[PATCH] VIRTIO_F_RING_PACKED_USED: packing used descriptors

On Tue, Nov 03, 2020 at 05:18:34PM +0000, Stefan Hajnoczi wrote:
> On Fri, Oct 30, 2020 at 11:01:10AM -0400, Michael S. Tsirkin wrote:
>
> On Fri, Oct 30, 2020 at 11:37:32AM +0000, Stefan Hajnoczi wrote:
> >
> On Thu, Oct 29, 2020 at 03:50:57AM -0400, Michael S. Tsirkin wrote:
> > > > +When after processing the used descriptor, the batch counter
> > > > +exceeds the ring size, the batch counter is decremented by the
> > > > +ring size; both device and driver then skip to the
> > > > +beginning (offset 0) of the ring until processing (reading for
> > > > +the driver and writing for the device) the next used descriptor.
> > >
> >
> This may be clearer:
> > >
> >
>
both device and driver then use ring index 0 for the next used
> >
>
descriptor
> > >
> >
> Or:
> > >
> >
>
The device writes the next used descriptor at ring index 0. The driver
> >
>
reads the next used descriptor from ring index 0.
> > >
> >
> I'm not sure I understand this scheme. Is this pseudo-code correct?
> > >
> >
>
batch_counter += num_used_descs
> >
>
if batch_counter
> ring_size:
> >
>
batch_counter -= ring_size
> >
>
used_idx = 0
> >
>
> yes
> >
> >
> >
> Can you share an explanation of how this scheme ensures that the
> >
> driver's next avail descriptor index never overtakes the device's next
> >
> used descriptor index?
> >
>
> Simply because to overtake it, it needs to wrap around to 0.
>
> That happens when batch counter wraps over to 0, if we make
>
> device wrap at that point all is well.
>
> What confuses me is that batch_counter is not reset to 0. It is set to
> batch_counter -= ring_size.
>
> I guess I need to get my crayons and draw pictures :-).
>
> Stefan

The idea is that when all available buffers have been used then
batch counter of the device matches the next avail index of the driver.