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RE: [Xen-devel] checksum offloading not disabled properly in responseto 'feature-no-csum-offload'

  • To: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>, "xen-devel" <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
  • Date: Sat, 12 Jan 2008 00:03:32 +1100
  • Delivery-date: Fri, 11 Jan 2008 05:04:09 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AchSuy1241X2K6a2T22bSYUUxHo6RABlrcgg
  • Thread-topic: [Xen-devel] checksum offloading not disabled properly in responseto 'feature-no-csum-offload'

> I think I've finally tracked down the problem I'm having with the PV
> windows drivers.
> Even though I set 'feature-no-csum-offload', and ethtool -k shows that
> it is off, wireshark in the windows domU says that the checksums are
> bad, until I issue the 'ethtool -K vifX.0 tx off' command, and then it
> all comes good. I suspect that Linux is disabling the feature without
> actually disabling the function...
> I haven't done enough testing yet to properly confirm this, but I'm
> reasonably confident that this is the problem.
> Anyone seen anything like this before?
> Ideally, I'd be able to convince Windows to not look at the received
> checksum, but even though I seem to be enabling all the right things
> setting all the right flags, Windows won't listen.

I found the _real_ cause of the problem, and it appears to be nothing to
do with checksum offloading, although some of the stuff I tested above
makes me wonder that there is still something I don't understand going
on somewhere.

When packets are created locally (eg on the same linux bridge as the
destination), they can be in several chunks, and the windows PV xennet
driver wasn't looking past the first chunk. This doesn't appear to
happen with non-local packets, which presumably get received off the
wire in a single chunk.

It always helps when you look for the problem in the right place :)



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