Signed-off-by: Kashyap Chamarthy <kcham...@redhat.com>
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+===================
+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,nowait
+
+(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": [
+ {
+ "type": "IvyBridge-IBRS-x86_64-cpu",
+ "vcpus-count": 1,
+ "props": {
+ "socket-id": 1,
+ "core-id": 0,
+ "thread-id": 0
+ }
+ },
+ {
+ "qom-path": "/machine/unattached/device[0]",
+ "type": "IvyBridge-IBRS-x86_64-cpu",
+ "vcpus-count": 1,
+ "props": {
+ "socket-id": 0,
+ "core-id": 0,
+ "thread-id": 0
+ }
+ }
+ ]
+ }
+ (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,
+ while hot-plugging a CPU into socket 1 requires passing the listed
+ properties to QMP ``device_add``:
+
+ (QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu socket-id=1
core-id=0 thread-id=0
+ {
+ "execute": "device_add",
+ "arguments": {
+ "socket-id": 1,
+ "driver": "IvyBridge-IBRS-x86_64-cpu",
+ "id": "cpu-2",
+ "core-id": 0,
+ "thread-id": 0
+ }
+ }
+ {
+ "return": {}
+ }
+ (QEMU)
+
+(5) Optionally, run QMP `query-cpus-fast` for some details about the
+ vCPUs::
+
+ (QEMU) query-cpus-fast
+ {
+ "execute": "query-cpus-fast",
+ "arguments": {}
+ }
+ {
+ "return": [
+ {
+ "qom-path": "/machine/unattached/device[0]",
+ "target": "x86_64",
+ "thread-id": 11534,
+ "cpu-index": 0,
+ "props": {
+ "socket-id": 0,
+ "core-id": 0,
+ "thread-id": 0
+ },
+ "arch": "x86"
+ },
+ {
+ "qom-path": "/machine/peripheral/cpu-2",
+ "target": "x86_64",
+ "thread-id": 12106,
+ "cpu-index": 1,
+ "props": {
+ "socket-id": 1,
+ "core-id": 0,
+ "thread-id": 0
+ },
+ "arch": "x86"
+ }
+ ]
+ }
+ (QEMU)
+
+vCPU hot-unplug
+---------------
+
+From the 'qmp-shell', invoke the QMP ``device_del`` command::
+
+ (QEMU) device_del id=cpu-2
+ {
+ "execute": "device_del",
+ "arguments": {
+ "id": "cpu-2"
+ }
+ }
+ {
+ "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
+ Interupt (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.
--
2.17.1
