Paravirt Linux CPU Hotplug

Why does not xl vcpu-set work as expected when I try to increase the number of VCPUs?

Most likely it is udev event is not handled correctly in the targeting domain.

To identify the problem, try to run `udevadm monitor` as root. If you can see similar output:

KERNEL[1327937839.577165] add      /devices/system/cpu/cpu1 (cpu)
UDEV  [1327937839.577368] add      /devices/system/cpu/cpu1 (cpu)
KERNEL[1327937839.577581] add      /devices/system/cpu/cpu2 (cpu)
UDEV  [1327937839.577684] add      /devices/system/cpu/cpu2 (cpu)

then CPUs are correctly added to system.

All you need to do is write to corresponding sysfs nodes to bring those CPUs online.

To automate this process, simply create a `cpu-online.rules` under /etc/udev/rules.d and restart udevd.

Content of `cpu-online.rules`:

SUBSYSTEM=="cpu",ACTION=="add",RUN+="/bin/sh -c '[ ! -e /sys$devpath/online ] || echo 1 > /sys$devpath/online'"

If you cannot see hotplug events, probably you're hitting other limitations such as max VCPU quotation, do check against Xen configuration documents.

If you've checked every options and configurations and believe that this problem is caused by a bug, please follow instructions in Reporting_Bugs_against_Xen to submit bug report.