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QEMU CPU Hotplug

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In the world of cloud compute, Scaling the compute resources without shutdown the compute instance is important. e.g. Add/Reduce cpu number, memory size, and so on.

In the version 1.5, QEMU has implemented the feature: CPU Hotplug, it allows virtual machine to add cpu core without shutdown.

Below is how to do it:

Before try this feature, please make sure your QEMU version >= 1.5.0, or this feature is not supported.

# ~/usr/bin/qemu-system-x86_64 --version
QEMU emulator version 1.6.50, Copyright (c) 2003-2008 Fabrice Bellard

Use QEMU command directly to enable this feature

# qemu-system-x86_64 -enable-kvm -m 512 -smp 1,maxcpus=4 ArchLinux.raw -serial \
   tcp::3333,nowait,server -qmp tcp::5555,nowait,server -nographic

the maxcpus means the maximum number of hotpluggable CPUs.

[root@myhost ~]# cat /proc/cpuinfo | grep processor
processor       : 0

After active the CPU we just add

# telnet localhost 5555
......
{ "execute": "qmp_capabilities" }  
{"return": {}}
{ "execute": "cpu-add", "arguments": { "id": 1 }}
{"return": {}}

[root@myhost ~]# cat /proc/cpuinfo | grep processor
processor       : 0
processor       : 1

Use libvirt to enable this feature

Make sure the version of libvirt >= 1.1.0, or else it wont work

[root@hdr245 ~]# libvirtd --version
libvirtd (libvirt) 1.1.1

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