Re: [RFC v2 13/13] vhost: Use VRING_AVAIL_F_NO_INTERRUPT at device call on shadow virtqueue

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

On Tue, Mar 16, 2021 at 9:08 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2021/3/16 上午3:48, Eugenio Pérez 写道:
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >   hw/virtio/vhost-shadow-virtqueue.c | 28 +++++++++++++++++++++++++++-
> >   1 file changed, 27 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
> > index 7df98fc43f..e3879a4622 100644
> > --- a/hw/virtio/vhost-shadow-virtqueue.c
> > +++ b/hw/virtio/vhost-shadow-virtqueue.c
> > @@ -71,10 +71,35 @@ typedef struct VhostShadowVirtqueue {
> >       /* Next head to consume from device */
> >       uint16_t used_idx;
> >
> > +    /* Cache for the exposed notification flag */
> > +    bool notification;
> > +
> >       /* Descriptors copied from guest */
> >       vring_desc_t descs[];
> >   } VhostShadowVirtqueue;
> >
> > +static void vhost_shadow_vq_set_notification(VhostShadowVirtqueue *svq,
> > +                                             bool enable)
> > +{
> > +    uint16_t notification_flag;
> > +
> > +    if (svq->notification == enable) {
> > +        return;
> > +    }
> > +
> > +    notification_flag = virtio_tswap16(svq->vdev, VRING_AVAIL_F_NO_INTERRUPT);
> > +
> > +    svq->notification = enable;
> > +    if (enable) {
> > +        svq->vring.avail->flags &= ~notification_flag;
> > +    } else {
> > +        svq->vring.avail->flags |= notification_flag;
> > +    }
> > +
> > +    /* Make sure device reads our flag */
> > +    smp_mb();
>
>
> This is a hint, so we don't need memory barrier here.
>
> Thanks
>

I will delete it for the next revision.

Thanks!

>
> > +}
> > +
> >   static void vhost_vring_write_descs(VhostShadowVirtqueue *svq,
> >                                       const struct iovec *iovec,
> >                                       size_t num, bool more_descs, bool write)
> > @@ -251,7 +276,7 @@ static void vhost_shadow_vq_handle_call_no_test(EventNotifier *n)
> >       do {
> >           unsigned i = 0;
> >
> > -        /* TODO: Use VRING_AVAIL_F_NO_INTERRUPT */
> > +        vhost_shadow_vq_set_notification(svq, false);
> >           while (true) {
> >               g_autofree VirtQueueElement *elem = vhost_shadow_vq_get_buf(svq);
> >               if (!elem) {
> > @@ -269,6 +294,7 @@ static void vhost_shadow_vq_handle_call_no_test(EventNotifier *n)
> >               svq->masked_notifier.signaled = true;
> >               event_notifier_set(svq->masked_notifier.n);
> >           }
> > +        vhost_shadow_vq_set_notification(svq, true);
> >       } while (vhost_shadow_vq_more_used(svq));
> >
> >       if (masked_notifier) {
>