docs/cpu-hotplug.rst

   1 ===================
   2 Virtual CPU hotplug
   3 ===================
   4
   5 A complete example of vCPU hotplug (and hot-unplug) using QMP
   6 ``device_add`` and ``device_del``.
   7
   8 vCPU hotplug
   9 ------------
  10
  11 (1) Launch QEMU as follows (note that the "maxcpus" is mandatory to
  12     allow vCPU hotplug)::
  13
  14       $ qemu-system-x86_64 -display none -no-user-config -m 2048 \
  15           -nodefaults -monitor stdio -machine pc,accel=kvm,usb=off \
  16           -smp 1,maxcpus=2 -cpu IvyBridge-IBRS \
  17           -qmp unix:/tmp/qmp-sock,server,nowait
  18
  19 (2) Run 'qmp-shell' (located in the source tree, under: "scripts/qmp/)
  20     to connect to the just-launched QEMU::
  21
  22       $> ./qmp-shell -p -v /tmp/qmp-sock
  23       [...]
  24       (QEMU)
  25
  26 (3) Find out which CPU types could be plugged, and into which sockets::
  27
  28       (QEMU) query-hotpluggable-cpus
  29       {
  30           "execute": "query-hotpluggable-cpus",
  31           "arguments": {}
  32       }
  33       {
  34           "return": [
  35               {
  36                   "type": "IvyBridge-IBRS-x86_64-cpu",
  37                   "vcpus-count": 1,
  38                   "props": {
  39                       "socket-id": 1,
  40                       "core-id": 0,
  41                       "thread-id": 0
  42                   }
  43               },
  44               {
  45                   "qom-path": "/machine/unattached/device[0]",
  46                   "type": "IvyBridge-IBRS-x86_64-cpu",
  47                   "vcpus-count": 1,
  48                   "props": {
  49                       "socket-id": 0,
  50                       "core-id": 0,
  51                       "thread-id": 0
  52                   }
  53               }
  54           ]
  55       }
  56       (QEMU)
  57
  58 (4) The ``query-hotpluggable-cpus`` command returns an object for CPUs
  59     that are present (containing a "qom-path" member) or which may be
  60     hot-plugged (no "qom-path" member).  From its output in step (3), we
  61     can see that ``IvyBridge-IBRS-x86_64-cpu`` is present in socket 0,
  62     while hot-plugging a CPU into socket 1 requires passing the listed
  63     properties to QMP ``device_add``:
  64
  65       (QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu socket-id=1 core-id=0 thread-id=0
  66       {
  67           "execute": "device_add",
  68           "arguments": {
  69               "socket-id": 1,
  70               "driver": "IvyBridge-IBRS-x86_64-cpu",
  71               "id": "cpu-2",
  72               "core-id": 0,
  73               "thread-id": 0
  74           }
  75       }
  76       {
  77           "return": {}
  78       }
  79       (QEMU)
  80
  81 (5) Optionally, run QMP `query-cpus-fast` for some details about the
  82     vCPUs::
  83
  84       (QEMU) query-cpus-fast
  85       {
  86           "execute": "query-cpus-fast",
  87           "arguments": {}
  88       }
  89       {
  90           "return": [
  91               {
  92                   "qom-path": "/machine/unattached/device[0]",
  93                   "target": "x86_64",
  94                   "thread-id": 11534,
  95                   "cpu-index": 0,
  96                   "props": {
  97                       "socket-id": 0,
  98                       "core-id": 0,
  99                       "thread-id": 0
 100                   },
 101                   "arch": "x86"
 102               },
 103               {
 104                   "qom-path": "/machine/peripheral/cpu-2",
 105                   "target": "x86_64",
 106                   "thread-id": 12106,
 107                   "cpu-index": 1,
 108                   "props": {
 109                       "socket-id": 1,
 110                       "core-id": 0,
 111                       "thread-id": 0
 112                   },
 113                   "arch": "x86"
 114               }
 115           ]
 116       }
 117       (QEMU)
 118
 119 vCPU hot-unplug
 120 ---------------
 121
 122 From the 'qmp-shell', invoke the QMP ``device_del`` command::
 123
 124       (QEMU) device_del id=cpu-2
 125       {
 126           "execute": "device_del",
 127           "arguments": {
 128               "id": "cpu-2"
 129           }
 130       }
 131       {
 132           "return": {}
 133       }
 134       (QEMU)
 135
 136 .. note::
 137     vCPU hot-unplug requires guest cooperation; so the ``device_del``
 138     command above does not guarantee vCPU removal -- it's a "request to
 139     unplug".  At this point, the guest will get a System Control
 140     Interupt (SCI) and calls the ACPI handler for the affected vCPU
 141     device.  Then the guest kernel will bring the vCPU offline and tell
 142     QEMU to unplug it.