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

[Xie, Huawei]

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 at 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.

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

/huawei