Hi Tian, On Mon, Sep 30, 2024 at 08:19:45AM +0000, Tian, Kevin wrote: > > 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: Makes sense, I see it’s used by a couple of other drivers also, so in that case it can be just a driver and not an 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... Thanks a lot for the feedback, I will work on RFC patches unless someone strongly disagrees with the approach. Thanks, Mostafa
