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

[Eugenio Perez Martin]

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!

> 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
> >
>