Re: [dpdk-dev] [PATCH] vhost: flush used->idx update before reading avail->flags

["Xie, Huawei" <huawei.xie@intel.com>]

> -----Original Message-----
> From: Linhaifeng [mailto:haifeng.lin@huawei.com]
> Sent: Friday, April 24, 2015 9:01 AM
> To: Xie, Huawei; dev@dpdk.org
> Cc: luke@snabb.co; mst@redhat.com
> Subject: Re: [PATCH] vhost: flush used->idx update before reading avail-
> >flags
> 
> 
> 
> On 2015/4/23 0:33, Huawei Xie wrote:
> > update of used->idx and read of avail->flags could be reordered.
> > memory fence should be used to ensure the order, otherwise guest could
> see a stale used->idx value after it toggles the interrupt suppression flag.
> >
> > Signed-off-by: Huawei Xie <huawei.xie@intel.com>
> > ---
> >  lib/librte_vhost/vhost_rxtx.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
> > index 510ffe8..6afba35 100644
> > --- a/lib/librte_vhost/vhost_rxtx.c
> > +++ b/lib/librte_vhost/vhost_rxtx.c
> > @@ -178,6 +178,9 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t
> queue_id,
> >  	*(volatile uint16_t *)&vq->used->idx += count;
> >  	vq->last_used_idx = res_end_idx;
> >
> > +	/* flush used->idx update before we read avail->flags. */
> > +	rte_mb();
> > +
> >  	/* Kick the guest if necessary. */
> >  	if (!(vq->avail->flags & VRING_AVAIL_F_NO_INTERRUPT))
> >  		eventfd_write((int)vq->callfd, 1);
> >
> 
> If not add memory fence what would happen? Packets loss or interrupt
> loss?How to test it ?

If those two store and load are reordered,  guest toggles the interrupt suppression flag, then it checks that there is no more work to do.
Actually there is.