Re: [RFC] Simple device assignment with VFIO platform

Mostafa Saleh
2024-09-30T13:33:39Z

Hi Eric,

On Mon, Sep 30, 2024 at 03:05:38PM +0200, Eric Auger wrote:
> Hi Mostafa,
>
> On 9/27/24 18:17, Mostafa Saleh wrote:
> > Hi All,
> >
> > Background
> > ==========
> > I have been looking into assigning simple devices which are not DMA
> > capable to VMs on Android using VFIO platform.
> >
> > I have been mainly looking with respect to Protected KVM (pKVM), which
> > would need some extra modifications mostly to KVM-VFIO, that is quite
> > early under prototyping at the moment, which have core pending pKVM
> > dependencies upstream as guest memfd[1] and IOMMUs support[2].
> >
> > However, this problem is not pKVM(or KVM) specific, and about the
> > design of VFIO.
> >
> > [1] https://lore.kernel.org/kvm/20240801090117.3841080-1-tabba@google.com/
> > [2] https://lore.kernel.org/kvmarm/20230201125328.2186498-1-jean-philippe@linaro.org/
> >
> > Problem
> > =======
> > At the moment, VFIO platform will deny a device from probing (through
> > vfio_group_find_or_alloc()), if it’s not part of an IOMMU group,
> > unless (CONFIG_VFIO_NOIOMMU is configured)
> >
> > As far as I understand the current solutions to pass through platform
> > devices that are not DMA capable are:
> > - Use VFIO platform + (CONFIG_VFIO_NOIOMMU): The problem with that, it
> > taints the kernel and this doesn’t actually fit the device description
> > as the device doesn’t only have an IOMMU, but it’s not DMA capable at
> > all, so the kernel should be safe with assigning the device without
> > DMA isolation.
> >
> > - Use VFIO mdev with an emulated IOMMU, this seems it could work. But
> > many of the code would be duplicate with the VFIO platform code as the
> > device is a platform device.
> >
> > - Use UIO: Can map MMIO to userspace which seems to be focused for
> > userspace drivers rather than VM passthrough and I can’t find its
> > support in Qemu.
> In case you did not have this reference, you may have a look at Alex'
> reply in
> https://patchew.org/QEMU/1518189456-2873-1-git-send-email-geert+renesas@glider.be/1518189456-2873-5-git-send-email-geert+renesas@glider.be/
> >
> > One other benefit from supporting this in VFIO platform, that we can
> > use the existing UAPI for platform devices (and support in VMMs)
> >
> > Proposal
> > ========
> > Extend VFIO platform to allow assigning devices without an IOMMU, this
> > can be possibly done by
> > - Checking device capability from the platform bus (would be something
> > ACPI/OF specific similar to how it configures DMA from
> > platform_dma_configure(), we can add a new function something like
> > platfrom_dma_capable())
> >
> > - Using emulated IOMMU for such devices
> > (vfio_register_emulated_iommu_dev()), instead of having intrusive
> > changes about IOMMUs existence.
> >
> > If that makes sense I can work on RFC(I don’t have any code at the moment)
> So if I understand correctly, assuming you are able to safely detect the
> device is not DMA capable you would use the
>
> vfio_register_emulated_iommu_dev() trick. Is that correct?

Yes, my guess the challenge here would be discovering that through the
platform bus.

Thanks,
Mostafa

>
> Thanks
>
> Eric
>
> >
> > Thanks,
> > Mostafa
> >
>