[Qemu-devel] [Bug 1195170] Re: cpu hot-add doesn't work with upstream qemu 1.5.50

[Paolo Bonzini]

You need to specify "-smp maxcpus=N" on the command-line, for example
"-smp cpus=2,maxcpus=4" will start the VM with 2 CPUs and let you add 2
more. Also, the argument to cpu-add is zero-based, so the range is
0..(maxcpus-1).

** Changed in: qemu
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1195170

Title:
  cpu hot-add doesn't work with upstream qemu 1.5.50

Status in QEMU:
  Invalid

Bug description:
  cpu hot-add doesn't work with upstream qemu 1.5.50. When we add the
  cpu [through QMP] it throws error as " {"error": {"class":
  "GenericError", "desc": "Unable to add CPU: 2, max allowed: 1"}} ",
  when we have booted the guest with two cpus.

  Host Kernel Version: 3.10.0-rc5+

  Libvirt Version: 1.0.6

  Qemu Version: 1.5.50

  Guest Kernel : 2.6.32-358.6.1.el6.x86_64

  
  Steps to Reproduce :

  Boot the guest with : qemu-system-x86_64 -drive
  file='/home/images/vm1.qcow2',format=qcow2,cache=none,boot=on -m 2048
  -smp 2,cores=2,threads=1,sockets=1 -qmp tcp:localhost:4444,server
  -monitor stdio -enable-kvm

  Start the  telnet localhost 4444 on another shell

  verify that there are two cpus :

  I verified through HMP 
  (qemu) info cpus 
  * CPU #0: pc=0x000000000000f081 thread_id=2034
    CPU #1: pc=0x00000000000f6497 (halted) thread_id=2031

  Hot add the CPU using the below command in QMP:

  { "execute": "cpu-add", "arguments": { "id": 2 } }
  {"error": {"class": "GenericError", "desc": "Unable to add CPU: 2, max 
allowed: 1"}}

  Notes:
  --------
  1. By default [without specifying the no of cpu], the guest boots up with 1 
cpu, when we try to add one more cpu, below is the error:

  { "execute": "cpu-add", "arguments": { "id": 1 } }
  {"error": {"class": "GenericError", "desc": "Unable to add CPU: 1, max 
allowed: 0"}}

  2. This issue is seen with or without enabling the KVM [i.e booting
  the guest without -enable-kvm option]

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