Re: [dpdk-dev] [PATCH] net/fm10k: fix Rx descriptor read timing

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Jul 04, 2016 at 03:51:08PM +0800, Wang Xiao W wrote:
> We find that when traffic is light, a few amount of packets will be
> wrongly parsed (e.g. packet type), however this issue will not happen
> when traffic is heavy.
> 
> The root cause is some fields in fm10k_rx_desc are read at wrong timing.
> When the input speed is slower than software's capability, fm10k scalar
> Rx function accesses descriptors closely to HW, so there's potential
> sequence: some fields like pkt_info in fm10k_rx_desc are read before HW
> writeback but some fields like DD bit are read after HW writeback, this
> will lead to the later packet parsing function using incorrect value.
> 
> This patch fixes this issue by reading and parsing Rx descriptor after
> DD bit is set.
> 
> Fixes: 4b61d3bfa941 ("fm10k: add receive and tranmit")
> Fixes: c82dd0a7bfa5 ("fm10k: add scatter receive")
> 
> Signed-off-by: Wang Xiao W <xiao.w.wang@intel.com>

Fix looks correct to me. The key part I think is that the read of the
descriptor at line "desc = q->hw_ring[next_dd];" is not atomic as it's
a read of 16B, allowing part of the descriptor to be read either side of
a HW write-back.

Acked-by: Bruce Richardson <bruce.richardson@intel.com>

> ---
>  drivers/net/fm10k/fm10k_rxtx.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/fm10k/fm10k_rxtx.c b/drivers/net/fm10k/fm10k_rxtx.c
> index dd92a91..5b2d04b 100644
> --- a/drivers/net/fm10k/fm10k_rxtx.c
> +++ b/drivers/net/fm10k/fm10k_rxtx.c
> @@ -114,10 +114,10 @@ fm10k_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts,
>  
>  	nb_pkts = RTE_MIN(nb_pkts, q->alloc_thresh);
>  	for (count = 0; count < nb_pkts; ++count) {
> +		if (!(q->hw_ring[next_dd].d.staterr & FM10K_RXD_STATUS_DD))
> +			break;
>  		mbuf = q->sw_ring[next_dd];
>  		desc = q->hw_ring[next_dd];
> -		if (!(desc.d.staterr & FM10K_RXD_STATUS_DD))
> -			break;
>  #ifdef RTE_LIBRTE_FM10K_DEBUG_RX
>  		dump_rxd(&desc);
>  #endif
> @@ -228,10 +228,10 @@ fm10k_recv_scattered_pkts(void *rx_queue, struct rte_mbuf **rx_pkts,
>  
>  	nb_seg = RTE_MIN(nb_pkts, q->alloc_thresh);
>  	for (count = 0; count < nb_seg; count++) {
> +		if (!(q->hw_ring[next_dd].d.staterr & FM10K_RXD_STATUS_DD))
> +			break;
>  		mbuf = q->sw_ring[next_dd];
>  		desc = q->hw_ring[next_dd];
> -		if (!(desc.d.staterr & FM10K_RXD_STATUS_DD))
> -			break;
>  #ifdef RTE_LIBRTE_FM10K_DEBUG_RX
>  		dump_rxd(&desc);
>  #endif
> -- 
> 1.9.3
>