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From: "Pucha, HimasekharX Reddy" <himasekharx.reddy.pucha@intel.com>
To: Erwan Velu <erwanaliasr1@gmail.com>
CC: "Velu, Erwan" <e.velu@criteo.com>, "linux-kernel@vger.kernel.org"
	<linux-kernel@vger.kernel.org>, Eric Dumazet <edumazet@google.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>, "Nguyen, Anthony L"
	<anthony.l.nguyen@intel.com>, "intel-wired-lan@lists.osuosl.org"
	<intel-wired-lan@lists.osuosl.org>, Jakub Kicinski <kuba@kernel.org>, "Paolo
 Abeni" <pabeni@redhat.com>, "David S. Miller" <davem@davemloft.net>
Subject: RE: [Intel-wired-lan] [PATCH v4 iwl-net] i40e: Prevent setting MTU if
 greater than MFS
Thread-Topic: [Intel-wired-lan] [PATCH v4 iwl-net] i40e: Prevent setting MTU
 if greater than MFS
Thread-Index: AQHadSYFc04RgPtII0iMq+CRVHx6vLFv3Slg
Date: Fri, 19 Apr 2024 14:26:04 +0000
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References: <20240313090719.33627-2-e.velu@criteo.com>
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>-----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@osuosl.org> On Behalf Of E=
rwan Velu
> Sent: Wednesday, March 13, 2024 2:37 PM
> Cc: Velu, Erwan <e.velu@criteo.com>; linux-kernel@vger.kernel.org; Eric D=
umazet <edumazet@google.com>; netdev@vger.kernel.org; Nguyen, Anthony L <an=
thony.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 i=
f greater than MFS
>
> Commit 6871a7de705 ("[intelxl] Use admin queue to set port MAC address an=
d maximum frame size") from iPXE project set the MFS to 0x600 =3D 1536.
> See https://github.com/ipxe/ipxe/commit/6871a7de705
>
> At boot time the i40e driver complains about it with the following messag=
e 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 wil=
l not be processed by the firmware generating tx_errors. The issue is prett=
y silent for users. i.e doing TCP in such context will generates lots of re=
transmissions 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 n=
etdev 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.
>=20
> 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) an=
d then set the MTU on PF0 to 9000 and it set it to 9000 with no errors and =
no messages in dmesg. =20

[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 th=
e default (9728)