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:

quoted

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.

quoted

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