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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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?


Wei.

> 
> thereby making it adhere to your limits just fine.

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