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:   Thu, 02 Feb 2017 05:59:24 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:     netdev@...r.kernel.org,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: "TCP: eth0: Driver has suspect GRO implementation, TCP
 performance may be compromised." message with "ethtool -K eth0 gro off"

On Thu, 2017-02-02 at 05:31 -0800, Eric Dumazet wrote:

> Anyway, I suspect the test is simply buggy ;)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 41dcbd568cbe2403f2a9e659669afe462a42e228..5394a39fcce964a7fe7075b1531a8a1e05550a54 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -164,7 +164,7 @@ static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb)
>  	if (len >= icsk->icsk_ack.rcv_mss) {
>  		icsk->icsk_ack.rcv_mss = min_t(unsigned int, len,
>  					       tcp_sk(sk)->advmss);
> -		if (unlikely(icsk->icsk_ack.rcv_mss != len))
> +		if (unlikely(icsk->icsk_ack.rcv_mss != len && skb_is_gso(skb)))
>  			tcp_gro_dev_warn(sk, skb);
>  	} else {
>  		/* Otherwise, we make more careful check taking into account,

This wont really help.

Our tcp_sk(sk)->advmss can be lower than the MSS used by the remote
peer.

ip ro add .... advmss 512

So the test is not universal.