[dpdk-dev] [PATCH] net/mlx5: fix number of segment calculation

[Yongseok Koh]

On Thu, Nov 09, 2017 at 06:04:32PM +0200, Ori Kam wrote:
> The CRC size should be taken into consideration when computing
> the number of mbuf segments for packet on the receive path.
> Large packets can be dropped due to extra CRC length.
> 
> Fixes: a1366b1a2be3 ("net/mlx5: add reference counter on DPDK Rx queues")
> Cc: stable at dpdk.org
> Cc: nelio.laranjeiro at 6wind.com
> 
> Signed-off-by: Ori Kam <orika at mellanox.com>
> ---
>  drivers/net/mlx5/mlx5_rxq.c |    7 +++++--
>  1 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/mlx5/mlx5_rxq.c b/drivers/net/mlx5/mlx5_rxq.c
> index 6b29aae..701925b 100644
> --- a/drivers/net/mlx5/mlx5_rxq.c
> +++ b/drivers/net/mlx5/mlx5_rxq.c
> @@ -887,6 +887,8 @@ struct mlx5_rxq_ctrl*
>  	const uint16_t desc_n =
>  		desc + priv->rx_vec_en * MLX5_VPMD_DESCS_PER_LOOP;
>  	unsigned int mb_len = rte_pktmbuf_data_room_size(mp);
> +	uint8_t crc_size =
> +			!!(dev->data->dev_conf.rxmode.hw_strip_crc == 0) << 2;

How about making it more explicit with ETHER_CRC_LEN? E.g.
	uint8_t crc_size = ETHER_CRC_LEN * 
			   (dev->data->dev_conf.rxmode.hw_strip_crc == 0);

>  
>  	tmpl = rte_calloc_socket("RXQ", 1,
>  				 sizeof(*tmpl) +
> @@ -900,12 +902,13 @@ struct mlx5_rxq_ctrl*
>  	/* Enable scattered packets support for this queue if necessary. */
>  	assert(mb_len >= RTE_PKTMBUF_HEADROOM);

You might want to make the same change for this assert?

>  	if (dev->data->dev_conf.rxmode.max_rx_pkt_len <=
> -	    (mb_len - RTE_PKTMBUF_HEADROOM)) {
> +	    (mb_len - RTE_PKTMBUF_HEADROOM - crc_size)) {
>  		tmpl->rxq.sges_n = 0;
>  	} else if (dev->data->dev_conf.rxmode.enable_scatter) {
>  		unsigned int size =
>  			RTE_PKTMBUF_HEADROOM +
> -			dev->data->dev_conf.rxmode.max_rx_pkt_len;
> +			dev->data->dev_conf.rxmode.max_rx_pkt_len +
> +			crc_size;

I think there's another bugs we didn't know. If scatter is required,
RTE_PKTMBUF_HEADROOM is also reserved per every chained mbufs. So, it looks like
mb_len should be "rte_pktmbuf_data_room_size(mp) - RTE_PKTMBUF_HEADROOM" when it
is declared in the beginning. Make sense?

> 		/*
> 		 * Determine the number of SGEs needed for a full packet
> 		 * and round it to the next power of two.
> 		 */
> 		sges_n = log2above((size / mb_len) + !!(size % mb_len));
> 		tmpl->rxq.sges_n = sges_n;

rxq.sges_n is 2bits, which means the max value is 3. So, if sges_n is larger
than 3, it would just take the last 2bits and it will result in false error
below. As we can't use sizeof() for bit-fields, this should be changed like:
		
 		/* Check the maximum value of the bit-field. */
 		tmpl->rxq.sges_n = -1;
 		tmpl->rxq.sges_n = RTE_MIN(tmpl->rxq.sges_n, sges_n);

> 		/* Make sure rxq.sges_n did not overflow. */
> 		size = mb_len * (1 << tmpl->rxq.sges_n);
> 		size -= RTE_PKTMBUF_HEADROOM;
> 		if (size < dev->data->dev_conf.rxmode.max_rx_pkt_len) {
> 			ERROR("%p: too many SGEs (%u) needed to handle"
> 			      " requested maximum packet size %u",
> 			      (void *)dev,
> 			      1 << sges_n,
> 			      dev->data->dev_conf.rxmode.max_rx_pkt_len);
> 			goto error;
> 		}

This may be unnecessary if we make right changes?


Thanks,
Yongseok