Bug 440035

Summary: why do Xen virtual interfaces try and do tx-checksumming?
Product: Red Hat Enterprise Linux 5 Reporter: Chris Stankaitis <cstankaitis>
Component: xenAssignee: Xen Maintainance List <xen-maint>
Status: CLOSED NOTABUG QA Contact: Virtualization Bugs <virt-bugs>
Severity: low Docs Contact:
Priority: low    
Version: 5.1   
Target Milestone: rc   
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Hardware: All   
OS: Linux   
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Fixed In Version: Doc Type: Bug Fix
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Last Closed: 2008-04-01 14:39:52 UTC Type: ---
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Description Chris Stankaitis 2008-04-01 14:32:53 UTC 
Description of problem:

Wondering why Xen's Virtual Interfaces virbr0 vifX.0 and on the Guest OS the vif
which eth0 etc map to have tx-checksumming turned on? given that these are not
actual physical NIC Cards which would/could offload the checksumming from the CPU??

The tx-checksumming was giving us big problems with SSH and FTP to our Xen Guest
and we ultimate had to add an ethtools command to our rc.local to turn it off.

Steps to Reproduce:
1. ethtool -k eth0

see that tx-checksumming  is on

you can see that is is on a virtual interface.

[root@dropbox ~]# cat /etc/modprobe.conf 
alias eth0 xennet
alias scsi_hostadapter xenblk


Expected results:

Since Xen bridge, virtual interfaces, and on guestOS virtual interfaces are not
physical NIC Cards which could legitimately offload the checksumming from the
CPU I would have expected that they would have tx-checksumming off by default.
Comment 1 Daniel Berrangé 2008-04-01 14:39:52 UTC 
The Xen VIF kernel driver *does* support tx-checksumming, in so much as it will
pass packets straight through to the host NIC for checksumming & fragmentation.
For guest<->guest traffic this allows checksumming/fragmentation overhead to be
avoided entirely. This results in a throughput improvements over > x100.


If you have specific scenarios in which tx checksumming causes problems please
enter a bug providing details of exactly what is broken & the steps to reproduce
the problem.