Subject: Re: Regression: too many packets with incorrect checksum

From: Cong Wang <[email-masked]>
Date: Thu, 09 May 2013 16:36:18 +0800

On Wed, 2013-05-08 at 19:51 +0800, Cong Wang wrote:
> Hi, all
> 
> The latest -net tree has some regression, I saw too many packets with
> incorrect checksum from tcpdump inside a KVM guest, see the bottom.
> (firewall is turned off on both sides.)
> 
> It seems this is not related with drivers, I tried virtio and e1000,
> both have the same problem. And UDP has the problem too. But turning off
> tx-checksum by `ethtool -K eth0 tx off` makes the problem disappear
> (just turning off TSO or GSO doesn't make any difference).
> 

Running another RHEL6 KVM guest, it doesn't have this problem, so it is
not related with KVM environment.

After adding some debug printk's inside dev_hard_start_xmit(),
->ip_summed is mostly CHECKSUM_PARTIAL, in this case, ->csum_start == 34
(kicking out headroom, of course) and ->csum_offset == 16 look correct
too.