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

[Eugenio Perez Martin <eperezma@redhat.com>]

On Mon, Oct 24, 2022 at 4:16 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Fri, Oct 21, 2022 at 4:15 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > 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.
>
> Yes, it's worth doing something similar sometime.
>

Maybe we could wrap them in some struct like virtio_le16 or virtio_16,
avoiding at least integer direct assignment? Wrappers like
cpu_to_virtio16 could return these structs and I think all compilers
should emit the same code as direct assignment.

Thanks!




> 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
> > > > >
> > > >
> >
>