Re: [PATCH 18/31] vhost: Shadow virtqueue buffers forwarding

[Eugenio Perez Martin]

On Sun, Jan 30, 2022 at 7:47 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2022/1/22 上午4:27, Eugenio Pérez 写道:
> > @@ -272,6 +590,28 @@ void vhost_svq_set_svq_kick_fd(VhostShadowVirtqueue 
> > *svq, int svq_kick_fd)
> >   void vhost_svq_stop(VhostShadowVirtqueue *svq)
> >   {
> >       event_notifier_set_handler(&svq->svq_kick, NULL);
> > +    g_autofree VirtQueueElement *next_avail_elem = NULL;
> > +
> > +    if (!svq->vq) {
> > +        return;
> > +    }
> > +
> > +    /* Send all pending used descriptors to guest */
> > +    vhost_svq_flush(svq, false);
>
>
> Do we need to wait for all the pending descriptors to be completed here?
>

No, this function does not wait, it only completes the forwarding of
the *used* descriptors.

The best example is the net rx queue in my opinion. This call will
check SVQ's vring used_idx and will forward the last used descriptors
if any, but all available descriptors will remain as available for
qemu's VQ code.

To skip it would miss those last rx descriptors in migration.

Thanks!

> Thanks
>
>
> > +
> > +    for (unsigned i = 0; i < svq->vring.num; ++i) {
> > +        g_autofree VirtQueueElement *elem = NULL;
> > +        elem = g_steal_pointer(&svq->ring_id_maps[i]);
> > +        if (elem) {
> > +            virtqueue_detach_element(svq->vq, elem, elem->len);
> > +        }
> > +    }
> > +
> > +    next_avail_elem = g_steal_pointer(&svq->next_guest_avail_elem);
> > +    if (next_avail_elem) {
> > +        virtqueue_detach_element(svq->vq, next_avail_elem,
> > +                                 next_avail_elem->len);
> > +    }
> >   }
>