[PATCH] virtio: Try to untangle DMA coherency

[mst@redhat.com (Michael S. Tsirkin)]

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?

-- 
MST