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===================
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Virtual CPU hotplug
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===================
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A complete example of vCPU hotplug (and hot-unplug) using QMP
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``device_add`` and ``device_del``.
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vCPU hotplug
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------------
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(1) Launch QEMU as follows (note that the "maxcpus" is mandatory to
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allow vCPU hotplug)::
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$ qemu-system-x86_64 -display none -no-user-config -m 2048 \
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-nodefaults -monitor stdio -machine pc,accel=kvm,usb=off \
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-smp 1,maxcpus=2 -cpu IvyBridge-IBRS \
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-qmp unix:/tmp/qmp-sock,server=on,wait=off
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(2) Run 'qmp-shell' (located in the source tree, under: "scripts/qmp/)
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to connect to the just-launched QEMU::
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$> ./qmp-shell -p -v /tmp/qmp-sock
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[...]
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(QEMU)
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(3) Find out which CPU types could be plugged, and into which sockets::
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(QEMU) query-hotpluggable-cpus
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{
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"execute": "query-hotpluggable-cpus",
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"arguments": {}
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}
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{
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"return": [
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{
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"type": "IvyBridge-IBRS-x86_64-cpu",
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"vcpus-count": 1,
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"props": {
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"socket-id": 1,
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"core-id": 0,
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"thread-id": 0
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}
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},
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{
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"qom-path": "/machine/unattached/device[0]",
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"type": "IvyBridge-IBRS-x86_64-cpu",
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"vcpus-count": 1,
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"props": {
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"socket-id": 0,
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"core-id": 0,
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"thread-id": 0
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}
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}
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]
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}
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(QEMU)
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(4) The ``query-hotpluggable-cpus`` command returns an object for CPUs
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that are present (containing a "qom-path" member) or which may be
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hot-plugged (no "qom-path" member). From its output in step (3), we
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can see that ``IvyBridge-IBRS-x86_64-cpu`` is present in socket 0,
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while hot-plugging a CPU into socket 1 requires passing the listed
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properties to QMP ``device_add``::
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(QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu socket-id=1 core-id=0 thread-id=0
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{
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"execute": "device_add",
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"arguments": {
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"socket-id": 1,
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"driver": "IvyBridge-IBRS-x86_64-cpu",
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"id": "cpu-2",
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"core-id": 0,
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"thread-id": 0
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}
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}
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{
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"return": {}
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}
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(QEMU)
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(5) Optionally, run QMP ``query-cpus-fast`` for some details about the
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vCPUs::
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(QEMU) query-cpus-fast
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{
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"execute": "query-cpus-fast",
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"arguments": {}
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}
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{
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"return": [
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{
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"qom-path": "/machine/unattached/device[0]",
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"target": "x86_64",
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"thread-id": 11534,
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"cpu-index": 0,
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"props": {
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"socket-id": 0,
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"core-id": 0,
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"thread-id": 0
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},
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"arch": "x86"
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},
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{
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"qom-path": "/machine/peripheral/cpu-2",
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"target": "x86_64",
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"thread-id": 12106,
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"cpu-index": 1,
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"props": {
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"socket-id": 1,
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"core-id": 0,
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"thread-id": 0
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},
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"arch": "x86"
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}
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]
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}
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(QEMU)
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vCPU hot-unplug
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---------------
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From the 'qmp-shell', invoke the QMP ``device_del`` command::
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(QEMU) device_del id=cpu-2
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{
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"execute": "device_del",
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"arguments": {
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"id": "cpu-2"
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}
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}
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{
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"return": {}
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}
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(QEMU)
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.. note::
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vCPU hot-unplug requires guest cooperation; so the ``device_del``
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command above does not guarantee vCPU removal -- it's a "request to
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unplug". At this point, the guest will get a System Control
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Interrupt (SCI) and calls the ACPI handler for the affected vCPU
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device. Then the guest kernel will bring the vCPU offline and tell
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QEMU to unplug it.
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