On Wed, Aug 01, 2018 at 09:16:38AM +0100, Will Deacon wrote: > On Tue, Jul 31, 2018 at 03:36:22PM -0500, Benjamin Herrenschmidt wrote: > > On Tue, 2018-07-31 at 10:30 -0700, Christoph Hellwig wrote: > > > > However the question people raise is that DMA API is already full of > > > > arch-specific tricks the likes of which are outlined in your post linked > > > > above. How is this one much worse? > > > > > > None of these warts is visible to the driver, they are all handled in > > > the architecture (possibly on a per-bus basis). > > > > > > So for virtio we really need to decide if it has one set of behavior > > > as specified in the virtio spec, or if it behaves exactly as if it > > > was on a PCI bus, or in fact probably both as you lined up. But no > > > magic arch specific behavior inbetween. > > > > The only arch specific behaviour is needed in the case where it doesn't > > behave like PCI. In this case, the PCI DMA ops are not suitable, but in > > our secure VMs, we still need to make it use swiotlb in order to bounce > > through non-secure pages. > > On arm/arm64, the problem we have is that legacy virtio devices on the MMIO > transport (so definitely not PCI) have historically been advertised by qemu > as not being cache coherent, but because the virtio core has bypassed DMA > ops then everything has happened to work. If we blindly enable the arch DMA > ops, we'll plumb in the non-coherent ops and start getting data corruption, > so we do need a way to quirk virtio as being "always coherent" if we want to > use the DMA ops (which we do, because our emulation platforms have an IOMMU > for all virtio devices). > > Will Right that's not very different from placing the device within the IOMMU domain but in fact bypassing the IOMMU. I wonder whether anyone ever needs a non coherent virtio-mmio. If yes we can extend PLATFORM_IOMMU to cover that or add another bit. What exactly do the non-coherent ops do that causes the corruption? -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
