[PATCH] net/mlx5: avoid setting kernel MTU if not needed

[PATCH] net/mlx5: avoid setting kernel MTU if not needed

[Maxime Coquelin]

This patch checks whether the Kernel MTU has the same value
as the requested one at port configuration time, and skip
setting it if it is the same.

Doing this, we can avoid the application to require
NET_ADMIN capability, as in v23.11.

Fixes: 10859ecf09c4 ("net/mlx5: fix MTU configuration")
Cc: stable@dpdk.org

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---

Hi Dariuz,

I set priv->mtu as it is done after the mlx5_set_mtu() call,
but I'm not sure it is necessary, as is the existing call to
mlx5_get_mtu() because it seems done in mlx5_dev_spawn().

---

 drivers/net/mlx5/mlx5_ethdev.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/mlx5/mlx5_ethdev.c b/drivers/net/mlx5/mlx5_ethdev.c
index 7708a0b808..f2ae75a8e1 100644
--- a/drivers/net/mlx5/mlx5_ethdev.c
+++ b/drivers/net/mlx5/mlx5_ethdev.c
@@ -647,6 +647,14 @@ mlx5_dev_set_mtu(struct rte_eth_dev *dev, uint16_t mtu)
 	ret = mlx5_get_mtu(dev, &kern_mtu);
 	if (ret)
 		return ret;
+
+	if (kern_mtu == mtu) {
+		priv->mtu = mtu;
+		DRV_LOG(DEBUG, "port %u adapter MTU was already set to %u",
+			dev->data->port_id, mtu);
+		return 0;
+	}
+
 	/* Set kernel interface MTU first. */
 	ret = mlx5_set_mtu(dev, mtu);
 	if (ret)
--
2.49.0

Re: [PATCH] net/mlx5: avoid setting kernel MTU if not needed

[David Marchand]

Hello,

On Wed, May 28, 2025 at 11:36 AM Maxime Coquelin
<maxime.coquelin@redhat.com> wrote:
>
> This patch checks whether the Kernel MTU has the same value
> as the requested one at port configuration time, and skip
> setting it if it is the same.
>
> Doing this, we can avoid the application to require
> NET_ADMIN capability, as in v23.11.
>
> Fixes: 10859ecf09c4 ("net/mlx5: fix MTU configuration")
> Cc: stable@dpdk.org
>
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>
> Hi Dariuz,
>
> I set priv->mtu as it is done after the mlx5_set_mtu() call,
> but I'm not sure it is necessary, as is the existing call to
> mlx5_get_mtu() because it seems done in mlx5_dev_spawn().

It seems there were some back and forth on this priv->mtu topic
between Nelio and other devs in the past.

Atm, I don't see the need for keeping such a cached mtu value in priv.
There is only one user of the value, and it is for configuration
operation that can do a query to the kernel.


-- 
David Marchand