[PATCH] net/mlx5: update stored Rx queue MTU when port MTU changes

[PATCH] net/mlx5: update stored Rx queue MTU when port MTU changes

[Shani Peretz]

Offending commit added rxq_ctrl->mtu to track MTU at queue
allocation time for validating shared RX queue compatibility.

When MTU changes via rte_eth_dev_set_mtu(), only priv->mtu was
updated, leaving rxq_ctrl->mtu with the old value. This caused
shared RX queue matching to fail with "mtu mismatch" errors when
ports reconfigured (e.g., enabling buffer_split offload), even
when all ports had identical MTU settings.

Update rxq_ctrl->mtu for all queues when port MTU changes to
keep metadata synchronized with actual port MTU.

Fixes: 4414eb800708 ("net/mlx5: store MTU at Rx queue allocation time")

Signed-off-by: Shani Peretz <shperetz@nvidia.com>
Acked-by: Dariusz Sosnowski <dsosnowski@nvidia.com>
---
 drivers/net/mlx5/mlx5_ethdev.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/drivers/net/mlx5/mlx5_ethdev.c b/drivers/net/mlx5/mlx5_ethdev.c
index 7747b0c869..7bec64c847 100644
--- a/drivers/net/mlx5/mlx5_ethdev.c
+++ b/drivers/net/mlx5/mlx5_ethdev.c
@@ -676,6 +676,7 @@ mlx5_dev_set_mtu(struct rte_eth_dev *dev, uint16_t mtu)
 {
 	struct mlx5_priv *priv = dev->data->dev_private;
 	uint16_t kern_mtu = 0;
+	unsigned int i;
 	int ret;
 
 	ret = mlx5_get_mtu(dev, &kern_mtu);
@@ -684,6 +685,17 @@ mlx5_dev_set_mtu(struct rte_eth_dev *dev, uint16_t mtu)
 
 	if (kern_mtu == mtu) {
 		priv->mtu = mtu;
+		/*
+		 * Update the MTU stored in RX queue control structures.
+		 * This is necessary for shared RX queues to correctly match
+		 * the MTU when ports try to join the shared group.
+		 */
+		for (i = 0; i < priv->rxqs_n; i++) {
+			struct mlx5_rxq_priv *rxq = mlx5_rxq_get(dev, i);
+
+			if (rxq != NULL && rxq->ctrl != NULL)
+				rxq->ctrl->mtu = mtu;
+		}
 		DRV_LOG(DEBUG, "port %u adapter MTU was already set to %u",
 			dev->data->port_id, mtu);
 		return 0;
@@ -698,6 +710,17 @@ mlx5_dev_set_mtu(struct rte_eth_dev *dev, uint16_t mtu)
 		return ret;
 	if (kern_mtu == mtu) {
 		priv->mtu = mtu;
+		/*
+		 * Update the MTU stored in RX queue control structures.
+		 * This is necessary for shared RX queues to correctly match
+		 * the MTU when ports try to join the shared group.
+		 */
+		for (i = 0; i < priv->rxqs_n; i++) {
+			struct mlx5_rxq_priv *rxq = mlx5_rxq_get(dev, i);
+
+			if (rxq != NULL && rxq->ctrl != NULL)
+				rxq->ctrl->mtu = mtu;
+		}
 		DRV_LOG(DEBUG, "port %u adapter MTU set to %u",
 			dev->data->port_id, mtu);
 		return 0;
-- 
2.43.0

Re: [PATCH] net/mlx5: update stored Rx queue MTU when port MTU changes

[Adrian Schollmeyer]

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Hi,

Am Montag, dem 17.11.2025 um 09:39 +0200 schrieb Shani Peretz:
> Offending commit added rxq_ctrl->mtu to track MTU at queue
> allocation time for validating shared RX queue compatibility.
> 
> When MTU changes via rte_eth_dev_set_mtu(), only priv->mtu was
> updated, leaving rxq_ctrl->mtu with the old value.

If you read the commit message of the offending patch, this was in fact
intentional. The MTU at allocation time is relevant when joining a Rx
share group, as it is (to the best of my knowledge) the only MTU
setting that actually has an influence on the size of the packet
buffers. Therefore, it is the value which is relevant for sharing
packet buffers.

I've tested your patch again with the prototype I used to discover the
original bug and unfortunately it reappears.

Best regards,
Adrian

-- 
Adrian Schollmeyer

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