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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, May 25, 2016 at 08:25:20AM +0000, Xie, Huawei wrote:
> On 5/25/2016 4:12 PM, Xie, Huawei 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))) {
> >  		uint32_t idx, desc_idx, used_idx;
> >  		struct vring_used_elem *uep;
> >  
> 
> Find this issue when do the code rework of RX/TX queue.
> As in other places, we also have loop retrieving the value of avial->idx
> or used->idx, i prefer to declare the index in vq structure as volatile
> to avoid potential issue.

It might be a good idea to wrap this in a macro
similar to ACCESS_ONCE in Linux.

> 
> Stephen:
> Another question is why we need a loop here?
> 
> /huawei

-- 
MST