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

[David Miller <davem@xxxxxxxxxxxxx>]

From: Wei Liu <wei.liu2@xxxxxxxxxx>
Date: Mon, 25 Mar 2013 18:32:06 +0000

> On Mon, Mar 25, 2013 at 04:18:09PM +0000, David Miller wrote:
>> From: Wei Liu <wei.liu2@xxxxxxxxxx>
>> 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@xxxxxxxxxx>
>> 
>> 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);                       
>>           
> 
> OK. But I see similar fix for a physical nic (commit b7e5887e0e414b), am
> I missing something here?
> 
> And the symptom is that if I don't reserve headroom I see skb->len =
> 65538. Can you shed some light on this?

See Eric's reply.  If a GRO frame is forwarded we don't make the GSO size
checks on the send size as we should.

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