Re: packed ring layout proposal v3

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

On Mon, Oct 30, 2017 at 06:30:56AM +0000, Ilya Lesokhin wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin [mailto:mst@redhat.com]
> > Sent: Monday, October 30, 2017 4:09 AM
> > To: Ilya Lesokhin <ilyal@mellanox.com>
> > Cc: virtio-dev@lists.oasis-open.org; virtualization@lists.linux-foundation.org
> > Subject: Re: packed ring layout proposal v3
> > 
> > On Sun, Oct 29, 2017 at 02:34:56PM +0000, Ilya Lesokhin wrote:
> > > > -----Original Message-----
> > > > From: Michael S. Tsirkin [mailto:mst@redhat.com]
> > > > Sent: Sunday, October 29, 2017 4:22 PM
> > > > To: Ilya Lesokhin <ilyal@mellanox.com>
> > > > Cc: virtio-dev@lists.oasis-open.org; virtualization@lists.linux-foundation.org
> > > > Subject: Re: packed ring layout proposal v3
> > > >
> > > > If you do this whats the point of the id? Just use descriptor offset like virtio 0
> > did.
> > > >
> > >
> > > I agree that ID is pointless when requests are completed in order.
> > >
> > > But I'm not sure what you mean by descriptor offset?
> > 
> > Where the descriptor is within the ring.
> > 
> 
> Using descriptor offset like virtio 0, won't work.
> In virtio 0, there was no reordering in the descriptor ring, so the offset was always unique.
> In the new spec, if descriptor in offset 2 completes before the descriptor in offset 1,
> It can be put in offset 1, but reusing offset 1 is not yet safe.
> 
> Also, please ignore my earlier comment about in-order completion,
> It invalidates the entire discussion.
> 
> 

Yes, using offsets only works if all descriptors are used and written back in
order. If they are, descriptor ID isn't necessary. If they aren't,
it's necessary.

-- 
MST