Re: [dpdk-dev] [PATCH] net/mlx5: add Rx checksum offload flag return bad

[Slava Ovsiienko <viacheslavo@nvidia.com>]

Hi, Jiawei

> -----Original Message-----
> From: Jiawei Zhu <17826875952@163.com>
> Sent: Monday, March 22, 2021 17:46
> To: dev@dpdk.org
> Cc: zhujiawei12@huawei.com; Matan Azrad <matan@nvidia.com>; Shahaf
> Shuler <shahafs@nvidia.com>; Slava Ovsiienko <viacheslavo@nvidia.com>
> Subject: [PATCH] net/mlx5: add Rx checksum offload flag return bad
> 
> From: Jiawei Zhu <zhujiawei12@huawei.com>
> 
> When open the rx checksum offload and receive the wrong checksum, add
> the ol_flags return bad. And it's not best to use multiplication and division
> here.

I'm sorry, there should be no any multiplications and divisions - the arguments
are constants (can be determined at compilation time) and ara power of 2,
hence compiler engages simple shifts. For example (I did rxq_cq_to_ol_flags not inline to
get the listing for x86-64):

  29              	rxq_cq_to_ol_flags:
  21              	/*
  22              	 * An architecture-optimized byte swap for a 16-bit value.
  23              	 *
  24              	 * Do not use this function directly. The preferred function is rte_bswap16().
  25              	 */
  26              	static inline uint16_t rte_arch_bswap16(uint16_t _x)
  27              	{
  28              		uint16_t x = _x;
  29 0034 86D6     		asm volatile ("xchgb %b[x1],%h[x2]"
  30              			      : [x1] "=Q" (x)
  37              	
  38 0036 89D0     	 movl %edx,%eax
  39 0038 6681E200 	 andw $512,%dx
  39      02
  40 003d 66250004 	 andw $1024,%ax
  41 0041 0FB7D2   	 movzwl %dx,%edx
  42 0044 0FB7C0   	 movzwl %ax,%eax
  43 0047 48C1EA02 	 shrq $2,%rdx
  44 004b 48C1E802 	 shrq $2,%rax
  45 004f 09D0     	 orl %edx,%eax
  46 0051 C3       	 ret

As we can see - there is no any mul/div and no any branches, that
improves the performance.

> 
> Signed-off-by: Jiawei Zhu <zhujiawei12@huawei.com>
> ---
>  drivers/net/mlx5/mlx5_rxtx.c  | 17 ++++++++++-------
> drivers/net/mlx5/mlx5_utils.h |  6 ------
>  2 files changed, 10 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/mlx5/mlx5_rxtx.c b/drivers/net/mlx5/mlx5_rxtx.c
> index e3ce9fd..9233af8 100644
> --- a/drivers/net/mlx5/mlx5_rxtx.c
> +++ b/drivers/net/mlx5/mlx5_rxtx.c
> @@ -1325,13 +1325,16 @@ enum mlx5_txcmp_code {
>  	uint32_t ol_flags = 0;
>  	uint16_t flags = rte_be_to_cpu_16(cqe->hdr_type_etc);
> 
> -	ol_flags =
> -		TRANSPOSE(flags,
> -			  MLX5_CQE_RX_L3_HDR_VALID,
> -			  PKT_RX_IP_CKSUM_GOOD) |
> -		TRANSPOSE(flags,
> -			  MLX5_CQE_RX_L4_HDR_VALID,
> -			  PKT_RX_L4_CKSUM_GOOD);
> +	if (flags & MLX5_CQE_RX_L3_HDR_VALID)
> +		ol_flags |= PKT_RX_IP_CKSUM_GOOD;
> +	else
> +		ol_flags |= PKT_RX_IP_CKSUM_BAD;
> +

If MLX5_CQE_RX_L3_HDR_VALID is not set - it does not always mean the sum is bad.
If might happen if HW just did not recognize the packet format (for example, for
some tunnels)

With best regards,
Slava


> +	if (flags & MLX5_CQE_RX_L4_HDR_VALID)
> +		ol_flags |= PKT_RX_IP_CKSUM_GOOD;
> +	else
> +		ol_flags |= PKT_RX_IP_CKSUM_BAD;
> +
>  	return ol_flags;
>  }
> 
> diff --git a/drivers/net/mlx5/mlx5_utils.h b/drivers/net/mlx5/mlx5_utils.h
> index 7a62187..2f71a23 100644
> --- a/drivers/net/mlx5/mlx5_utils.h
> +++ b/drivers/net/mlx5/mlx5_utils.h
> @@ -44,12 +44,6 @@
>  #define NB_SEGS(m) ((m)->nb_segs)
>  #define PORT(m) ((m)->port)
> 
> -/* Transpose flags. Useful to convert IBV to DPDK flags. */ -#define
> TRANSPOSE(val, from, to) \
> -	(((from) >= (to)) ? \
> -	 (((val) & (from)) / ((from) / (to))) : \
> -	 (((val) & (from)) * ((to) / (from))))
> -
>  /*
>   * For the case which data is linked with sequence increased index, the
>   * array table will be more efficiect than hash table once need to serarch
> --
> 1.8.3.1
>