Re: [PATCH v2 3/8] xen/arm, doc: Add a DT property to specify IOMMU for Dom0less domUs

[Henry Wang <xin.wang2@amd.com>]

Hi Julien,

On 5/20/2024 8:41 AM, Henry Wang wrote:
> Hi Julien,
>
> Thanks for spending time on the review!
>
> On 5/19/2024 6:17 PM, Julien Grall wrote:
>> Hi Henry,
>>
>> On 16/05/2024 11:03, Henry Wang wrote:
>>> diff --git a/docs/misc/arm/device-tree/booting.txt 
>>> b/docs/misc/arm/device-tree/booting.txt
>>> index bbd955e9c2..61f9082553 100644
>>> --- a/docs/misc/arm/device-tree/booting.txt
>>> +++ b/docs/misc/arm/device-tree/booting.txt
>>> @@ -260,6 +260,19 @@ with the following properties:
>>>       value specified by Xen command line parameter 
>>> gnttab_max_maptrack_frames
>>>       (or its default value if unspecified, i.e. 1024) is used.
>>>   +- passthrough
>>> +
>>> +    A string property specifying whether IOMMU mappings are enabled 
>>> for the
>>> +    domain and hence whether it will be enabled for passthrough 
>>> hardware.
>>> +    Possible property values are:
>>> +
>>> +    - "enabled"
>>> +    IOMMU mappings are enabled for the domain.
>>> +
>>> +    - "disabled"
>>> +    IOMMU mappings are disabled for the domain and so hardware may 
>>> not be
>>> +    passed through. This option is the default if this property is 
>>> missing.
>>
>> Looking at the code below, it seems like the default will depend on 
>> whether the partial device-tree is present. Did I misunderstand?
>
> I am not sure if I understand the "partial device tree" in above 
> comment correctly. The "passthrough" property is supposed to be placed 
> in the dom0less domU domain node exactly the same way as the other 
> dom0less domU properties (such as "direct-map" etc.). This way we can 
> control the XEN_DOMCTL_CDF_iommu is set or not for each dom0less domU 
> separately.

Oh I think I get your points, you meant the XEN_DOMCTL_CDF_iommu will 
still be set if the passthrough dt property is "disabled", but user 
provides a partial device tree. Yes you are correct. I will update the 
doc to explain a bit more details as below. Thanks for pointing it out.

  - "enabled"
     IOMMU mappings are enabled for the domain. Note that this option is 
the
     default if the user provides the device partial passthrough device 
tree
     for the domain.

  - "disabled"
     IOMMU mappings are disabled for the domain and so hardware may not be
     passed through. This option is the default if this property is missing
     and the user does not provide the device partial device tree for 
the domain.

Kind regards,
Henry