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

[Linhaifeng <haifeng.lin-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

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-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> ---
>  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 ?