# 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.