On 4 March 2015 at 23:29, Catalin Marinas [off-list ref] wrote:

I disagree it is 100% a host-side issue. It is a host-side issue _if_
the host tells the guest that the (virtual) device is non-coherent (or,
more precisely, it does not explicitly tell the guest that the device is
coherent). If the guest thinks the (virtual) device is non-coherent
because of information passed by the host, I fully agree that the host
needs to manage the cache coherency.

However, the host could also pass a "dma-coherent" property in the DT
given to the guest and avoid any form of cache maintenance. If the guest
does not honour such coherency property, it's a guest problem and it
needs fixing in the guest. This isn't any different from a real physical
device behaviour.

Right, and we should do that for things like virtio, because we want
the performance. But we also have devices (like vga framebuffers)
which shouldn't be handled as cacheable, so we need to be able to
deal with both situations.

-- PMM