Re: [PATCH 2/2] virtio: clean up space checks on xmit

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

On Thu, Dec 03, 2015 at 05:12:54PM -0800, Stephen Hemminger wrote:
> The space check for transmit ring only needs a single conditional.
> I.e only need to recheck for space if there was no space in first check.
> 
> This can help performance and simplifies loop.

This is a good cleanup. Besides that, I have few minor comments below.

> 
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> ---
>  drivers/net/virtio/virtio_rxtx.c | 68 +++++++++++++++++-----------------------
>  1 file changed, 29 insertions(+), 39 deletions(-)
> 
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index 466fee6..7cb932a 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -832,7 +832,6 @@ uint16_t
>  virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
>  {
>  	struct virtqueue *txvq = tx_queue;
> -	struct rte_mbuf *txm;
>  	uint16_t nb_used, nb_tx;
>  	int error;
>  
> @@ -846,58 +845,49 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
>  	if (likely(nb_used > txvq->vq_nentries - txvq->vq_free_thresh))
>  		virtio_xmit_cleanup(txvq, nb_used);
>  
> -	nb_tx = 0;
> -
> -	while (nb_tx < nb_pkts) {
> +	for (nb_tx = 0; nb_tx < nb_pkts; nb_tx++) {
> +		struct rte_mbuf *txm = tx_pkts[nb_tx];
>  		/* Need one more descriptor for virtio header. */
> -		int need = tx_pkts[nb_tx]->nb_segs - txvq->vq_free_cnt + 1;
> +		int need = txm->nb_segs - txvq->vq_free_cnt + 1;

While reviewing the code, I found the var name `need' is not properly
taken. Maybe `need_cleanup' is better, and it's better to be defined
as bool type. What do you think of that?

(And yes, it has nothing to do with your patch, I just found it we
can rename it to a better name to improve the code readability a bit.
If you agree, would you submit a patch, or should I do it?)

>  
> -		/*Positive value indicates it need free vring descriptors */
> +		/* Positive value indicates it need free vring descriptors */
>  		if (unlikely(need > 0)) {
>  			nb_used = VIRTQUEUE_NUSED(txvq);
>  			virtio_rmb();
>  			need = RTE_MIN(need, (int)nb_used);
>  
>  			virtio_xmit_cleanup(txvq, need);
> -			need = (int)tx_pkts[nb_tx]->nb_segs -
> -				txvq->vq_free_cnt + 1;
> +			need = txm->nb_segs - txvq->vq_free_cnt + 1;
> +			if (unlikely(need > 0)) {
> +				PMD_TX_LOG(ERR,
> +					   "No free tx descriptors to transmit");
> +				break;
> + 			}
   ^

Minor nit: I found a leading white space there.

Otherwise, it looks good:

Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>

	--yliu