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

[Dariusz Sosnowski <dsosnowski@nvidia.com>]

Hi,

On Wed, May 28, 2025 at 02:12:37PM +0200, David Marchand wrote:
> 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().

Correct, this additional update of priv->mtu is not needed here.
It can be removed.

> 
> 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.

I agree. It's not really needed, especially since the same value is also
stored in dev->data->mtu, so the kernel query can easily be replaced
with reading dev->data->mtu.

I'll prepare a patch later which does that.
Thanks for bringing that up.

For posperity - this cached value is used only during
rte_eth_rx_queue_setup() while validating shared RX queue
configuration.

Best regards,
Dariusz Sosnowski