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cpu-hotplug.rst (4133B)

      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=on,wait=off
     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     Interrupt (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.