RE: [RFC PATCH V2] x86/VMBus: Confidential VMBus for dynamic DMA buffer transition

RE: [RFC PATCH V2] x86/VMBus: Confidential VMBus for dynamic DMA buffer transition

[Michael Kelley]

From: Tianyu Lan <ltykernel@gmail.com> Sent: Tuesday, February 10, 2026 8:21 AM
> 
> Hyper-V provides Confidential VMBus to communicate between
> device model and device guest driver via encrypted/private
> memory in Confidential VM. The device model is in OpenHCL
> (https://openvmm.dev/guide/user_guide/openhcl.html) that
> plays the paravisor rule.
> 
> For a VMBUS device, there are two communication methods to

s/VMBUS/VMBus/

> talk with Host/Hypervisor. 1) VMBus Ring buffer 2) dynamic
> DMA transition.

I'm not sure what "dynamic DMA transition" is. Maybe just
"DMA transfers"?  Also, do the same substitution further
down in this commit message.

> The Confidential VMBus Ring buffer has been
> upstreamed by Roman Kisel(commit 6802d8af).

It's customary to use 12 character commit IDs, which would be
6802d8af47d1 in this case.

> 
> The dynamic DMA transition of VMBus device normally goes
> through DMA core and it uses SWIOTLB as bounce buffer in
> CVM

"CVM" is Microsoft-speak. The Linux terminology is "a CoCo VM".

> to communicate with Host/Hypervisor. The Confidential
> VMBus device may use private/encrypted memory to do DMA
> and so the device swiotlb(bounce buffer) isn't necessary.

The phrase "isn't necessary" does not capture the real issue
here. Saying "isn't necessary" makes it sound like this patch is
just avoids unnecessary work, so that it is a performance
improvement. But that's not the case.

The real issue is that swiotlb memory is decrypted. So bouncing
through the swiotlb exposes to the host what is supposed to be
confidential data passed on the Confidential VMBus. Disabling
the swiotlb bouncing in this case is a hard requirement to preserve
confidentially.

So I would reword the sentences as something like this:

The Confidential VMBus device can do DMA directly to
private/encrypted memory. Because the swiotlb is decrypted
memory, the DMA transfer must not be bounced through the
swiotlb, so as to preserve confidentiality. This is different from
the default for Linux CoCo VMs, so disable the VMBus device's
use of swiotlb.

> To disable device's swiotlb, set device->dma_io_tlb_mem
> to NULL in VMBus driver and is_swiotlb_force_bounce()
> always returns false.
> 
> Suggested-by: Michael Kelley <mhklinux@outlook.com>
> Signed-off-by: Tianyu Lan <tiala@microsoft.com>
> ---
> Change since v1:
>        Use device.dma_io_tlb_mem to disable device bounce buffer
> 
>  drivers/hv/vmbus_drv.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index a53af6fe81a6..58dab8cc3fcb 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -2133,11 +2133,15 @@ int vmbus_device_register(struct hv_device *child_device_obj)
>  	child_device_obj->device.dma_mask = &child_device_obj->dma_mask;
>  	dma_set_mask(&child_device_obj->device, DMA_BIT_MASK(64));
> 
> +	device_initialize(&child_device_obj->device);
> +	if (child_device_obj->channel->co_external_memory)
> +		child_device_obj->device.dma_io_tlb_mem = NULL;
> +

Doing this as part of the VMBus bus driver makes sense. While directly
setting device.dma_io_tlb_mem to NULL should work, it would be better
to add a function to the swiotlb code to do this, and then call that function
here, passing the device as an argument. The need to disable swiotlb on a
device will likely arise in similar contexts (such as TDISP), and it would be
better to have a swiotlb function for that purpose. This use case may be
a bit ahead of the TDISP work, and having a swiotlb function in place will
help ensure that duplicate mechanisms aren't created as everything
comes together.

See my earlier comments in [1] about the key point in the commit message,
and about adding a swiotlb_dev_disable() function to the swiotlb code.

Michael

[1] https://lore.kernel.org/linux-hyperv/SN6PR02MB4157DAE6D8CC6BA11CA87298D4DCA@SN6PR02MB4157.namprd02.prod.outlook.com/

>  	/*
>  	 * Register with the LDM. This will kick off the driver/device
>  	 * binding...which will eventually call vmbus_match() and vmbus_probe()
>  	 */
> -	ret = device_register(&child_device_obj->device);
> +	ret = device_add(&child_device_obj->device);
>  	if (ret) {
>  		pr_err("Unable to register child device\n");
>  		put_device(&child_device_obj->device);
> --
> 2.50.1

Re: [RFC PATCH V2] x86/VMBus: Confidential VMBus for dynamic DMA buffer transition

[Robin Murphy]

On 2026-02-11 6:00 pm, Michael Kelley wrote:
> From: Tianyu Lan <ltykernel@gmail.com> Sent: Tuesday, February 10, 2026 8:21 AM
>>
>> Hyper-V provides Confidential VMBus to communicate between
>> device model and device guest driver via encrypted/private
>> memory in Confidential VM. The device model is in OpenHCL
>> (https://openvmm.dev/guide/user_guide/openhcl.html) that
>> plays the paravisor rule.
>>
>> For a VMBUS device, there are two communication methods to
> 
> s/VMBUS/VMBus/
> 
>> talk with Host/Hypervisor. 1) VMBus Ring buffer 2) dynamic
>> DMA transition.
> 
> I'm not sure what "dynamic DMA transition" is. Maybe just
> "DMA transfers"?  Also, do the same substitution further
> down in this commit message.
> 
>> The Confidential VMBus Ring buffer has been
>> upstreamed by Roman Kisel(commit 6802d8af).
> 
> It's customary to use 12 character commit IDs, which would be
> 6802d8af47d1 in this case.
> 
>>
>> The dynamic DMA transition of VMBus device normally goes
>> through DMA core and it uses SWIOTLB as bounce buffer in
>> CVM
> 
> "CVM" is Microsoft-speak. The Linux terminology is "a CoCo VM".
> 
>> to communicate with Host/Hypervisor. The Confidential
>> VMBus device may use private/encrypted memory to do DMA
>> and so the device swiotlb(bounce buffer) isn't necessary.
> 
> The phrase "isn't necessary" does not capture the real issue
> here. Saying "isn't necessary" makes it sound like this patch is
> just avoids unnecessary work, so that it is a performance
> improvement. But that's not the case.
> 
> The real issue is that swiotlb memory is decrypted. So bouncing
> through the swiotlb exposes to the host what is supposed to be
> confidential data passed on the Confidential VMBus. Disabling
> the swiotlb bouncing in this case is a hard requirement to preserve
> confidentially.

Yeah, this really isn't a Hyper-V problem. Indeed as things stand, 
"swiotlb=force" could potentially break confidentiality for any 
environment trying to invent a notion of private DMA, and perhaps we 
could throw a big warning about that, but really the answer there is 
"Don't run your confidential workload with 'swiotlb=force'. Why would 
you even do that? Debug your drivers in a regular VM or bare-metal with 
full debug visibility like a normal person..."

The fact is we do not have a proper notion of trusted/private DMA yet, 
and this is not the way to add it. The current assumption is very much 
that all DMA is untrusted in the CoCo sense, because initially it was 
only virtual devices emulated by a hypervisor, thus had to be bounced 
through shared memory anyway. AMD SEV with a stage 1 IOMMU in the guest 
can allow an assigned physical device to access a suitably-aligned 
encrypted buffer directly, but that's still effectively just putting the 
buffer into a temporarily shared state for that device, it merely skips 
sharing it with the rest of the system. !force_dma_unencrypted() doesn't 
mean "we trust this device's DMA", it just means "we don't have to use 
explicitly-decrypted pages to accommodate untrusted/shared DMA here", 
plus it also serves double-duty for host encryption which doesn't share 
the same trust model anyway.

I assumed this would follow the TDISP stuff, but if Hyper-V has an 
alternative device-trusting mechanism already then there's no need to 
wait. We want some common device property (likely consolidating the 
current PCI external-facing port notion of trustedness plus whatever 
TDISP wants), with which we can then make proper decisions in all the 
right DMA API paths - and if it can end up replacing the horrible 
force_dma_unencrypted() as well then all the better! I'd totally 
forgotten about the previous discussion that Michael referred to (which 
I had to track down[1]), but it looks like all the main points were 
already covered there and we were approaching a consensus, so really I 
guess someone just needs to give it a go.

Thanks,
Robin.

[1] 
https://lore.kernel.org/linux-hyperv/20250409000835.285105-6-romank@linux.microsoft.com/

> 
> So I would reword the sentences as something like this:
> 
> The Confidential VMBus device can do DMA directly to
> private/encrypted memory. Because the swiotlb is decrypted
> memory, the DMA transfer must not be bounced through the
> swiotlb, so as to preserve confidentiality. This is different from
> the default for Linux CoCo VMs, so disable the VMBus device's
> use of swiotlb.
> 
>> To disable device's swiotlb, set device->dma_io_tlb_mem
>> to NULL in VMBus driver and is_swiotlb_force_bounce()
>> always returns false.
>>
>> Suggested-by: Michael Kelley <mhklinux@outlook.com>
>> Signed-off-by: Tianyu Lan <tiala@microsoft.com>
>> ---
>> Change since v1:
>>         Use device.dma_io_tlb_mem to disable device bounce buffer
>>
>>   drivers/hv/vmbus_drv.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
>> index a53af6fe81a6..58dab8cc3fcb 100644
>> --- a/drivers/hv/vmbus_drv.c
>> +++ b/drivers/hv/vmbus_drv.c
>> @@ -2133,11 +2133,15 @@ int vmbus_device_register(struct hv_device *child_device_obj)
>>   	child_device_obj->device.dma_mask = &child_device_obj->dma_mask;
>>   	dma_set_mask(&child_device_obj->device, DMA_BIT_MASK(64));
>>
>> +	device_initialize(&child_device_obj->device);
>> +	if (child_device_obj->channel->co_external_memory)
>> +		child_device_obj->device.dma_io_tlb_mem = NULL;
>> +
> 
> Doing this as part of the VMBus bus driver makes sense. While directly
> setting device.dma_io_tlb_mem to NULL should work, it would be better
> to add a function to the swiotlb code to do this, and then call that function
> here, passing the device as an argument. The need to disable swiotlb on a
> device will likely arise in similar contexts (such as TDISP), and it would be
> better to have a swiotlb function for that purpose. This use case may be
> a bit ahead of the TDISP work, and having a swiotlb function in place will
> help ensure that duplicate mechanisms aren't created as everything
> comes together.
> 
> See my earlier comments in [1] about the key point in the commit message,
> and about adding a swiotlb_dev_disable() function to the swiotlb code.
> 
> Michael
> 
> [1] https://lore.kernel.org/linux-hyperv/SN6PR02MB4157DAE6D8CC6BA11CA87298D4DCA@SN6PR02MB4157.namprd02.prod.outlook.com/
> 
>>   	/*
>>   	 * Register with the LDM. This will kick off the driver/device
>>   	 * binding...which will eventually call vmbus_match() and vmbus_probe()
>>   	 */
>> -	ret = device_register(&child_device_obj->device);
>> +	ret = device_add(&child_device_obj->device);
>>   	if (ret) {
>>   		pr_err("Unable to register child device\n");
>>   		put_device(&child_device_obj->device);
>> --
>> 2.50.1

Re: [RFC PATCH V2] x86/VMBus: Confidential VMBus for dynamic DMA buffer transition

[Aneesh Kumar K.V]

Robin Murphy <robin.murphy@arm.com> writes:

> On 2026-02-11 6:00 pm, Michael Kelley wrote:
>> From: Tianyu Lan <ltykernel@gmail.com> Sent: Tuesday, February 10, 2026 8:21 AM
>>>
>>> Hyper-V provides Confidential VMBus to communicate between
>>> device model and device guest driver via encrypted/private
>>> memory in Confidential VM. The device model is in OpenHCL
>>> (https://openvmm.dev/guide/user_guide/openhcl.html) that
>>> plays the paravisor rule.
>>>
>>> For a VMBUS device, there are two communication methods to
>> 
>> s/VMBUS/VMBus/
>> 
>>> talk with Host/Hypervisor. 1) VMBus Ring buffer 2) dynamic
>>> DMA transition.
>> 
>> I'm not sure what "dynamic DMA transition" is. Maybe just
>> "DMA transfers"?  Also, do the same substitution further
>> down in this commit message.
>> 
>>> The Confidential VMBus Ring buffer has been
>>> upstreamed by Roman Kisel(commit 6802d8af).
>> 
>> It's customary to use 12 character commit IDs, which would be
>> 6802d8af47d1 in this case.
>> 
>>>
>>> The dynamic DMA transition of VMBus device normally goes
>>> through DMA core and it uses SWIOTLB as bounce buffer in
>>> CVM
>> 
>> "CVM" is Microsoft-speak. The Linux terminology is "a CoCo VM".
>> 
>>> to communicate with Host/Hypervisor. The Confidential
>>> VMBus device may use private/encrypted memory to do DMA
>>> and so the device swiotlb(bounce buffer) isn't necessary.
>> 
>> The phrase "isn't necessary" does not capture the real issue
>> here. Saying "isn't necessary" makes it sound like this patch is
>> just avoids unnecessary work, so that it is a performance
>> improvement. But that's not the case.
>> 
>> The real issue is that swiotlb memory is decrypted. So bouncing
>> through the swiotlb exposes to the host what is supposed to be
>> confidential data passed on the Confidential VMBus. Disabling
>> the swiotlb bouncing in this case is a hard requirement to preserve
>> confidentially.
>
> Yeah, this really isn't a Hyper-V problem. Indeed as things stand, 
> "swiotlb=force" could potentially break confidentiality for any 
> environment trying to invent a notion of private DMA, and perhaps we 
> could throw a big warning about that, but really the answer there is 
> "Don't run your confidential workload with 'swiotlb=force'. Why would 
> you even do that? Debug your drivers in a regular VM or bare-metal with 
> full debug visibility like a normal person..."
>
> The fact is we do not have a proper notion of trusted/private DMA yet, 
> and this is not the way to add it. The current assumption is very much 
> that all DMA is untrusted in the CoCo sense, because initially it was 
> only virtual devices emulated by a hypervisor, thus had to be bounced 
> through shared memory anyway. AMD SEV with a stage 1 IOMMU in the guest 
> can allow an assigned physical device to access a suitably-aligned 
> encrypted buffer directly, but that's still effectively just putting the 
> buffer into a temporarily shared state for that device, it merely skips 
> sharing it with the rest of the system. !force_dma_unencrypted() doesn't 
> mean "we trust this device's DMA", it just means "we don't have to use 
> explicitly-decrypted pages to accommodate untrusted/shared DMA here", 
> plus it also serves double-duty for host encryption which doesn't share 
> the same trust model anyway.
>
> I assumed this would follow the TDISP stuff, but if Hyper-V has an 
> alternative device-trusting mechanism already then there's no need to 
> wait. We want some common device property (likely consolidating the 
> current PCI external-facing port notion of trustedness plus whatever 
> TDISP wants), with which we can then make proper decisions in all the 
> right DMA API paths - and if it can end up replacing the horrible 
> force_dma_unencrypted() as well then all the better! I'd totally 
> forgotten about the previous discussion that Michael referred to (which 
> I had to track down[1]), but it looks like all the main points were 
> already covered there and we were approaching a consensus, so really I 
> guess someone just needs to give it a go.
>

With my device-assignment–related changes, I have made the following
update. It may be a slightly stronger requirement to enforce that 
trusted device cannot use SWIOTLB, but it simplifies the overall design.
I also have a prototype, that added two default swiotlb, ie,

static struct io_tlb_mem io_tlb_default_mem;
static struct io_tlb_mem io_tlb_default_shared_mem;

Looking at that change, I would suggest we avoid doing this unless we
are certain that there is a requirement for a trusted device to use
SWIOTLB bouncing.

diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index b27de03f2466..07ef149bd9fc 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -292,6 +292,9 @@ bool swiotlb_free(struct device *dev, struct page *page, size_t size);
 
 static inline bool is_swiotlb_for_alloc(struct device *dev)
 {
+       if (device_cc_accepted(dev))
+               return false;
+
        return dev->dma_io_tlb_mem->for_alloc;
 }
 #else
diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 34fe14b987f0..a89a7ac07499 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -159,6 +159,14 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
  */
 static bool dma_direct_use_pool(struct device *dev, gfp_t gfp)
 {
+       /*
+        * Atomic pools are marked decrypted and are used if we require require
+        * updation of pfn mem encryption attributes or for DMA non-coherent
+        * device allocation. Both is not true for trusted device.
+        */
+       if (device_cc_accepted(dev))
+               return false;
+
        return !gfpflags_allow_blocking(gfp) && !is_swiotlb_for_alloc(dev);
 }
 
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index a862712f4dc6..6d9f0c869c6f 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -1643,6 +1643,9 @@ bool is_swiotlb_active(struct device *dev)
 {
        struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
 
+       if (device_cc_accepted(dev))
+               return false;
+
        return mem && mem->nslabs;
 }

Re: [RFC PATCH V2] x86/VMBus: Confidential VMBus for dynamic DMA buffer transition

[Tianyu Lan]

On Mon, Feb 16, 2026 at 6:21 PM Aneesh Kumar K.V
<aneesh.kumar@kernel.org> wrote:
>
> Robin Murphy <robin.murphy@arm.com> writes:
>
> > On 2026-02-11 6:00 pm, Michael Kelley wrote:
> >> From: Tianyu Lan <ltykernel@gmail.com> Sent: Tuesday, February 10, 2026 8:21 AM
> >>>
> >>> Hyper-V provides Confidential VMBus to communicate between
> >>> device model and device guest driver via encrypted/private
> >>> memory in Confidential VM. The device model is in OpenHCL
> >>> (https://openvmm.dev/guide/user_guide/openhcl.html) that
> >>> plays the paravisor rule.
> >>>
> >>> For a VMBUS device, there are two communication methods to
> >>
> >> s/VMBUS/VMBus/
> >>
> >>> talk with Host/Hypervisor. 1) VMBus Ring buffer 2) dynamic
> >>> DMA transition.
> >>
> >> I'm not sure what "dynamic DMA transition" is. Maybe just
> >> "DMA transfers"?  Also, do the same substitution further
> >> down in this commit message.
> >>
> >>> The Confidential VMBus Ring buffer has been
> >>> upstreamed by Roman Kisel(commit 6802d8af).
> >>
> >> It's customary to use 12 character commit IDs, which would be
> >> 6802d8af47d1 in this case.
> >>
> >>>
> >>> The dynamic DMA transition of VMBus device normally goes
> >>> through DMA core and it uses SWIOTLB as bounce buffer in
> >>> CVM
> >>
> >> "CVM" is Microsoft-speak. The Linux terminology is "a CoCo VM".
> >>
> >>> to communicate with Host/Hypervisor. The Confidential
> >>> VMBus device may use private/encrypted memory to do DMA
> >>> and so the device swiotlb(bounce buffer) isn't necessary.
> >>
> >> The phrase "isn't necessary" does not capture the real issue
> >> here. Saying "isn't necessary" makes it sound like this patch is
> >> just avoids unnecessary work, so that it is a performance
> >> improvement. But that's not the case.
> >>
> >> The real issue is that swiotlb memory is decrypted. So bouncing
> >> through the swiotlb exposes to the host what is supposed to be
> >> confidential data passed on the Confidential VMBus. Disabling
> >> the swiotlb bouncing in this case is a hard requirement to preserve
> >> confidentially.
> >
> > Yeah, this really isn't a Hyper-V problem. Indeed as things stand,
> > "swiotlb=force" could potentially break confidentiality for any
> > environment trying to invent a notion of private DMA, and perhaps we
> > could throw a big warning about that, but really the answer there is
> > "Don't run your confidential workload with 'swiotlb=force'. Why would
> > you even do that? Debug your drivers in a regular VM or bare-metal with
> > full debug visibility like a normal person..."
> >
> > The fact is we do not have a proper notion of trusted/private DMA yet,
> > and this is not the way to add it. The current assumption is very much
> > that all DMA is untrusted in the CoCo sense, because initially it was
> > only virtual devices emulated by a hypervisor, thus had to be bounced
> > through shared memory anyway. AMD SEV with a stage 1 IOMMU in the guest
> > can allow an assigned physical device to access a suitably-aligned
> > encrypted buffer directly, but that's still effectively just putting the
> > buffer into a temporarily shared state for that device, it merely skips
> > sharing it with the rest of the system. !force_dma_unencrypted() doesn't
> > mean "we trust this device's DMA", it just means "we don't have to use
> > explicitly-decrypted pages to accommodate untrusted/shared DMA here",
> > plus it also serves double-duty for host encryption which doesn't share
> > the same trust model anyway.
> >
> > I assumed this would follow the TDISP stuff, but if Hyper-V has an
> > alternative device-trusting mechanism already then there's no need to
> > wait. We want some common device property (likely consolidating the
> > current PCI external-facing port notion of trustedness plus whatever
> > TDISP wants), with which we can then make proper decisions in all the
> > right DMA API paths - and if it can end up replacing the horrible
> > force_dma_unencrypted() as well then all the better! I'd totally
> > forgotten about the previous discussion that Michael referred to (which
> > I had to track down[1]), but it looks like all the main points were
> > already covered there and we were approaching a consensus, so really I
> > guess someone just needs to give it a go.
> >
>
> With my device-assignment–related changes, I have made the following
> update. It may be a slightly stronger requirement to enforce that
> trusted device cannot use SWIOTLB, but it simplifies the overall design.
> I also have a prototype, that added two default swiotlb, ie,
>
> static struct io_tlb_mem io_tlb_default_mem;
> static struct io_tlb_mem io_tlb_default_shared_mem;
>
> Looking at that change, I would suggest we avoid doing this unless we
> are certain that there is a requirement for a trusted device to use
> SWIOTLB bouncing.
>

Hi Robin & Aneesh:
     Thanks for your suggestion and draft patch. Later response due to
holiday. We may combine the Aneesh's change with Michael's suggestion
that DMA core exposes DMA core API of disabling swiotlb allocation and force
using swiotlb and latform or subsystem(e.g, TSM module) maycall them
according to user case.

> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index b27de03f2466..07ef149bd9fc 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -292,6 +292,9 @@ bool swiotlb_free(struct device *dev, struct page *page, size_t size);
>
>  static inline bool is_swiotlb_for_alloc(struct device *dev)
>  {
> +       if (device_cc_accepted(dev))
> +               return false;
> +
>         return dev->dma_io_tlb_mem->for_alloc;
>  }
>  #else
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 34fe14b987f0..a89a7ac07499 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -159,6 +159,14 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
>   */
>  static bool dma_direct_use_pool(struct device *dev, gfp_t gfp)
>  {
> +       /*
> +        * Atomic pools are marked decrypted and are used if we require require
> +        * updation of pfn mem encryption attributes or for DMA non-coherent
> +        * device allocation. Both is not true for trusted device.
> +        */
> +       if (device_cc_accepted(dev))
> +               return false;
> +
>         return !gfpflags_allow_blocking(gfp) && !is_swiotlb_for_alloc(dev);
>  }
>
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index a862712f4dc6..6d9f0c869c6f 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1643,6 +1643,9 @@ bool is_swiotlb_active(struct device *dev)
>  {
>         struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
>
> +       if (device_cc_accepted(dev))
> +               return false;
> +
>         return mem && mem->nslabs;
>  }



--
Thanks
Tianyu Lan

Re: [RFC PATCH V2] x86/VMBus: Confidential VMBus for dynamic DMA buffer transition

[Tianyu Lan]

On Thu, Feb 12, 2026 at 2:00 AM Michael Kelley <mhklinux@outlook.com> wrote:
>
> From: Tianyu Lan <ltykernel@gmail.com> Sent: Tuesday, February 10, 2026 8:21 AM
> >
> > Hyper-V provides Confidential VMBus to communicate between
> > device model and device guest driver via encrypted/private
> > memory in Confidential VM. The device model is in OpenHCL
> > (https://openvmm.dev/guide/user_guide/openhcl.html) that
> > plays the paravisor rule.
> >
> > For a VMBUS device, there are two communication methods to
>
> s/VMBUS/VMBus/
>
> > talk with Host/Hypervisor. 1) VMBus Ring buffer 2) dynamic
> > DMA transition.
>
> I'm not sure what "dynamic DMA transition" is. Maybe just
> "DMA transfers"?  Also, do the same substitution further
> down in this commit message.
>
> > The Confidential VMBus Ring buffer has been
> > upstreamed by Roman Kisel(commit 6802d8af).
>
> It's customary to use 12 character commit IDs, which would be
> 6802d8af47d1 in this case.
>
> >
> > The dynamic DMA transition of VMBus device normally goes
> > through DMA core and it uses SWIOTLB as bounce buffer in
> > CVM
>
> "CVM" is Microsoft-speak. The Linux terminology is "a CoCo VM".
>
> > to communicate with Host/Hypervisor. The Confidential
> > VMBus device may use private/encrypted memory to do DMA
> > and so the device swiotlb(bounce buffer) isn't necessary.
>
> The phrase "isn't necessary" does not capture the real issue
> here. Saying "isn't necessary" makes it sound like this patch is
> just avoids unnecessary work, so that it is a performance
> improvement. But that's not the case.
>
> The real issue is that swiotlb memory is decrypted. So bouncing
> through the swiotlb exposes to the host what is supposed to be
> confidential data passed on the Confidential VMBus. Disabling
> the swiotlb bouncing in this case is a hard requirement to preserve
> confidentially.
>
> So I would reword the sentences as something like this:
>
> The Confidential VMBus device can do DMA directly to
> private/encrypted memory. Because the swiotlb is decrypted
> memory, the DMA transfer must not be bounced through the
> swiotlb, so as to preserve confidentiality. This is different from
> the default for Linux CoCo VMs, so disable the VMBus device's
> use of swiotlb.
>
> > To disable device's swiotlb, set device->dma_io_tlb_mem
> > to NULL in VMBus driver and is_swiotlb_force_bounce()
> > always returns false.
> >
> > Suggested-by: Michael Kelley <mhklinux@outlook.com>
> > Signed-off-by: Tianyu Lan <tiala@microsoft.com>
> > ---
> > Change since v1:
> >        Use device.dma_io_tlb_mem to disable device bounce buffer
> >
> >  drivers/hv/vmbus_drv.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> > index a53af6fe81a6..58dab8cc3fcb 100644
> > --- a/drivers/hv/vmbus_drv.c
> > +++ b/drivers/hv/vmbus_drv.c
> > @@ -2133,11 +2133,15 @@ int vmbus_device_register(struct hv_device *child_device_obj)
> >       child_device_obj->device.dma_mask = &child_device_obj->dma_mask;
> >       dma_set_mask(&child_device_obj->device, DMA_BIT_MASK(64));
> >
> > +     device_initialize(&child_device_obj->device);
> > +     if (child_device_obj->channel->co_external_memory)
> > +             child_device_obj->device.dma_io_tlb_mem = NULL;
> > +
>
> Doing this as part of the VMBus bus driver makes sense. While directly
> setting device.dma_io_tlb_mem to NULL should work, it would be better
> to add a function to the swiotlb code to do this, and then call that function
> here, passing the device as an argument. The need to disable swiotlb on a
> device will likely arise in similar contexts (such as TDISP), and it would be
> better to have a swiotlb function for that purpose. This use case may be
> a bit ahead of the TDISP work, and having a swiotlb function in place will
> help ensure that duplicate mechanisms aren't created as everything
> comes together.
>
> See my earlier comments in [1] about the key point in the commit message,
> and about adding a swiotlb_dev_disable() function to the swiotlb code.
>
> Michael

Hi Michael:
     Thanks for your review. Will add swiotlb_dev_disable() in the next version.

>
> [1] https://lore.kernel.org/linux-hyperv/SN6PR02MB4157DAE6D8CC6BA11CA87298D4DCA@SN6PR02MB4157.namprd02.prod.outlook.com/
>
> >       /*
> >        * Register with the LDM. This will kick off the driver/device
> >        * binding...which will eventually call vmbus_match() and vmbus_probe()
> >        */
> > -     ret = device_register(&child_device_obj->device);
> > +     ret = device_add(&child_device_obj->device);
> >       if (ret) {
> >               pr_err("Unable to register child device\n");
> >               put_device(&child_device_obj->device);
> > --
> > 2.50.1



-- 
Thanks
Tianyu Lan