Message-ID: <43f198b5-60f8-40f5-a2cd-ff21b31a91d4@redhat.com>
Date: Wed, 20 Aug 2025 18:29:27 +0200
From: Eric Auger <eric.auger@...hat.com>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: Alex Williamson <alex.williamson@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, clg@...hat.com
Subject: Re: [PATCH 2/2] vfio/platform: Mark for removal

Hi Mostafa,

On 8/20/25 5:20 PM, Mostafa Saleh wrote:
> Hi Eric,
>
> On Tue, Aug 19, 2025 at 11:58:32AM +0200, Eric Auger wrote:
>> Hi Mostafa,
>>
>> On 8/18/25 7:33 PM, Mostafa Saleh wrote:
>>> On Mon, Aug 18, 2025 at 10:52:42AM -0600, Alex Williamson wrote:
>>>> On Fri, 15 Aug 2025 16:59:37 +0000
>>>> Mostafa Saleh <smostafa@...gle.com> wrote:
>>>>
>>>>> Hi Alex,
>>>>>
>>>>> On Wed, Aug 06, 2025 at 11:03:12AM -0600, Alex Williamson wrote:
>>>>>> vfio-platform hasn't had a meaningful contribution in years.  In-tree
>>>>>> hardware support is predominantly only for devices which are long since
>>>>>> e-waste.  QEMU support for platform devices is slated for removal in
>>>>>> QEMU-10.2.  Eric Auger presented on the future of the vfio-platform
>>>>>> driver and difficulties supporting new devices at KVM Forum 2024,
>>>>>> gaining some support for removal, some disagreement, but garnering no
>>>>>> new hardware support, leaving the driver in a state where it cannot
>>>>>> be tested.
>>>>>>
>>>>>> Mark as obsolete and subject to removal.  
>>>>> Recently(this year) in Android, we enabled VFIO-platform for protected KVM,
>>>>> and it’s supported in our VMM (CrosVM) [1].
>>>>> CrosVM support is different from Qemu, as it doesn't require any device
>>>>> specific logic in the VMM, however, it relies on loading a device tree
>>>>> template in runtime (with “compatiable” string...) and it will just
>>>>> override regs, irqs.. So it doesn’t need device knowledge (at least for now)
>>>>> Similarly, the kernel doesn’t need reset drivers as the hypervisor handles that.
>>>> I think what we attempt to achieve in vfio is repeatability and data
>>>> integrity independent of the hypervisor.  IOW, if we 'kill -9' the
>>>> hypervisor process, the kernel can bring the device back to a default
>>>> state where the device isn't wedged or leaking information through the
>>>> device to the next use case.  If the hypervisor wants to support
>>>> enhanced resets on top of that, that's great, but I think it becomes
>>>> difficult to argue that vfio-platform itself holds up its end of the
>>>> bargain if we're really trusting the hypervisor to handle these aspects.
>>> Sorry I was not clear, we only use that in Android for ARM64 and pKVM,
>>> where the hypervisor in this context means the code running in EL2 which
>>> is more privileged than the kernel, so it should be trusted.
>>> However, as I mentioned that code is not upstream yet, so it's a valid
>>> concern that the kernel still needs a reset driver.
>>>
>>>>> Unfortunately, there is no upstream support at the moment, we are making
>>>>> some -slow- progress on that [2][3]
>>>>>
>>>>> If it helps, I have access to HW that can run that and I can review/test
>>>>> changes, until upstream support lands; if you are open to keeping VFIO-platform.
>>>>> Or I can look into adding support for existing upstream HW(with platforms I am
>>>>> familiar with as Pixel-6)
>>>> Ultimately I'll lean on Eric to make the call.  I know he's concerned
>>>> about testing, but he raised that and various other concerns whether
>>>> platform device really have a future with vfio nearly a year ago and
>>>> nothing has changed.  Currently it requires a module option opt-in to
>>>> enable devices that the kernel doesn't know how to reset.  Is that
>>>> sufficient or should use of such a device taint the kernel?  If any
>>>> device beyond the few e-waste devices that we know how to reset taint
>>>> the kernel, should this support really even be in the kernel?  Thanks,
>>> I think with the way it’s supported at the moment we need the kernel
>>> to ensure that reset happens.
>> Effectively my main concern is I cannot test vfio-platform anymore. We
>> had some CVEs also impacting the vfio platform code base and it is a
>> major issue not being able to test. That's why I was obliged, last year,
>> to resume the integration of a new device (the tegra234 mgbe), nobody
>> seemed to be really interested in and this work could not be upstreamed
>> due to lack of traction and its hacky nature.
>>
>> You did not really comment on which kind of devices were currently
>> integrated. Are they within the original scope of vfio (with DMA
>> capabilities and protected by an IOMMU)? Last discussion we had in
>> https://lore.kernel.org/all/ZvvLpLUZnj-Z_tEs@google.com/ led to the
>> conclusion that maybe VFIO was not the best suited framework.
> At the moment, Android device assignement only supports DMA capable
> devices which are behind an IOMMU, and we use VFIO-platform for that,
> most of our use cases are accelerators.
>
> In that thread, I was looking into adding support for simpler devices
> (such as sensors) but as discussed that won’t be done through
> VFIO-platform.
>
> Ignoring Android, as I mentioned, I can work on adding support for
> existing upstream platforms (preferably ARM64, that I can get access to)
> such as Pixel-6, which should make it easier to test.
>
> Also, we have some interest on adding new features such as run-time
> power management.

OK fair enough. If Alex agrees then we can wait for those efforts. Also
I think it would make sense to formalize the way you reset the devices
(I understand the hyp does that under the hood).
>
>> In case we keep the driver in, I think we need to get a garantee that
>> you or someone else at Google commits to review and test potential
>> changes with a perspective to take over its maintenance.
> I can’t make guarantees on behalf of Google, but I can contribute in
> reviewing/testing/maintenance of the driver as far as I am able to.
> If you want, you can add me as reviewer to the driver.

I understand. I think the usual way then is for you to send a patch to
update the Maintainers file.

Thanks

Eric
>
> Thanks,
> Mostafa
>
>
>> Thanks
>>
>> Eric
>>
>>> But maybe instead of having that specific reset handler for VFIO, we
>>> can rely on the “shutdown” method already existing in "platform_driver"?
>>> I believe that should put the device in a state where it can be re-probed
>>> safely. Although not all devices implement that but it seems more generic
>>> and scalable.
>>>
>>> Thanks,
>>> Mostafa
>>>
>>>> Alex
>>>>

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