/lib/udev/rules.d/40-vm-hotadd.rules is in udev 237-3ubuntu10.
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 8 9 10 11 12 13 14 | # On Hyper-V and Xen Virtual Machines we want to add memory and cpus as soon as they appear
ATTR{[dmi/id]sys_vendor}=="Microsoft Corporation", ATTR{[dmi/id]product_name}=="Virtual Machine", GOTO="vm_hotadd_apply"
ATTR{[dmi/id]sys_vendor}=="Xen", GOTO="vm_hotadd_apply"
GOTO="vm_hotadd_end"
LABEL="vm_hotadd_apply"
# Memory hotadd request
SUBSYSTEM=="memory", ACTION=="add", DEVPATH=="/devices/system/memory/memory[0-9]*", TEST=="state", ATTR{state}="online"
# CPU hotadd request
SUBSYSTEM=="cpu", ACTION=="add", DEVPATH=="/devices/system/cpu/cpu[0-9]*", TEST=="online", ATTR{online}="1"
LABEL="vm_hotadd_end"
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