RE: [Intel-wired-lan] [PATCH v4 iwl-net] i40e: Prevent setting MTU if greater than MFS

["Pucha, HimasekharX Reddy" <himasekharx.reddy.pucha@intel.com>]

>-----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@osuosl.org> On Behalf Of Erwan Velu
> Sent: Wednesday, March 13, 2024 2:37 PM
> Cc: Velu, Erwan <e.velu@criteo.com>; linux-kernel@vger.kernel.org; Eric Dumazet <edumazet@google.com>; netdev@vger.kernel.org; Nguyen, Anthony L <anthony.l.nguyen@intel.com>; intel-wired-lan@lists.osuosl.org; Jakub Kicinski <kuba@kernel.org>; Paolo Abeni <pabeni@redhat.com>; David S. Miller <davem@davemloft.net>
> Subject: [Intel-wired-lan] [PATCH v4 iwl-net] i40e: Prevent setting MTU if greater than MFS
>
> 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.
> 
> Signed-off-by: Erwan Velu <e.velu@criteo.com>
> ---
>  drivers/net/ethernet/intel/i40e/i40e_main.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>


With patch when we set the MFS to 1700 (5888) in the NVM (as seen below) and then set the MTU on PF0 to 9000 and it set it to 9000 with no errors and no messages in dmesg.  

[root@localhost user]# ip link set mtu 9000 dev enp131s0f0np0
[root@localhost user]# ip link show dev enp131s0f0np0
9: enp131s0f0np0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether 00:00:00:00:01:00 brd ff:ff:ff:ff:ff:ff

dmesg when loading the driver:
[257.035823] 140e 0000:83:00.1: MFS for port 1 (5888) has been set below the default (9728)