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X-Received-From: 59.151.112.132 Subject: [Qemu-devel] [PATCH v2] docs: add cpu-hotplug.txt 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: Dou Liyang , ehabkost@redhat.com, armbru@redhat.com, bharata@linux.vnet.ibm.com, famz@redhat.com, imammedo@redhat.com, david@gibson.dropbear.id.au Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" X-Virus-Scanned: ClamAV using ClamSMTP This document describes how to use cpu hotplug in QEMU. Signed-off-by: Dou Liyang --- Change log v1 -> v2: From Fam's advice: 1. Fix some comment. Change log v1: From Igor's advice: 1. Remove any mentioning of apic-id from the document. 2. Remove the "device_del qom_path" from the CPU hot-unplug. 3. Fix some comment. docs/cpu-hotplug.txt | 127 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 127 insertions(+) create mode 100644 docs/cpu-hotplug.txt diff --git a/docs/cpu-hotplug.txt b/docs/cpu-hotplug.txt new file mode 100644 index 0000000..6a2f6f8 --- /dev/null +++ b/docs/cpu-hotplug.txt @@ -0,0 +1,127 @@ +QEMU CPU hotplug +================ + +This document explains how to use the CPU hotplug feature in QEMU, +which regards the CPU as a divece. + +the -device/device_add and device_del based on CPUs are merged since +2.7 + +Guest support is required for CPU hotplug to work. + +CPU hot-plug +------------ + +In order to be able to hotplug CPUs, QEMU has to be told the maximum +amount of CPUs which the guest can grow. This is done at startup time +by means of the -smp command-line option, which has the following +format: + + -smp [cpus=]n[,maxcpus=cpus][,cores=cores][,threads=threads] + [,sockets=sockets] + +Where, + + - "cpus" set the number of CPUs to 'n' [default=1]. + - "maxcpus" set the maximum number of CPUs, including offline VCPUs + for hotplug, etc. + - "cores" set the number of CPU cores on one socket. + - "threads set the number of threads on one CPU core. + - "sockets set the number of discrete sockets in the system. On + sPAPR, sockets have no real meaning, And it has no real effect. + +For example, the following command-line: + + qemu [...] -smp 3,maxcpus=10,sockets=2,cores=2,threads=2 + +Creates a guest with 3 VCPUs and it supports up to 10 VCPUs. The +CPU topology is sockets (2) * cores (2) * threads (2) and can't be +greater than maxcpus. When the guest is booted, the guest will see +3 VCPUs. More on this below. + +Query possible available CPU objects +------------------------------------ + +The VCPUs should be hotplugged by socket/core/thread-ids parameters +of the possible available CPUs objects. + +Before adding the VCPUs, we should know those topology parameters, +so that we can find out the available place (socket,core,thread) +for a new VCPU. + +There are two ways to obtain them: + +1. from the properties advertised by QEMU via the QMP command +query-hotpluggable-cpus. +2. from the corresponding HMP command "info hotpluggable-cpus". + +For example, a monitor command can be used to list the possible CPU +objects: + + (qemu) info hotpluggable-cpus + +Select the hotpluggable CPUs including "CPUInstance Properties" for +hotpluging. Such as this: + + ... + type: "qemu64-x86_64-cpu" + vcpus_count: "1" + CPUInstance Properties: + socket-id: "0" + core-id: "1" + thread-id: "0" + ... + +Hotplug CPUs +------------ + +A monitor command can be used to hotplug CPUs: + + - "device_add": creates a VCPU device and inserts it into the + specific place as a device + +For example, the following command adds a VCPU which has the id cpu1 +in the last position of the guest's CPU sockets which was discussed +earlier by using the socket/core/thread-ids: + + (qemu) device_add qemu64-x86_64-cpu,id=cpu9,socket-id=2,core-id=0, + thread-id=1 + +Where, + + - "qemu64-x86_64-cpu" is the CPU model. + - "id" is the unique identifier in the device sets. It is required. + - "socket-id/core-id/thread-id" represent the designated position + which is obtained form the above possible list of CPUs. + +It's also possible to start a guest with cpu cold-plugged into the +specific place (socket,core,thread). + +In the following command-lines example, a guest which has 3 VCPUs is +created: + + qemu [...] -smp 1,maxcpus=10,sockets=2,cores=2,threads=2 \ + -device qemu64-x86_64-cpu,id=cpu1,socket-id=1, \ + core-id=1,thread-id=0 \ + -device qemu64-x86_64-cpu,id=cpu2,socket-id=1, \ + core-id=1,thread-id=1 \ + +Two VCPUs are cold-plugged by "-device" parameter. They are in the +same socket and core with different thread-ids. After that, the +guest has additional seven VCPUs to be hot-plugged when needed. + +CPU hot-unplug +-------------- + +In order to be able to hot unplug CPU device, QEMU removes CPUs device +by using the ids which were assigned when you hot plugged the CPUs. + +A monitor command can be used to hot unplug CPUs: + + - "device_del": deletes a CPU device + +For example, assuming that the CPU device with id "cpu1" exists and has +been offline, the following command tries to remove it. + + (qemu) device_del cpu1 +