On 9 February 2017 at 18:49, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > On Thu, Feb 09, 2017 at 06:31:18PM +0000, Will Deacon wrote: >> On Thu, Feb 09, 2017 at 08:17:16PM +0200, Michael S. Tsirkin wrote: >> > On Thu, Feb 02, 2017 at 04:40:49PM +0000, Will Deacon wrote: >> > > On Thu, Feb 02, 2017 at 06:30:28PM +0200, Michael S. Tsirkin wrote: >> > > > I am inclined to say, for 4.10 let's revert >> > > > c7070619f3408d9a0dffbed9149e6f00479cf43b since what it fixes is not a >> > > > regression in 4.10. >> > > >> > > No complaints there, as long as we can keep working to fix this for 4.11 >> > > and onwards. You'll also need to cc stable on the revert. >> > > >> > > > So I think we can defer the fix to 4.11. >> > > > I think we still want f7f6634d23830ff74335734fbdb28ea109c1f349 >> > > > for hosts with virtio 1 support. >> > > > >> > > > All this will hopefully push hosts to just implement virtio 1. >> > > > For mmio the changes are very small: several new registers, >> > > > that's all. You want this for proper 64 bit dma mask anyway. >> > > >> > > As I've said, virtio 1 will have exactly the same issue unless we start >> > > requiring firmware to advertise dma-coherent/_CCA for virtio-mmio >> > > devices correctly. >> > > >> > >> > OK I read up on _CCA in ACPI spec. It says: >> > The _CCA object returns whether or not a bus-master device supports >> > hardware managed cache coherency. Expected values are 0 to indicate it >> > is not supported, and 1 to indicate that it is supported. >> > >> > So if host is cache coherent, and guest thinks it isn't, we incur >> > unnecessary overhead by wasting coherent memory. >> > I get that but you said it actually breaks - why does it? >> >> It breaks because QEMU doesn't set _CCA for virtio-mmio devices, and that >> only becomes a problem when we use the DMA API, because that results in the >> guest taking out a non-cacheable mapping. On ARM (and other archs such as >> Power), having a mismatch between a cacheable and a non-cacheable mapping >> can result in a loss of coherency between the two (for example, if the >> non-cacheable gues accesses bypass the cache, but the cacheable host >> accesses allocate in the cache). >> >> Will > > I see. And I guess using a cacheable mapping is significantly faster. > I would say we want to typically use cacheable for virtio then, > whether we bypass the IOMMU or not. I guess this is why we always set > _CCA/DT correctly, right? > The point is that you don't get to choose: if the hardware is DMA coherent, the CPU should use cacheable mappings, or you may lose data. If the hardware is non-coherent, the CPU should use non-cacheable mappings for the same reason. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
