netdev - Re: [PATCH v3 bpf-next 3/8] veth: Avoid drops by oversized packets when XDP is enabled

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Message-ID: <709ccfa0-fcd8-bd48-1e85-2960bbe232dd@lab.ntt.co.jp>
Date:   Tue, 24 Jul 2018 18:39:09 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH v3 bpf-next 3/8] veth: Avoid drops by oversized packets
 when XDP is enabled

On 2018/07/24 10:56, Toshiaki Makita wrote:
> On 2018/07/24 9:27, Jakub Kicinski wrote:
>> On Mon, 23 Jul 2018 00:13:03 +0900, Toshiaki Makita wrote:
>>> From: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>>>
>>> All oversized packets including GSO packets are dropped if XDP is
>>> enabled on receiver side, so don't send such packets from peer.
>>>
>>> Drop TSO and SCTP fragmentation features so that veth devices themselves
>>> segment packets with XDP enabled. Also cap MTU accordingly.
>>>
>>> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>>
>> Is there any precedence for fixing up features and MTU like this?  Most
>> drivers just refuse to install the program if settings are incompatible.
> 
> I don't know any precedence. I can refuse the program on installing it
> when features and MTU are not appropriate. Is it preferred?
> Note that with current implementation wanted_features are not touched so
> features will be restored when the XDP program is removed. MTU will not
> be restored though, as I do not remember the original MTU.

I just recalled that virtio_net used to refused XDP when guest offload
features are incompatible but now it dynamically fixup them on
installing an XDP program.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3f93522ffab2d46a36b57adf324a54e674fc9536

-- 
Toshiaki Makita