/lib/udev/rules.d/z10_xen-vcpu-hotplug.rules is in xe-guest-utilities 6.2.0-1120+dsf1-0ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | # Hotplug CPU support
ATTR{[dmi/id]sys_vendor}!="Xen", GOTO="xen_hotadd_end"
# CPU hotadd request
SUBSYSTEM=="cpu", ACTION=="add", DEVPATH=="/devices/system/cpu/cpu[0-9]*", TEST=="online", ATTR{online}="1"
LABEL="xen_hotadd_end"
|