> I recall our original code is virtio_wmb(). 
> Use store fence to ensure all updates to entries before updating the index.
> Why do we need virtio_rmb() here and add virtio_wmb after 
> vq_update_avail_idx()?

Store fence is unnecessary, Intel CPU's are cache coherent, please read
the virtio Linux ring header file for explanation. A full fence WMB
is more expensive and causes CPU stall

> >     vq->vq_ring.avail->idx = vq->vq_avail_idx;
> >  }
> > 
> > @@ -255,7 +264,7 @@ static inline void
> >  virtqueue_notify(struct virtqueue *vq)
> >  {
> >     /*
> > -    * Ensure updated avail->idx is visible to host. mb() necessary?
> > +    * Ensure updated avail->idx is visible to host.
> >      * For virtio on IA, the notificaiton is through io port operation
> >      * which is a serialization instruction itself.
> >      */
> > --
> > 1.8.4.2
> 

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