Re: [dpdk-dev] [PATCH] virtio: use volatile to get used->idx in the loop

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Wed, Jun 01, 2016 at 05:40:08AM +0000, Xie, Huawei wrote:
> On 5/30/2016 4:20 PM, Yuanhan Liu wrote:
> > On Wed, May 25, 2016 at 12:16:41AM +0800, Huawei Xie wrote:
> >> There is no external function call or any barrier in the loop,
> >> the used->idx would only be retrieved once.
> >>
> >> Signed-off-by: Huawei Xie <huawei.xie@intel.com>
> >> ---
> >>  drivers/net/virtio/virtio_ethdev.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/virtio_ethdev.c
> >> index c3fb628..f6d6305 100644
> >> --- a/drivers/net/virtio/virtio_ethdev.c
> >> +++ b/drivers/net/virtio/virtio_ethdev.c
> >> @@ -204,7 +204,8 @@ virtio_send_command(struct virtqueue *vq, struct virtio_pmd_ctrl *ctrl,
> >>  		usleep(100);
> >>  	}
> >>  
> >> -	while (vq->vq_used_cons_idx != vq->vq_ring.used->idx) {
> >> +	while (vq->vq_used_cons_idx !=
> >> +	       *((volatile uint16_t *)(&vq->vq_ring.used->idx))) {
> > I'm wondering maybe we could fix VIRTQUEUE_NUSED (which has no such
> > qualifier) and use this macro here?
> >
> > If you check the reference of that macro, you might find similar
> > issues, say, it is also used inside the while-loop of
> > virtio_recv_mergeable_pkts().
> >
> > 	--yliu
> >  
> >
> 
> Yes, seems it has same issue though haven't confirmed with asm code.

So, move the "volatile" qualifier to VIRTQUEUE_NUSED?

	--yliu