Subject: Re: [PATCH v4 iwl-net] i40e: Prevent setting MTU if greater than MFS

From: Simon Horman <[email-masked]>
Date: Mon, 18 Mar 2024 17:45:03 +0000

On Wed, Mar 13, 2024 at 10:07:16AM +0100, Erwan Velu wrote:
> Commit 6871a7de705 ("[intelxl] Use admin queue to set port MAC address
> and maximum frame size") from iPXE project set the MFS to 0x600 = 1536.
> See https://github.com/ipxe/ipxe/commit/6871a7de705
> 
> 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
> 
> If the MTU size is increased, the driver accepts it but large packets will
> not 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, 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.
> 
> A typical trace looks like:
> [  377.548696] i40e 0000:5d:00.0 eno5: Error changing mtu to 9000, Max is 1500. MFS is too small.
> 

Hi Erwan, all,

As a fix, I think this patch warrants a fixes tag.
Perhaps this one is appropriate?

Fixes: 41c445ff0f48 ("i40e: main driver core")

> Signed-off-by: Erwan Velu <[email-masked]>
> ---
>  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 f86578857e8a..85ecf2f3de18 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
> @@ -2946,7 +2946,7 @@ 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) {

I am fine with this patch, so please take what follows as a suggestion
for improvement, possibly as a follow-up. Not as a hard requirement from
my side.

The part of this function between the two hunks of this patch is:

		netdev_err(netdev, "Error changing mtu to %d, Max is %d\n",
			   new_mtu, frame_size - I40E_PACKET_HDR_PAD);

My reading is that with this patch two different limits are
checked wrt maximum MTU size:

1. A VSI level limit, which relates to RX buffer size
2. A PHY level limit that relates to the MFS

That seems fine to me. But the log message for 1 (above) does
not seem particularly informative wrt which limit has been exceeded.

> @@ -2955,6 +2955,14 @@ 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);
> +		return -EINVAL;
> +	}
> +
>  	netdev_dbg(netdev, "changing MTU from %d to %d\n",
>  		   netdev->mtu, new_mtu);
>  	netdev->mtu = new_mtu;
> -- 
> 2.44.0
> 
>