Unsupported delivery mode 7

[Chris Webb <chris.webb@elastichosts.com>]

We're running kvm-78 in production on Linux 2.6.27 x86_64 on dual quad-core
Opteron 'Barcelona' machines. Our kvm modules are built from the kvm-78
sources rather than the older version bundled with the kernel, and we're
using the NPT features of the processors.

For the most part, everything is performing very well and running reliably.
However, occasionally a guest will hang as it starts (or is reset) with a
large number of messages of the form

  Unsupported delivery mode 7

in the dmesg. Following this, killing and relaunching the qemu process is
usually sufficient to get a working guest.

I'm aware that our versions of the kvm kernel modules and userspace are not
the latest release, but because we're running long-lived guests on behalf of
clients, it's quite a major operation to upgrade. Does this look like a
known bug which has already been fixed or should I try to reproduce it
properly on a test machine with an ability to debug, use magic sysrq, etc?
(It seems impossible to reproduce on my lower spec desktop machine, for what
it's worth. Normally I'd reproduce kernel problems in a KVM virtual
machine---but that's obviously not an option here!)

Am I right in suspecting it might be connected to interrupt delivery
following page migration when a guest moves from one processor to another,
and that a workaround might be to taskset guests to one or other physical
CPU until we're able to upgrade to a more recent version of KVM?

Many thanks in advance for any advice anyone can offer.

Cheers,

Chris.