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Subject: Re: [virtio] Re: [virtio-dev] [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.