Re: [PATCH net v2] net/mlx5: Correctly set gso_size when LRO is used

[Tariq Toukan <ttoukan.linux@gmail.com>]

On 15/07/2025 23:20, Christoph Paasch via B4 Relay wrote:
> From: Christoph Paasch <cpaasch@openai.com>
> 
> gso_size is expected by the networking stack to be the size of the
> payload (thus, not including ethernet/IP/TCP-headers). However, cqe_bcnt
> is the full sized frame (including the headers). Dividing cqe_bcnt by
> lro_num_seg will then give incorrect results.
> 
> For example, running a bpftrace higher up in the TCP-stack
> (tcp_event_data_recv), we commonly have gso_size set to 1450 or 1451 even
> though in reality the payload was only 1448 bytes.
> 
> This can have unintended consequences:
> - In tcp_measure_rcv_mss() len will be for example 1450, but. rcv_mss
> will be 1448 (because tp->advmss is 1448). Thus, we will always
> recompute scaling_ratio each time an LRO-packet is received.
> - In tcp_gro_receive(), it will interfere with the decision whether or
> not to flush and thus potentially result in less gro'ed packets.
> 
> So, we need to discount the protocol headers from cqe_bcnt so we can
> actually divide the payload by lro_num_seg to get the real gso_size.
> 
> v2:
>   - Use "(unsigned char *)tcp + tcp->doff * 4 - skb->data)" to compute header-len
>     (Tariq Toukan <tariqt@nvidia.com>)
>   - Improve commit-message (Gal Pressman <gal@nvidia.com>)
> 
> Fixes: e586b3b0baee ("net/mlx5: Ethernet Datapath files")
> Signed-off-by: Christoph Paasch <cpaasch@openai.com>
> ---
>   drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> index 84b1ab8233b8107f0d954ea29c33601b279a2c27..7462514c7f3d1606339ede13a6207c1629ab65a3 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> @@ -1154,8 +1154,9 @@ static void mlx5e_lro_update_tcp_hdr(struct mlx5_cqe64 *cqe, struct tcphdr *tcp)
>   	}
>   }
>   
> -static void mlx5e_lro_update_hdr(struct sk_buff *skb, struct mlx5_cqe64 *cqe,
> -				 u32 cqe_bcnt)
> +static unsigned int mlx5e_lro_update_hdr(struct sk_buff *skb,
> +					 struct mlx5_cqe64 *cqe,
> +					 u32 cqe_bcnt)
>   {
>   	struct ethhdr	*eth = (struct ethhdr *)(skb->data);
>   	struct tcphdr	*tcp;
> @@ -1205,6 +1206,8 @@ static void mlx5e_lro_update_hdr(struct sk_buff *skb, struct mlx5_cqe64 *cqe,
>   		tcp->check = tcp_v6_check(payload_len, &ipv6->saddr,
>   					  &ipv6->daddr, check);
>   	}
> +
> +	return (unsigned int)((unsigned char *)tcp + tcp->doff * 4 - skb->data);
>   }
>   
>   static void *mlx5e_shampo_get_packet_hd(struct mlx5e_rq *rq, u16 header_index)
> @@ -1561,8 +1564,9 @@ static inline void mlx5e_build_rx_skb(struct mlx5_cqe64 *cqe,
>   		mlx5e_macsec_offload_handle_rx_skb(netdev, skb, cqe);
>   
>   	if (lro_num_seg > 1) {
> -		mlx5e_lro_update_hdr(skb, cqe, cqe_bcnt);
> -		skb_shinfo(skb)->gso_size = DIV_ROUND_UP(cqe_bcnt, lro_num_seg);
> +		unsigned int hdrlen = mlx5e_lro_update_hdr(skb, cqe, cqe_bcnt);
> +
> +		skb_shinfo(skb)->gso_size = DIV_ROUND_UP(cqe_bcnt - hdrlen, lro_num_seg);
>   		/* Subtract one since we already counted this as one
>   		 * "regular" packet in mlx5e_complete_rx_cqe()
>   		 */
> 
> ---
> base-commit: 0e9418961f897be59b1fab6e31ae1b09a0bae902
> change-id: 20250714-cpaasch-pf-925-investigate-incorrect-gso_size-on-cx-7-nic-852b450a8dad
> 
> Best regards,

Reviewed-by: Tariq Toukan <tariqt@nvidia.com>

Thanks.