netdev - Re: "TCP: eth0: Driver has suspect GRO implementation, TCP performance may be compromised." message with "ethtool -K eth0 gro off"

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Date:   Wed, 22 Mar 2017 06:47:42 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     Markus Trippelsdorf <markus@...ppelsdorf.de>,
        netdev@...r.kernel.org
Subject: Re: "TCP: eth0: Driver has suspect GRO implementation, TCP
 performance may be compromised." message with "ethtool -K eth0 gro off"

On Mon, 2017-03-20 at 16:27 -0300, Marcelo Ricardo Leitner wrote:
>  This warning is a hint, and can not assume senders are not dumb.
> 
> Agreed. But we can make it consider such cases. What about the following
> patch? (untested)
> 
> I think we can directly account for the size of the timestamps in there,
> as that won't make a difference to congestion control in case it's
> wrong, and also validate against MTU if we have it. I didn't subtract
> the headers from MTU on purpose, as dealing with ipv4/ipv6 there is
> not worth for the same reason.
> 
> This should silent this false-positive.


Note that the problem could have its origin on a middle box,
not on the host terminating the TCP flow.

So we can try hard, but we can't eliminate false positives.

Maybe replace the 12 by MAX_TCP_OPTION_SPACE ?