DT using Xen toolstack:

[snip]

         xen_dummy_iommu {
                 compatible = "xen,dummy-iommu";
                 #iommu-cells = <0x01>;
                 phandle = <0xfde9>;
         };
         virtio@2000000 {
                 compatible = "virtio,mmio";
                 reg = <0x00 0x2000000 0x00 0x200>;
                 interrupts = <0x00 0x01 0xf01>;
                 interrupt-parent = <0xfde8>;
                 dma-coherent;
                 iommus = <0xfde9 0x01>;
         };

[snip]


And got:

virtio-mmio 2000000.virtio: deferred probe timeout, ignoring dependency


>
> Rob

-- 
Regards,

Oleksandr Tyshchenko