Re: [RFC] /dev/ioasid uAPI proposal

[Jason Gunthorpe]

On Fri, Jun 04, 2021 at 09:11:03AM +0800, Jason Wang wrote:
> > nor do any virtio drivers implement the required platform specific
> > cache flushing to make no-snoop TLPs work.
> 
> I don't get why virtio drivers needs to do that. I think DMA API should hide
> those arch/platform specific stuffs from us.

It is not arch/platform stuff. If the device uses no-snoop then a
very platform specific recovery is required in the device driver.

It is not part of the normal DMA API, it is side APIs like
flush_agp_cache() or wbinvd() that are used by GPU drivers only.

If drivers/virtio doesn't explicitly call these things it doesn't
support no-snoop - hence no VDPA device can ever use no-snoop.

Since VIRTIO_F_ACCESS_PLATFORM doesn't trigger wbinvd on x86 it has
nothing to do with no-snoop.

Jason