Re: [virtio] Re: [virtio-dev] [PATCH] VIRTIO_F_RING_PACKED_USED: packing used descriptors

[Stefan Hajnoczi <stefanha@redhat.com>]

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

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