libvirt users - Sep 2010 - Clock Sources under RHEL/Centos 5.x with KVM and libvirt

I'm wondering under what circumstances the kvm-clock source is exposed to a
guest.

I've got a bunch of KVM servers running Centos 5.5 and some of the guests
have are defaulting to jiffies or apic whilst some are picking up kvm-clock.
These guests are a mix of 32bit and 64bit Centos 5.5.

Looking at the virtual machine definition file there doesn't appear to be a
difference between the guests with kvm-clock and those without.

Any where else to look, or what configuration will expose kvm-clock to a guest.

Steve

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Steven Ellis - Bulletin.Net Inc - http://www.bulletin.net