Re: [virtio-comment] Hardware friendly proposals from Intel for packed-ring-layout

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 24, 2017 at 02:10:34PM +0200, Paolo Bonzini wrote:
> On 24/08/2017 13:53, Tiwei Bie wrote:
> > 
> > * In addition to the DESC_HW flag, each virtio queue has a tail pointer
> >     - Driver creates suitable (i.e. multiple of cacheline) descriptors,
> >       then performs MMIO write to tail pointer.
> 
> If I understand correctly, the tail pointer is the value that is written
> to the MMIO register.  If that is the case, this is unfortunately bad
> for virtualization.  Virt prefers a doorbell register where the value
> doesn't matter.

I think it's fine actually, this tail pointer merely duplicates data
that is already available in form of DESC_HW flag.  Virt can just ignore
the value.