I have a quad core machine which is running debian with a xen kernel. I am running a couple linux servers on there which don't do a lot of CPU intesive stuff, but I have there a windows server 2008 vm, which seems to be sucking the life out of one single vcpu. Is there a way to give it more vcpus? vcpus = 2 for example does nothing, which does the charm on the linux vms. Even xm list still shows the vm with one vcpu.
When Windows installed selects the HAL library appropriate the the hardware your installing on. If you install XP on a single CPU system then the uniprocessor HAL is selected. If you then add a second vcpu to the guest then the windows HAL won't see it.
You'll need to replace Window's hal or try re-installing the guest.