Re: [RFC] /dev/ioasid uAPI proposal

[Jason Gunthorpe]

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