netdev - Re: [Patch net] mlx5: fixup checksum for ethernet padding

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Message-ID: <0e6a8a08-b181-b17a-1330-0159808e9a42@gmail.com>
Date:   Tue, 27 Nov 2018 17:25:51 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] mlx5: fixup checksum for ethernet padding

On 11/27/2018 04:07 PM, Cong Wang wrote:
> On Tue, Nov 27, 2018 at 3:48 PM Eric Dumazet <eric.dumazet@...il.com> wrote:

>>
>> But the padding might be added on normal packets (say 1000 bytes + 3 bytes of padding) ?
> 
> I never see other padding cases than ETH_ZLEN. Does ethernet standard
> require padding for other cases? I only read the section "3.2.8 Pad field" in
> the standard.
> 

Padding can be done by senders, eg using AF_PACKET,
added at the tail of a regular IP/IP6 frame of 1000 or 6000 bytes.

Note that mlx5 will presumably set CHECKSUM_UNNECESSARY for standard protocols,
so if you want to reproduce the issue, you might need to find an IP frame that
mlx5 is not able to checksum validate.

Maybe a non-first fragment will do the trick.