[PATCH] virtio: Try to untangle DMA coherency

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

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.
> 
> Will

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?

-- 
MST