Xen users - Jun 2008 - 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.

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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@lists.xensource.com
[mailto:xen-users-bounces@lists.xensource.com] On Behalf Of David P
Olshefski
Sent: June 16, 2008 4:33 PM
To: xen-users@lists.xensource.com
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.


_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users