On Tue, Jun 08, 2021 at 10:54:59AM +0200, Enrico Weigelt, metux IT consult wrote:

Maybe the device as well as the transport could announce their
capability (which IMHO should go via the virtio protocol), and if both
are capable, the (guest's) virtio subsys tells the driver whether it's
usable for a specific device. Perhaps we should also have a mechanism
to tell the device that it's actually used.

The usage should be extremely narrow, like 

"If the driver issues a GPU command with flag X then the resulting
DMAs will be no-snoop and the driver must re-establish coherency at
the right moment"

It is not a general idea, but something baked directly into the device
protocol that virtio carries.

The general notion of no-nsoop should ideally be carried in the PCIe
extended config space flag. If 0 then no-snoop should never be issued,
expected, or requested.

Jason