Re: [PATCH net v3] net/mlx5e: do not HW-GRO coalesce small frames

[Tariq Toukan <tariqt@nvidia.com>]

On 06/08/2026 9:56, Glenn Judd wrote:
> When hardware GRO (SHAMPO) coalesces a small IPv4/TCP segment that was
> padded up to the 60-byte minimum Ethernet frame, the trailing padding is
> folded into the merged payload causing padding to be delivered
> to the user as payload.
> 
> Detecting and reproducing the issue: the selftest
> tools/testing/selftests/drivers/net/gro.py subtest
> hw_ipv4_data_lrg_1byte sends {100, 1} expecting to receive {101}.
> In current code, it receives {106} (100 + 1 payload + 5 pad) instead.
> 
> This patch avoids giving the user padding as payload by simply not
> coalescing small packets (which fails the subtest; the same approach
> and behavior as sw gro). This gains code simplicity at the cost of
> more computation (passing an extra skb up the stack) for small packets
> that could be coalesced.
> 
> The threshold is chosen as ETH_ZLEN + 2 * VLAN_HLEN. This is the largest
> frame that may still contain minimum-frame padding (+ 2 VLAN tags), so
> anything larger is safe to consider for coalesce. (We do not include
> ETH_FCS_LEN in that threshold computation as netdev_fix_features()
> drops NETIF_F_GRO_HW whenever NETIF_F_RXFCS is set, so retained FCS
> can't reach this path.)
> 
> Fixes: 92552d3abd32 ("net/mlx5e: HW_GRO cqe handler implementation")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-5
> Assisted-by: Meta:internal-AI-tooling
> Signed-off-by: Glenn Judd <gmj@meta.com>
> ---
> Notes:
>      v3: Per reviewer request, redesigned to simply not coalesce
>          small packets. Dropping the v2 padding detection/removal.
>      v2: https://lore.kernel.org/netdev/20260803214412.1714151-1-gmj@meta.com/
>      v1: https://lore.kernel.org/netdev/20260731185442.2778723-1-gmj@meta.com/
> 
>   drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> index 04af54b704d8..68b4dc49ccea 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> @@ -2263,6 +2263,11 @@ static void mlx5e_handle_rx_cqe_mpwrq_shampo(struct mlx5e_rq *rq, struct mlx5_cq
>   
>   	data_offset = wqe_offset & (page_size - 1);
>   	page_idx = wqe_offset >> rq->mpwqe.page_shift;
> +	if (unlikely(cqe_bcnt <= ETH_ZLEN + 2 * VLAN_HLEN)) {
> +		match = false;
> +		flush = true;
> +	}
> +
>   	if (*skb &&
>   	    !(match && mlx5e_hw_gro_skb_has_enough_space(*skb, data_bcnt,
>   							 page_size))) {
> 

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

Thanks for your patch.