> From: Mostafa Saleh <smostafa@xxxxxxxxxx> > 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@xxxxxxxxxx/ > [2] https://lore.kernel.org/kvmarm/20230201125328.2186498-1-jean- > philippe@xxxxxxxxxx/ > > 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...
