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

[Glenn Judd <gmj@meta.com>]

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))) {

-- 
2.53.0-Meta