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Description of problem:
currently, qemu monitor haven't any tips.

Version-Release number of selected component (if applicable):
host kernel:
# uname -r
2.6.32-257.el6.x86_64

qemu vesion:
qemu-kvm-0.12.1.2-2.265.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1./usr/libexec/qemu-kvm -M rhel6.3.0 -cpu host --enable-kvm -m 4G -smp
1,maxcpus=161 -name rhel6.3 -uuid ddcbfb49-3411-1701-3c36-6bdbc00bedbb -rtc
base=utc,clock=host,driftfix=slew -drive
file=/dev/vg-54/lvtest,if=none,id=ide,format=qcow2,cache=none,werror=stop,rerror=stop
-device ide-drive,drive=ide,id=drive-ide0-0-0,bootindex=1 -netdev
tap,id=hostnet1 -device e1000,netdev=hostnet1,id=net1,mac=86:12:50:a4:32:72
-spice port=5911,disable-ticketing -vga qxl -device sga -chardev
socket,id=serial0,path=/var/test1,server,nowait -device
isa-serial,chardev=serial0 -balloon virtio -monitor
unix:/tmp/monitor2,server,nowait -monitor stdio

2.hotplug duplicate vcpu from monitor
cpu_set 0 online

3.
  
Actual results:
monitor haven't any warning message, and hotplug fail(guest don't get new cpu).

Expected results:
qemu have duplicate warning message. 

Additional info:

Attempt to hotplug already existing vcpu is a nop, it practically does nothing except of sending SCI to guest and guest checks for hot-plugged cpus and since cpu was already marked in bios as present, guest doesn't see anything to hot-plug.

Since it's not critical, I'd say ignore this for RHEL6.3 and implement/fix it upstream first.

This request was not resolved in time for the current release.
Red Hat invites you to ask your support representative to
propose this request, if still desired, for consideration in
the next release of Red Hat Enterprise Linux.

This request was erroneously removed from consideration in Red Hat Enterprise Linux 6.4, which is currently under development.  This request will be evaluated for inclusion in Red Hat Enterprise Linux 6.4.

Supported libvirt interface does it correctly, i.e. it doesn't try to hotplug existing vCPU. So fixing it for 6.3 is so not important and customers shouldn't be affected by it.
Closing it  as WONTFIX.