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

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

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.


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