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Re: [Xen-devel] network misbehaviour with gplpv and 2.6.30

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] network misbehaviour with gplpv and 2.6.30
From: Paul Durrant <paul.durrant@xxxxxxxxxx>
Date: Tue, 21 Jul 2009 11:13:49 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Lyon <andrew.lyon@xxxxxxxxx>, Andrew
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James Harper wrote:

Are you saying that ring slot n
has only NETRXF_extra_info and *not* NETRXF_more_data?

Yes. From the debug I have received from Andrew Lyon, NETRXF_more_data
is _never_ set.

From what Andrew tells me (and it's not unlikely that I misunderstood),
the packets in question come from a physical machine external to the
machine running xen. I can't quite understand how that could be as they
are 'large' packets (>1514 byte total packet length) which should only
be locally originated. Unless he's running with jumbo frames (are you

It's not unusual for h/w drivers to support 'LRO', i.e. they re-assemble consecutive in-order TCP segments into a large packet before passing up the stack. I believe that these would manifest themselves as TSOs coming into the transmit side of netback, just as locally originated large packets would.

I've asked for some more debug info but he's in a different timezone to
me and probably isn't awake yet. I'm less and less inclined to think
that this is actually a problem with GPLPV and more a problem with
netback (or a physical network driver) in 2.6.30, but a tcpdump in Dom0,
HVM without GPLPV and maybe in a Linux DomU should tell us more.

Yes, a tcpdump of what's being passed into netback in dom0 should tell us what's happening.


Paul Durrant, Software Engineer

Citrix Systems (R&D) Ltd.
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