Re: [PATCH] net/mlx5: fix number of segment calculation

[Adrien Mazarguil <adrien.mazarguil@6wind.com>]

Hi Yongseok,

On Thu, Nov 09, 2017 at 02:30:30PM -0800, Yongseok Koh wrote:
> 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@dpdk.org
> > Cc: nelio.laranjeiro@6wind.com
> > 
> > Signed-off-by: Ori Kam <orika@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?

RTE_PKTMBUF_HEADROOM is actually only reserved on the first segment,
i.e. once per mbuf chain, it should be fine.

> > 		/*
> > 		 * 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:

The name is perhaps confusing, sges_n is documented as a log 2 value, 1 << 3
means 8 segments at most. Assuming default mbuf size, this allows up to
17280 bytes per packet excluding headroom.

You're right exceeding 3 will remove the extra bits and since sizeof() can't
be used, that's precisely the reason for the subsequent check, which makes
sure the stored value is enough for a max_rx_pkt_len-sized packet after
converting it back to a number of bytes.

> 		
>  		/* 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?

I think it has to be kept as a safety check even if the max number of SGEs
is increased, at least as long as it's stored as a bit-field value.

-- 
Adrien Mazarguil
6WIND