On Wed, Aug 01, 2018 at 10:05:35AM +0100, Will Deacon wrote: > Hi Christoph, > > 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. > > Apologies: it's me that wants the DMA ops enabled to handle legacy devices > behind an IOMMU, but see below. > > > > 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. > > That's true, although that doesn't exist in the legacy virtio spec, and we > have an existing emulation platform which puts legacy virtio devices behind > an IOMMU. Currently, Linux is unable to boot on this platform unless the > IOMMU is configured as bypass. If we can use the coherent IOMMU DMA ops, > then it works perfectly. > > > 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. > > As above -- that means we hang during boot because we get stuck trying to > bring up a virtio-block device whose DMA is aborted by the IOMMU. The easy > answer is "just upgrade to latest virtio and advertise the presence of the > IOMMU". I'm pushing for that in future platforms, but it seems a shame not > to support the current platform, especially given that other systems do have > hacks in mainline to get virtio working. > > > 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. 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 agree that this makes sense for future revisions of virtio (or perhaps > it can just be a clarification to virtio 1.0), but we're still left in the > dark with legacy devices and it would be nice to have them work on the > systems which currently exist, even if it's a legacy-only hack in the arch > code. > > Will Myself I'm sympathetic to this use-case and I see more uses to this than just legacy support. But more work is required IMHO. Will post tomorrow though - it's late here ... -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
