Re: [RFC PATCH 02/12] vhost: Move SVQ queue rewind to the destination

[Eugenio Perez Martin]

On Mon, Jul 18, 2022 at 7:50 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Sat, Jul 16, 2022 at 7:34 PM Eugenio Pérez <eperezma@redhat.com> wrote:
> >
> > Migration with SVQ already migrate the inflight descriptors,
>
> How is this done?
>

Migration between vhost-vdpa with x-svq=on maintain the guest's
visible state in VirtQueues, and they already have all the protocols
to migrate them.

We cannot migrate if we cannot restore the state / inflight in the
destination, but since we need x-svq=on at this point, we can restore
both of them. Extra care will be needed when ASID is introduced.

> > so the
> > destination can perform the work.
> >
> > This makes easier to migrate between backends or to recover them in
> > vhost devices that support set in flight descriptors.
> >
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >  hw/virtio/vhost-vdpa.c | 24 +++++++++++-------------
> >  1 file changed, 11 insertions(+), 13 deletions(-)
> >
> > diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> > index 18820498b3..4458c8d23e 100644
> > --- a/hw/virtio/vhost-vdpa.c
> > +++ b/hw/virtio/vhost-vdpa.c
> > @@ -1178,7 +1178,18 @@ static int vhost_vdpa_set_vring_base(struct 
> > vhost_dev *dev,
> >                                         struct vhost_vring_state *ring)
> >  {
> >      struct vhost_vdpa *v = dev->opaque;
> > +    VirtQueue *vq = virtio_get_queue(dev->vdev, ring->index);
> >
> > +    /*
> > +     * vhost-vdpa devices does not support in-flight requests. Set all of 
> > them
> > +     * as available.
> > +     *
> > +     * TODO: This is ok for networking, but other kinds of devices might
> > +     * have problems with these retransmissions.
> > +     */
> > +    while (virtqueue_rewind(vq, 1)) {
> > +        continue;
> > +    }
> >      if (v->shadow_vqs_enabled) {
> >          /*
> >           * Device vring base was set at device start. SVQ base is handled 
> > by
> > @@ -1197,19 +1208,6 @@ static int vhost_vdpa_get_vring_base(struct 
> > vhost_dev *dev,
> >      int ret;
> >
> >      if (v->shadow_vqs_enabled) {
> > -        VirtQueue *vq = virtio_get_queue(dev->vdev, ring->index);
> > -
> > -        /*
> > -         * Setting base as last used idx, so destination will see as 
> > available
> > -         * all the entries that the device did not use, including the 
> > in-flight
> > -         * processing ones.
> > -         *
> > -         * TODO: This is ok for networking, but other kinds of devices 
> > might
> > -         * have problems with these retransmissions.
> > -         */
> > -        while (virtqueue_rewind(vq, 1)) {
> > -            continue;
> > -        }
>
> I think it's not a good idea to partially revert the code that was
> just introduced in the previous patch (unless it can be backported to
> -stable independently).
>
> We probably need to squash the changes.
>

You definitely have a point. Maybe it's better to remove the "Fixes"
tag? -stable version cannot enable SVQ anyway (it doesn't have the
parameter).

I'll send a v2 when we have all the comments sorted out. Maybe a
better alternative is to delay the x-svq parameter to the top of both
series if both of them go in?

Thanks!

> Thanks
>
> >          ring->num = virtio_queue_get_last_avail_idx(dev->vdev, 
> > ring->index);
> >          return 0;
> >      }
> > --
> > 2.31.1
> >
>