Message-ID: <aKYvS3qgV_dW1woo@google.com>
Date: Wed, 20 Aug 2025 20:25:47 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Eric Auger <eric.auger@...hat.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 Eric,

On Wed, Aug 20, 2025 at 06:29:27PM +0200, Eric Auger wrote:
> 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).

I think currently - with some help from the platform bus- we can rely on
the existing shutdown method, instead of specific hooks.
As the hypervisor logic will only be for ARM64 (at least for now), I can
look more into this.

But I think the top priority would be to establish a decent platform to
test with, I will start looking into Pixel-6 (although that would need
to land IOMMU support for it upstream first). I also have a morello
board with SMMUv3, but I think it's all PCI.

> >
> >> 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.

I see, I will send one shortly.

Thanks,
Mostafa

> 
> 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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