netdev - Re: [PATCH 2/6] xen-netfront: reduce gso_max_size to account for ethernet header

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Message-Id: <20130325.121809.2023305728304341952.davem@davemloft.net>
Date:	Mon, 25 Mar 2013 12:18:09 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	wei.liu2@...rix.com
Cc:	xen-devel@...ts.xen.org, netdev@...r.kernel.org,
	ian.campbell@...rix.com, annie.li@...cle.com,
	konrad.wilk@...cle.com, david.vrabel@...rix.com
Subject: Re: [PATCH 2/6] xen-netfront: reduce gso_max_size to account for
 ethernet header

From: Wei Liu <wei.liu2@...rix.com>
Date: Mon, 25 Mar 2013 11:08:18 +0000

> The maximum packet including ethernet header that can be handled by netfront /
> netback wire format is 65535. Reduce gso_max_size accordingly.
> 
> Drop skb and print warning when skb->len > 65535. This can 1) save the effort
> to send malformed packet to netback, 2) help spotting misconfiguration of
> netfront in the future.
> 
> Signed-off-by: Wei Liu <wei.liu2@...rix.com>

This is effectively the default already, you don't need to change this
value explicitly.

->gso_max_size is set by default to 65536 and then TCP performs this
calculation:

                xmit_size_goal = ((sk->sk_gso_max_size - 1) -                          
                                  inet_csk(sk)->icsk_af_ops->net_header_len -          
                                  inet_csk(sk)->icsk_ext_hdr_len -                     
                                  tp->tcp_header_len);                                 

thereby making it adhere to your limits just fine.
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