On Thu, Feb 09, 2017 at 08:49:41PM +0200, Michael S. Tsirkin 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). > > > 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?
At the moment, _CCA/DT is pretty much never set correctly for virtio-mmio (that is, it isn't set even though the device is cache coherent). If it *was* set correctly, then we wouldn't have needed to revert my patch. Robin's patch to only use the DMA API if _CCA/DT is present would work (although the thing that he posted was buggy iirc). Will _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
