From: Wei Liu <redacted>
Date: Mon, 25 Mar 2013 18:32:06 +0000

On Mon, Mar 25, 2013 at 04:18:09PM +0000, David Miller wrote:

quoted

From: Wei Liu <redacted>
Date: Mon, 25 Mar 2013 11:08:18 +0000

quoted

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 <redacted>

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.