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Tue, 25 Sep 2018 16:02:59 +0000 (UTC) From: Kashyap Chamarthy To: qemu-devel@nongnu.org Date: Tue, 25 Sep 2018 18:02:48 +0200 Message-Id: <20180925160248.30801-3-kchamart@redhat.com> In-Reply-To: <20180925160248.30801-1-kchamart@redhat.com> References: <20180925160248.30801-1-kchamart@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.27 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Tue, 25 Sep 2018 16:03:01 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PATCH v2 2/2] docs: Document vCPU hotplug procedure X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: imammedo@redhat.com, Kashyap Chamarthy , armbru@redhat.com, ehabkost@redhat.com Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" Signed-off-by: Kashyap Chamarthy --- docs/cpu-hotplug.rst | 140 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 140 insertions(+) create mode 100644 docs/cpu-hotplug.rst diff --git a/docs/cpu-hotplug.rst b/docs/cpu-hotplug.rst new file mode 100644 index 0000000000..8f9e63a9f7 --- /dev/null +++ b/docs/cpu-hotplug.rst @@ -0,0 +1,140 @@ +=================== +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 \ + -blockdev node-name=node-Base,driver=qcow2,file.driver=file,file.filename=./base.qcow2 \ + -device virtio-blk,drive=node-Base,id=virtio0 -qmp unix:/tmp/qmp-sock,server,nowait + +(2) Run 'qmp-shell' (located in the source tree) to connect to the + just-launched QEMU:: + + $> ./qmp/qmp-shell -p -v /tmp/qmp-sock + [...] + (QEMU) + +(3) Check which socket is free to allow hotplugging a CPU:: + + (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) We can see that socket 1 is free, so use `device_add` to hotplug + "IvyBridge-IBRS-x86_64-cpu":: + + (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 tells + QEMU to unplug it.