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X-Received-From: 59.151.112.132 Subject: [Qemu-devel] [PATCH v4] 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 , drjones@redhat.com, ehabkost@redhat.com, armbru@redhat.com, bharata@linux.vnet.ibm.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: Andrew Jones Signed-off-by: Dou Liyang --- Change log v3 -> v4 From David's advice 1. add spapr examples 2. Fix some comment From drew's advice 1. Fix some syntax Change log v2 -> v3: From drew's advice: 1. modify the examples. 2. Fix some syntax. 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 | 132 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 132 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..0c1bfe9 --- /dev/null +++ b/docs/cpu-hotplug.txt @@ -0,0 +1,132 @@ +QEMU CPU hotplug +================ + +This document explains how to use the CPU hotplug feature in QEMU, +which regards the CPU as a device, using -device/device_add and +device_del. + +QEMU support was merged for 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 +number of CPUs which the guest can have. 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" sets the number of CPUs to 'n' [default=1]. + - "maxcpus" sets the maximum number of CPUs, including offline VCPUs + for hotplug. + - "sockets" sets the number of discrete sockets in the system. + - "cores" sets the number of CPU cores on one socket. + - "threads" sets the number of threads on one CPU core. + +For example, the following command-line: + + qemu [...] -smp 3,maxcpus=8,sockets=2,cores=2,threads=2 + +creates a guest with 3 VCPUs and supports up to 8 VCPUs. The CPU +topology is sockets (2) * cores (2) * threads (2) and can't be +greater than maxcpus. When the guest finishes loading, the guest +will see 3 VCPUs. More on this below. + +Query available CPU objects +--------------------------- + +To add a VCPUs, it must be identified by socket-id, core-id, and/or +thread-id parameters. + +Before adding the VCPUs, we should know those topology parameters, +so that we can find the available location (socket,core,thread) for +a new VCPU. + +Using the corresponding HMP command "info hotpluggable-cpus" to obtain +them. + +For example, the following command-line: + + (qemu) info hotpluggable-cpus + +lists the hotpluggable CPUs including "CPUInstance Properties" for +hotplugging. Such as this: + + ... + type: "qemu64-x86_64-cpu" + vcpus_count: "1" + CPUInstance Properties: + socket-id: "0" + core-id: "1" + thread-id: "0" + ... + +Or + + ... + type: "POWER7_v2.3-spapr-cpu-core" + vcpus_count: "1" + CPUInstance Properties: + core-id: "2" + ... + +Different platforms may have different "CPUInstance Properties". + +Hotplug CPUs +------------ + +A monitor command can be used to hotplug CPUs: + + - "device_add": creates a CPU device and inserts it into the + specific location. + +For example, the following command adds a VCPU, which has the id cpu1, +to a specific location in the topology (socket=0,core=1,thread=1): + + (qemu) device_add qemu64-x86_64-cpu,id=cpu1,socket-id=0,core-id=1,thread-id=1 + +Where, + + - "qemu64-x86_64-cpu" is the CPU model. + - "id" is the unique identifier in the device set. + - "socket-id/core-id/thread-id" represent the designated location, + which is obtained form the above possible list of CPUs. + +It's also possible to start a guest with a cpu cold-plugged into a +specific location (socket,core,thread). + +In the following command line example, a guest which has 3 VCPUs is +created: + + qemu [...] -smp 1,maxcpus=8,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, which are in the +same socket and core, but with different thread-ids. After that, the +guest has an additional five VCPUs to be hot-plugged when needed. + +CPU hot-unplug +-------------- + +In order to be able to hot unplug a CPU device, QEMU removes CPU +devices by using the ids which were assigned when hotplugging the +CPU device. + +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, +then the following command tries to remove it. + + (qemu) device_del cpu1 +