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

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Message-ID: <1364230472.29473.21.camel@edumazet-glaptop>
Date:	Mon, 25 Mar 2013 09:54:32 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	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 12:18 -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.

However, GRO can build packets up to 65535 bytes, not including the
Ethernet header.

For such packets, it seems xen-netfront needs a segmentation.

And we might have other providers as well (UFO for example ?), but I
have not checked.


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