netdev - Re: [Intel-wired-lan] [PATCH v3] i40e: Prevent setting MTU if greater than MFS

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Date: Tue, 12 Mar 2024 10:48:19 +0100
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: Erwan Velu <erwanaliasr1@...il.com>
Cc: Erwan Velu <e.velu@...teo.com>, linux-kernel@...r.kernel.org,
 Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
 Tony Nguyen <anthony.l.nguyen@...el.com>, intel-wired-lan@...ts.osuosl.org,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 "David S. Miller" <davem@...emloft.net>
Subject: Re: [Intel-wired-lan] [PATCH v3] i40e: Prevent setting MTU if greater
 than MFS

Dear Erwan,


Thank you very umch for your patch.

Am 12.03.24 um 10:42 schrieb Erwan Velu:
> Commit 6871a7de705b6f6a4046f0d19da9bcd689c3bc8e from iPXE project is
> setting the MFS to 0x600 = 1536.

Please add a link, as most people do not have the iPXE archive checked 
out. Maybe also add the commit message summary.

> At boot time the i40e driver complains about it with
> the following message but continues.
> 
> 	MFS for port 1 has been set below the default: 600

Hmm, but 1536 > 600. So the log message is incorrect?

> If the MTU size is increased, the driver accept it but large packets will not

accept*s*

> be processed by the firmware generating tx_errors. The issue is pretty
> silent for users. i.e doing TCP in such context will generates lots of
> retransmissions until the proper window size (below 1500) will be used.
> 
> To fix this case, it would have been ideal to increase the MFS,
> via i40e_aqc_opc_set_mac_config, incoming patch will take care of it.
> 
> At least, this commit prevents setting up an MTU greater than the current MFS.
> It will avoid being in the position of having an MTU set to 9000 on the
> netdev with a firmware refusing packets larger than 1536.

Maybe add the new log message.

One last formal nit: Please use a line length limit of 75 characters per 
line.

> Signed-off-by: Erwan Velu <e.velu@...teo.com>
> ---
>   drivers/net/ethernet/intel/i40e/i40e_main.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
> index 89a3401d20ab..225b2fd0449e 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
> @@ -2950,7 +2950,7 @@ static int i40e_change_mtu(struct net_device *netdev, int new_mtu)
>   	struct i40e_netdev_priv *np = netdev_priv(netdev);
>   	struct i40e_vsi *vsi = np->vsi;
>   	struct i40e_pf *pf = vsi->back;
> -	int frame_size;
> +	int frame_size, mfs, max_mtu;
>   
>   	frame_size = i40e_max_vsi_frame_size(vsi, vsi->xdp_prog);
>   	if (new_mtu > frame_size - I40E_PACKET_HDR_PAD) {
> @@ -2959,6 +2959,14 @@ static int i40e_change_mtu(struct net_device *netdev, int new_mtu)
>   		return -EINVAL;
>   	}
>   
> +	mfs = pf->hw.phy.link_info.max_frame_size;
> +	max_mtu = mfs - I40E_PACKET_HDR_PAD;
> +	if (new_mtu > max_mtu) {
> +		netdev_err(netdev, "Error changing mtu to %d, Max is %d. MFS is too small.\n",
> +			   new_mtu, max_mtu);

The other log messages capitalize MTU.

> +		return -EINVAL;
> +	}
> +
>   	netdev_dbg(netdev, "changing MTU from %d to %d\n",
>   		   netdev->mtu, new_mtu);
>   	netdev->mtu = new_mtu;


The rest looks reasonable.


Kind regards,

Paul