Yesterday I reproduced this problem on a (new) IBM x3550 M3 with the latest
squeeze packages, and installing
http://ftp.cyconet.org/debian/archive/unofficial/xen/4.0.1-5/xen-hypervisor-4.0-amd64_4.0.1-5_amd64.deb
fixed it.
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2. That which causes joy or happiness.