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

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Date:	Mon, 25 Mar 2013 20:49:09 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Miller <davem@...emloft.net>
CC:	<eric.dumazet@...il.com>, <wei.liu2@...rix.com>,
	<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

On Mon, 2013-03-25 at 12:59 -0400, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Mon, 25 Mar 2013 09:54:32 -0700
> 
> > On Mon, 2013-03-25 at 12:18 -0400, David Miller wrote:
> > 
> >> 
> >> 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.
> > 
> > For locally generated TCP traffic this is the case.
> 
> Right, and also any other piece of code that is not interpreting the
> gso_max_size value the same way (as "(x - 1) - sizeof_headers") would
> need to be fixed.
[...]

tcp_tso_should_defer() also ignores headers, though I don't think the
check is particularly critical in that case.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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