cpu_hotplug.txt

==================================cpu hotplug==================================
1.cpu hotplug testing. In any case, here's how to verify it:
<qemu-kvm-command-line>...-smp 2,sockets=2,cores=1,threads=1,maxcpus=X
 * switch to the monitor.
 * type 'cpu_set <n+1> online'.
 * A new cpu should appear on /sys/devices/system/cpu.
 * bring it online by writting 1 to the "online" file inside that directory on guest.
 * Try adding as many cpus(X) as possible, and make sure we at least do not crash. 
2.How do i logically offline a CPU?
# echo 0 > /sys/devices/system/cpu/cpuX/online
Once the logical offline is successful, check
# cat /proc/interrupts
You should now not see the CPU that you removed. Also online file will report the state as 0 when a cpu if offline and 1 when its online.
To display the current cpu state.
# cat /sys/devices/system/cpu/cpuX/online
3.Q: Why can't i remove CPU0 on some systems?
A: Some architectures may have some special dependency on a certain CPU. In current ACPI specifications, we didn't have a way to change the target CPU. Hence if the current ACPI version doesn't support such re-direction, we disable that CPU by making it not-removable. In such cases you will also notice that the online file is missing under cpu0.