=================== 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=on,wait=off

(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": [
          {
              "props": {
                  "core-id": 1,
                  "socket-id": 0,
                  "thread-id": 0
              },
              "type": "IvyBridge-IBRS-x86_64-cpu",
              "vcpus-count": 1
          },
          {
              "props": {
                  "core-id": 0,
                  "socket-id": 0,
                  "thread-id": 0
              },
              "qom-path": "/machine/unattached/device[0]",
              "type": "IvyBridge-IBRS-x86_64-cpu",
              "vcpus-count": 1
          }
      ]
  }
  (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 core 0, while hot-plugging a CPU into socket 0 core 1 requires passing the listed properties to QMP device_add::

  (QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu socket-id=0 core-id=1 thread-id=0
  {
      "execute": "device_add",
      "arguments": {
          "core-id": 1,
          "driver": "IvyBridge-IBRS-x86_64-cpu",
          "id": "cpu-2",
          "socket-id": 0,
          "thread-id": 0
      }
  }
  {
      "return": {}
  }
  (QEMU)

(5) Optionally, run QMP query-cpus-fast for some details about the vCPUs::

  (QEMU) query-cpus-fast
  {
      "arguments": {}
      "execute": "query-cpus-fast",
  }
  {
      "return": [
          {
              "cpu-index": 0,
              "props": {
                  "core-id": 0,
                  "socket-id": 0,
                  "thread-id": 0
              },
              "qom-path": "/machine/unattached/device[0]",
              "target": "x86_64",
              "thread-id": 28957
          },
          {
              "cpu-index": 1,
              "props": {
                  "core-id": 1,
                  "socket-id": 0,
                  "thread-id": 0
              },
              "qom-path": "/machine/peripheral/cpu-2",
              "target": "x86_64",
              "thread-id": 29095
          }
      ]
  }
  (QEMU)

vCPU hot-unplug

From the 'qmp-shell', invoke the QMP device_del command::

  (QEMU) device_del id=cpu-2
  {
      "arguments": {
          "id": "cpu-2"
      }
      "execute": "device_del",
  }
  {
      "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.