I have a Xen 4.0.1 DomU running Debian Squeeze with the stock 2.6.32-5 Kernel. When I add CPUs via xm-vcpu-set they correctly show up in


but are not set online and therefore are not available to the system until I manually set them online via

echo 1 > /sys/devices/system/cpu/cpuX/online

Removing vcpus works as advertised. Any I idea? Am I missing something or is this a bug / outstanding feature?

1 Answer 1


Seems like pv_ops kernel do not take the CPUs added by the hypervisor online by default, one has to create an udev-rule for it like:

ACTION=="add", KERNEL=="cpu[0-9]*", RUN+="/bin/bash -c '/bin/sleep 10; echo 1 > /sys/devices/system/cpu/%k/online'"

( I put it in /etc/udev/rules.d/40-xen.rules )

To be honest I have no idea if this is already installed in a apckage I'm missing in my installation, but this works for me.

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