RE: [RFC] Simple device assignment with VFIO platform

Tian, Kevin
2024-09-30T08:19:49Z

> From: Mostafa Saleh <smostafa@google.com>
> Sent: Saturday, September 28, 2024 12:17 AM
>
> 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.

emulated IOMMU is not tied to mdev:

/*
* Virtual device without IOMMU backing. The VFIO core fakes up an
* iommu_group as the iommu_group sysfs interface is part of the
* userspace ABI. The user of these devices must not be able to
* directly trigger unmediated DMA.
*/
VFIO_EMULATED_IOMMU,

Except it's not a virtual device, it does match the last sentence that
such device cannot trigger unmediated DMA.

>
> - 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.
>
> 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)

This sounds the best option out of my head now...