RHEL Engineering is moving the tracking of its product development work on RHEL 6 through RHEL 9 to Red Hat Jira (issues.redhat.com). If you're a Red Hat customer, please continue to file support cases via the Red Hat customer portal. If you're not, please head to the "RHEL project" in Red Hat Jira and file new tickets here. Individual Bugzilla bugs in the statuses "NEW", "ASSIGNED", and "POST" are being migrated throughout September 2023. Bugs of Red Hat partners with an assigned Engineering Partner Manager (EPM) are migrated in late September as per pre-agreed dates. Bugs against components "kernel", "kernel-rt", and "kpatch" are only migrated if still in "NEW" or "ASSIGNED". If you cannot log in to RH Jira, please consult article #7032570. That failing, please send an e-mail to the RH Jira admins at rh-issues@redhat.com to troubleshoot your issue as a user management inquiry. The email creates a ServiceNow ticket with Red Hat. Individual Bugzilla bugs that are migrated will be moved to status "CLOSED", resolution "MIGRATED", and set with "MigratedToJIRA" in "Keywords". The link to the successor Jira issue will be found under "Links", have a little "two-footprint" icon next to it, and direct you to the "RHEL project" in Red Hat Jira (issue links are of type "https://issues.redhat.com/browse/RHEL-XXXX", where "X" is a digit). This same link will be available in a blue banner at the top of the page informing you that that bug has been migrated.

Description of problem:
Use setvcpus to hotplug vcpu for a guest, then the guest's CPU usage is 100% and guest hang there.
BTW, if set vcpu number to 0, it seems the error message is not correct.

Version-Release number of selected component (if applicable):
libvirt-0.9.10-21.el6_3.1.x86_64&libvirt-0.9.10-21.el6.x86_64
qemu-kvm-rhev-0.12.1.2-2.295.el6.x86_64
kernel-2.6.32-276.el6.x86_64


How reproducible:
100%

Steps to Reproduce:
1. Set the maximum cpu number to 4
#virsh dumpxml test
......
<vcpu placement='static' current='1'>4</vcpu>
.....
2. hotplug vcpus 
#virsh setvcpus test 2 

check the guest xml again
#virsh dumpxml test
......
<vcpu placement='static' current='2'>4</vcpu>
.....

3. After add step2, guest's CPU usage up to 100% and cann't conduct any operation in guest.

4. set vcpus to 0
# virsh setvcpus  test 0
error: invalid argument: virDomainSetVcpus
  
Actual results:
As steps 3&4.

Expected results:
Hotplug vcpu can succeed, and the error message should be improved when set vcpu to 0.

Additional info:
No error meesages in libvirtd.log

This bug can always reproduced with rhel6.2 guest(release version), but can't reproduce with rhel6.3 guest.

There's either a bug or the the guest OS is missing support for CPU hotplug, so that it cannot handle the CPU that has emerged in the emulated system. This part of this bug isn't a problem with libvirt. 

As of the error message, that probably is worth tweaking a little bit.

I opened BZ#834365 to track improvement of the error message and I'm reassigning this to kernel to track the guest hang.

6.2 guest doesn't support cpu hotplug, and issue is fixed in 6.3

*** This bug has been marked as a duplicate of bug 788562 ***