Summary

There are 2 ways to hotplug CPU in QEMU:

dedicated legacy interface: cpu-add QMP command
generic device-add/device-del interface for hot-(un)plugging CPUs

Owner

Name: Igor Mammedov
Email: imammedo@redhat.com

cpu-add interface

Summary

{ 'command': 'cpu-add', 'data': {'id': 'int'} }

ID - a number in range [0..max-cpus)
Available since: 1.5
Supported targets: i386-softmmu, x86_64-softmmu, s399x (since 2.6)

Description

Command is a legacy solution for CPU hot-add and gives a simplified interface for it. It provides an opportunity to implement the feature for targets that currently can't implement CPU hot-add using device_add command due to their present design. Later targets that implement it could rewrite it to become a wrapper over device_add when it becomes usable for target.

Usage example

1. start QEMU with QMP socket available and with startup amount of CPUs less than maxcpus

./qemu-system-x86_64 -qmp unix:/tmp/qmp-sock,server,nowait -smp 1,maxcpus=4

2. Connect to QMP socket using qmp-shell command

./QMP/qmp-shell /tmp/qmp-sock

3. Add CPUs issuing cpu-add command in qmp-shell command prompt

cpu-add id=1

4. Optionally online newly added CPU inside guest

Linux kernel doesn't online hot-added CPUs automatically. Once CPU is hot-added it should be onlined using an appropriate udev script or manually by issuing a following command:

echo 1 > /sys/devices/system/cpu/cpu1/online

Sample udev script: Add the following line to /etc/udev/rules.d/99-hotPlugCPU.rules

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

Current limitations

migration target should be started with initial CPU count '-smp XX' that includes hot-added CPUs on migration source side.
CPU shouldn't be hot-plugged during migration.
adding CPUs should be done in successive order from lower to higher IDs in [0..max-cpus) range.
It's possible to add arbitrary CPUs in random order, however that would cause migration to fail on its target side.

device_add/device_del interface

Summary

{ 'command': 'device_add', 'data': {'driver': 'str', 'id': 'str', ... }}

mandatory properties for every CPU:
- driver: cpu model type name
- id: unique device name
target
socket-id: socket number in range [0..max sockets)
core-id: core number in range [0..max cores)
thread-id: thread-id in range [..max threads)