On Thu, Oct 21, 2021 at 02:26:00AM +0000, Tian, Kevin wrote: > > I'll leave it to Jean to confirm. If only coherent DMA can be used in > > the guest on other platforms, suppose VFIO should not blindly set > > IOMMU_CACHE and in concept it should deny assigning a non-coherent > > device since no co-ordination with guest exists today. > > Jean, what's your opinion? Yes a sanity check to prevent assigning non-coherent devices would be good, though I'm not particularly worried about non-coherent devices. PCIe on Arm should be coherent (according to the Base System Architecture). So vfio-pci devices should be coherent, but vfio-platform and mdev are case-by-case (hopefully all coherent since it concerns newer platforms). More worrying, I thought we disabled No-Snoop for VFIO but I was wrong, it's left enabled. On Arm I don't think userspace can perform the right cache maintenance operations to maintain coherency with a device that issues No-Snoop writes. Userspace can issue clean+invalidate but not invalidate alone, so there is no equivalent to arch_sync_dma_for_cpu(). I think the worse that can happen is the device owner shooting itself in the foot by using No-Snoop, but would it hurt to disable it? Thanks, Jean
