Virtual CPU hotplug

A complete example of vCPU hotplug (and hot-unplug) using QMP device_add and device_del.

vCPU hotplug

  1. 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,nowait
    
  2. 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)
    
  3. 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)
    
  4. 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 that IvyBridge-IBRS-x86_64-cpu is present in socket 0, while hot-plugging a CPU into socket 1 requires passing the listed properties to QMP device_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)
    
  5. 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.