Re: [PATCH 5 of 5] virtio: expose added descriptors immediately

[Rusty Russell <rusty@rustcorp.com.au>]

On Sun, 13 Nov 2011 23:03:14 +0200, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Thu, Nov 03, 2011 at 06:12:53PM +1030, Rusty Russell wrote:
> > A virtio driver does virtqueue_add_buf() multiple times before finally
> > calling virtqueue_kick(); previously we only exposed the added buffers
> > in the virtqueue_kick() call.  This means we don't need a memory
> > barrier in virtqueue_add_buf(), but it reduces concurrency as the
> > device (ie. host) can't see the buffers until the kick.
> > 
> > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> 
> In the past I played with a patch like this, but I didn't see a
> performance gain either way. Do you see any gain?

No, but I haven't run it on real hardware.  lguest may see a win with
this in theory, since the virtqueue processing is fully async, so I'll
run some tests.

> I'm a bit concerned that with this patch, a buggy driver that
> adds more than 2^16 descriptors without a kick
> would seem to work sometimes. Let's add WARN_ON(vq->num_added > (1 << 16))?

Hmm, I guess it could wait for the add to fail before doing a kick, but
noone does that at the moment, so I've added a slight variant:

	WARN_ON(vq->num_added > vq->vring.num);

Thanks,
Rusty.