[dpdk-dev] [PATCH v2 02/24] virtio: Use weaker barriers

[Xie, Huawei]

> -----Original Message-----
> From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> Sent: Tuesday, January 27, 2015 5:59 PM
> To: Xie, Huawei
> Cc: Ouyang, Changchun; dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v2 02/24] virtio: Use weaker barriers
> 
> 
> > 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
> 


I mean virtio_wmb rather than virtio_rmb should be used here, 
and both of them are defined as compiler barrier.

The following code is linux virtio driver for adding buffer to vring.
/* Put entry in available array (but don't update avail->idx until they
	 * do sync). */
	avail = (vq->vring.avail->idx & (vq->vring.num-1));
	vq->vring.avail->ring[avail] = head;

	/* Descriptors and available array need to be set before we expose the
	 * new available array entries. */
	virtio_wmb(vq->weak_barriers);
	vq->vring.avail->idx++;

> > >  	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
> >