This is a known problem with Xen, and has been broken for some time. It is recommendable to disable tx checksum offloading in all pv domU’s.
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of David P Olshefski
Sent: June 16, 2008 4:33 PM
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] problems with
csum
After booting two VMs on two separate physical
machines, all works fine, and I can execute iperf to obtain about 923Mbps over
the Gb link which connects the two physical
hosts.
After
applying a few simple iptable rules to perform DNAT and SNAT in dom0, the
performance dropped to 500Kbps.....looking at the packet stream using ethereal,
I found that
the
packets had csum errors (hence lots of retransmissions). The errors are
in the TCP check sum (IP csum was fine).
It
seems like the problem goes away when I turn off csum offload inside the VM:
domU> ethtool -K eth0 tx off
Can
anyone shed some light on this for me?
Dave O.
