Virtual CPU hotplug
A complete example of vCPU hotplug (and hot-unplug) using QMP
device_add
and device_del
.
vCPU hotplug
Launch QEMU as follows (note that the “maxcpus” is mandatory to allow vCPU hotplug):
$ qemu-system-x86_64 -display none -no-user-config -m 2048 \ -nodefaults -monitor stdio -machine pc,accel=kvm,usb=off \ -smp 1,maxcpus=2 -cpu IvyBridge-IBRS \ -qmp unix:/tmp/qmp-sock,server=on,wait=off
Run ‘qmp-shell’ (located in the source tree, under: “scripts/qmp/) to connect to the just-launched QEMU:
$> ./qmp-shell -p -v /tmp/qmp-sock [...] (QEMU)
Find out which CPU types could be plugged, and into which sockets:
(QEMU) query-hotpluggable-cpus { "execute": "query-hotpluggable-cpus", "arguments": {} } { "return": [ { "type": "IvyBridge-IBRS-x86_64-cpu", "vcpus-count": 1, "props": { "socket-id": 1, "core-id": 0, "thread-id": 0 } }, { "qom-path": "/machine/unattached/device[0]", "type": "IvyBridge-IBRS-x86_64-cpu", "vcpus-count": 1, "props": { "socket-id": 0, "core-id": 0, "thread-id": 0 } } ] } (QEMU)
The
query-hotpluggable-cpus
command returns an object for CPUs that are present (containing a “qom-path” member) or which may be hot-plugged (no “qom-path” member). From its output in step (3), we can see thatIvyBridge-IBRS-x86_64-cpu
is present in socket 0, while hot-plugging a CPU into socket 1 requires passing the listed properties to QMPdevice_add
:(QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu socket-id=1 core-id=0 thread-id=0 { "execute": "device_add", "arguments": { "socket-id": 1, "driver": "IvyBridge-IBRS-x86_64-cpu", "id": "cpu-2", "core-id": 0, "thread-id": 0 } } { "return": {} } (QEMU)
Optionally, run QMP
query-cpus-fast
for some details about the vCPUs:(QEMU) query-cpus-fast { "execute": "query-cpus-fast", "arguments": {} } { "return": [ { "qom-path": "/machine/unattached/device[0]", "target": "x86_64", "thread-id": 11534, "cpu-index": 0, "props": { "socket-id": 0, "core-id": 0, "thread-id": 0 }, "arch": "x86" }, { "qom-path": "/machine/peripheral/cpu-2", "target": "x86_64", "thread-id": 12106, "cpu-index": 1, "props": { "socket-id": 1, "core-id": 0, "thread-id": 0 }, "arch": "x86" } ] } (QEMU)
vCPU hot-unplug
From the ‘qmp-shell’, invoke the QMP device_del
command:
(QEMU) device_del id=cpu-2
{
"execute": "device_del",
"arguments": {
"id": "cpu-2"
}
}
{
"return": {}
}
(QEMU)
Note
vCPU hot-unplug requires guest cooperation; so the device_del
command above does not guarantee vCPU removal – it’s a “request to
unplug”. At this point, the guest will get a System Control
Interrupt (SCI) and calls the ACPI handler for the affected vCPU
device. Then the guest kernel will bring the vCPU offline and tell
QEMU to unplug it.