This document describes how to use cpu hotplug in QEMU.
Signed-off-by: Andrew Jones <drjo...@redhat.com>
Signed-off-by: Dou Liyang <douly.f...@cn.fujitsu.com>
---
Change log v4 -> v5
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+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 4,maxcpus=8,sockets=2,cores=2,threads=2
+
+creates a guest with 4 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 4 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 all hot-pluggable CPUs. Such as this:
+
+ ...
+ type: "qemu64-x86_64-cpu"
+ vcpus_count: "1"
+ CPUInstance Properties:
+ socket-id: "1"
+ core-id: "0"
+ 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", which
+will be used in hot-plugging below.
+
+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=1,core=0,thread=0):
+
+ (qemu) device_add qemu64-x86_64-cpu,id=cpu1,socket-id=1,core-id=0,thread-id=0
+
+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 2,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 four VCPUs to be hot-plugged when needed.
+
+Above is about the x86-64 architecture. On other architectures, we
+just need modify the command parameters based on the "CPUInstance
+Properties".
+
+For example, the following command adds a VCPU in a sPAPR hardware
+System:
+
+ (qemu) device_add POWER7_v2.3-spapr-cpu-core,id=cpu1,core-id=2
+
+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
+
--
2.5.5
