netdev - Re: Question related to GSO6 checksum magic

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Date:   Wed, 12 Feb 2020 21:13:43 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Question related to GSO6 checksum magic

On 11.02.2020 22:01, Alexander Duyck wrote:
> On Tue, 2020-02-11 at 20:48 +0100, Heiner Kallweit wrote:
>> Few network drivers like Intel e1000e or r8169 have the following in the
>> GSO6 tx path:
>>
>> ipv6_hdr(skb)->payload_len = 0;
>> tcp_hdr(skb)->check = ~csum_ipv6_magic(&ipv6_hdr(skb)->saddr,
>> 				       &ipv6_hdr(skb)->daddr,
>> 				       0, IPPROTO_TCP, 0);
>> (partially also w/o the payload_len assignment)
>>
>> This sounds like we should factor it out to a helper.
>> The code however leaves few questions to me, but I'm not familiar enough
>> with the net core low-level details to answer them:
>>
>> - This code is used in a number of drivers, so is it something that
>>   should be moved to the core? If yes, where would it belong to?
>>
>> - Is clearing payload_len needed? IOW, can it be a problem if drivers
>>   miss this?
>>
>> Thanks, Heiner
> 
> The hardware is expecting the TCP header to contain the partial checksum
> minus the length. It does this because it reuses the value when it
> computes the checksum for the header of outgoing TCP frames and it will
> add the payload length as it is segmenting the frames.
> 
Thanks, that helped a lot!

> An alternative approach would be to pull the original checksum value out
> and simply do the checksum math to subtract the length from it. If I am
> not mistaken there are some drivers that take that approach for some of
> the headers.
> 
> - Alex
> 
Heiner