netdev - Re: GRO disabled with IPv4 options

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Date:   Thu, 16 Nov 2017 14:34:51 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cristian Klein <cklein@...umu.se>
Cc:     netdev@...r.kernel.org, herbert@...dor.apana.org.au,
        Ahmed Ali-Eldin <ahmeda@...umu.se>
Subject: Re: GRO disabled with IPv4 options

On Thu, 2017-11-16 at 16:12 +0100, Cristian Klein wrote:
> [CC-ing Herbert Xu, who is to 'git blame' for the code in question. :)]
> 
> Dear all,
> 
> We are working on a research prototype which, among others, adds a new 
> IPv4 option. During testing we noticed that the packets captured by 
> tcpdump shrank from 10s of KBs to the MTU, which indicates that Generic 
> Receive Offload (GRO) got disabled.
> 
> Upon further investigation, we found the following line in 
> `inet_gro_receive`:
> 
> 	if (*(u8 *)iph != 0x45)
> 		goto out_unlock;
> 
> in plain English, don't do GRO if any IPv4 options are present.
> 
> Does somebody know the rationale for this? Is it because IPv4 options 
> are rarely used, hence implementing GRO in that case does not pay off or 
> are there some caveats? Specifically would it make sense to do GRO when 
> the IPv4 options are byte-identical in consecutive packets?

I guess nobody really uses IPV4 options, and you are the first caring
enough ;)