On Wed, Aug 01, 2018 at 01:36:39AM -0700, Christoph Hellwig wrote: > On Wed, Aug 01, 2018 at 09:16:38AM +0100, Will Deacon wrote: > > 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, > > No one is suggesting that as far as I can tell. > > > 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). > > >From all that I've gather so far: no you do not want that. We really > need to figure out virtio "dma" interacts with the host / device. > > If you look at the current iommu spec it does talk of physical address > with a little careveout for VIRTIO_F_IOMMU_PLATFORM. > > So between that and our discussion in this thread and its previous > iterations I think we need to stick to the current always physical, > bypass system dma ops mode of virtio operation as the default. > > We just need to figure out how to deal with devices that deviate > from the default. One things is that VIRTIO_F_IOMMU_PLATFORM really > should become VIRTIO_F_PLATFORM_DMA to cover the cases of non-iommu > dma tweaks (offsets, cache flushing), which seems well in spirit of > the original design. Well I wouldn't say that. VIRTIO_F_IOMMU_PLATFORM is for guest programmable protection which is designed for things like userspace drivers but still very much which a CPU doing the accesses. I think VIRTIO_F_IO_BARRIER needs to be extended to VIRTIO_F_PLATFORM_DMA. > The other issue is VIRTIO_F_IO_BARRIER > which is very vaguely defined, and which needs a better definition. > And last but not least we'll need some text explaining the challenges > of hardware devices - I think VIRTIO_F_PLATFORM_DMA + VIRTIO_F_IO_BARRIER > is what would basically cover them, but a good description including > an explanation of why these matter. I think VIRTIO_F_IOMMU_PLATFORM + VIRTIO_F_PLATFORM_DMA but yea. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
