Re: [Qemu-devel] [PATCH v2] hw/virtio/vring/event_idx: fix the vring_avail_event error

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

On Fri, Oct 31, 2014 at 10:57:55AM +0000, Stefan Hajnoczi wrote:
> On Fri, Oct 31, 2014 at 12:40:16AM +0000, Bin Wu wrote:
> > The event idx in virtio is an effective way to reduce the number of
> > interrupts and exits of the guest. When the guest puts an request
> > into the virtio ring, it doesn't exit immediately to inform the
> > backend. Instead, the guest checks the "avail" event idx to determine
> > the notification.
> > 
> > In virtqueue_pop, when a request is poped, the current avail event
> > idx should be set to the number of vq->last_avail_idx.
> > 
> > Signed-off-by: Bin Wu <wu.wubin@huawei.com>
> > ---
> > V2 -> V1:
> > update the same code in hw/virtio/dataplane/vring.c (Stefan)
> > ---
> >  hw/virtio/dataplane/vring.c | 8 ++++----
> >  hw/virtio/virtio.c          | 2 +-
> >  2 files changed, 5 insertions(+), 5 deletions(-)
> 
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> By the way, did you see that kernel drivers/vhost/vhost.c doesn't update
> the field for each pop?  Instead it only updates when notify is
> re-enabled.  I wonder if that approach is better than what QEMU does.
> 
> Stefan

Yes you should not move event idx if you don't want events.