Re: [PATCH 2/4] vhost: toggle device callbacks using used event idx

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

On Fri, Oct 21, 2022 at 09:45:14AM +0200, Eugenio Perez Martin wrote:
> On Fri, Oct 21, 2022 at 5:40 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Thu, Oct 20, 2022 at 11:53 PM Eugenio Pérez <eperezma@redhat.com> wrote:
> > >
> > > Actually use the new field of the used ring and tell the device if SVQ
> > > wants to be notified.
> > >
> > > The code is not reachable at the moment.
> > >
> > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > ---
> > >  hw/virtio/vhost-shadow-virtqueue.c | 18 +++++++++++++++---
> > >  1 file changed, 15 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
> > > index a518f84772..f5c0fad3fc 100644
> > > --- a/hw/virtio/vhost-shadow-virtqueue.c
> > > +++ b/hw/virtio/vhost-shadow-virtqueue.c
> > > @@ -369,15 +369,27 @@ static bool vhost_svq_more_used(VhostShadowVirtqueue *svq)
> > >   */
> > >  static bool vhost_svq_enable_notification(VhostShadowVirtqueue *svq)
> > >  {
> > > -    svq->vring.avail->flags &= ~cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> > > -    /* Make sure the flag is written before the read of used_idx */
> > > +    if (virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> > > +        uint16_t *used_event = (uint16_t *)&svq->vring.avail->ring[svq->vring.num];
> > > +        *used_event = svq->shadow_used_idx;
> >
> > Do we need to care about the endian here?
> >
> > E.g vduse has:
> >
> >     *((uint16_t *)&vq->vring.used->ring[vq->vring.num]) = htole16(val);
> >
> 
> Good catch, I forgot about endianness.
> 
> I'll review the series, thanks!

It's generally a waste that we don't use endian-ness annotations
the way linux does.


> > Thanks
> >
> > > +    } else {
> > > +        svq->vring.avail->flags &= ~cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> > > +    }
> > > +
> > > +    /* Make sure the event is enabled before the read of used_idx */
> > >      smp_mb();
> > >      return !vhost_svq_more_used(svq);
> > >  }
> > >
> > >  static void vhost_svq_disable_notification(VhostShadowVirtqueue *svq)
> > >  {
> > > -    svq->vring.avail->flags |= cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> > > +    /*
> > > +     * No need to disable notification in the event idx case, since used event
> > > +     * index is already an index too far away.
> > > +     */
> > > +    if (!virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> > > +        svq->vring.avail->flags |= cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
> > > +    }
> > >  }
> > >
> > >  static uint16_t vhost_svq_last_desc_of_chain(const VhostShadowVirtqueue *svq,
> > > --
> > > 2.31.1
> > >
> >