Re: [RFC] Simple device assignment with VFIO platform

[Yi Liu <yi.l.liu@intel.com>]

On 2024/9/28 00:17, 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)

so your device does not have an IOMMU and also it does not do DMA at all?

> 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.

you need to set the vfio_noiommu parameter as well. yes, this would give
your device a fake iommu group. But the kernel would say this taints it.

> 
> - 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.

QEMU is for device passthrough, it makes sense it needs to use the VFIO
without noiommu instead of UIO. The below link has more explanations.

https://wiki.qemu.org/Features/VT-d

As the introduction of vfio cdev, you may compile the vfio group out
by CONFIG_VFIO_GROUP==n. Supposedly, you will not be blocked by the
vfio_group_find_or_alloc(). But you might be blocked due to no present
iommu. You may have a try though.

> 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.

is it the mdev approach listed in the above?

-- 
Regards,
Yi Liu