Re: [dpdk-stable] [PATCH v2] net/virtio: fix wrong TX pkt length stats

[Yuanhan Liu <yliu@fridaylinux.org>]

On Mon, Oct 23, 2017 at 02:40:36PM +0800, Zhiyong Yang wrote:
> In the function virtqueue_enqueue_xmit(), when can_push is true,
> vtnet_hdr_size is added to pkt_len by calling rte_pktmbuf_prepend.
> So, virtio header length should be subtracted before calling stats
> function.
> 
> Fixes: 58169a9c8153 ("net/virtio: support Tx checksum offload")
> 
> Cc: stable@dpdk.org
> Cc: yliu@fridaylinux.org
> Cc: maxime.coquelin@redhat.com
> Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>
> Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
> 
> Changes in V2:
> Put code in the function virtqueue_enqueue_xmit() according to
> yuanhan's comments.
> 
>  drivers/net/virtio/virtio_rxtx.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index 2cf82fef4..f28751e07 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -396,6 +396,13 @@ virtqueue_enqueue_xmit(struct virtnet_tx *txvq, struct rte_mbuf *cookie,
>  		vq->vq_desc_tail_idx = idx;
>  	vq->vq_free_cnt = (uint16_t)(vq->vq_free_cnt - needed);
>  	vq_update_avail_ring(vq, head_idx);
> +
> +	/* when can_push is true, vtnet_hdr_size is added to pkt_len
> +	 * of mbuf. It should be subtracted in order to make stats function
> +	 * work in the right way.
> +	 */
> +	if (can_push)
> +		cookie->pkt_len -= head_size;

I actually meant to put it inside the "if (can_push)" clause, so that you
don't have to add yet another if. The another reason I wanted you to do
the move is, rte_pktmbuf_prepend (which did the magic) is exactly invoked
inside this if. Such move puts logical related code tight together. At least,
you could make the comment simpler: you don't have to say "when can_push
is true ...". Instead, it could be:

    /*
     * rte_pktmbuf_prepend() counts the hdr size to the pkt length,
     * which is wrong. Below subtract restores the correct pkt size.
     */

	--yliu