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.