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[Matt Riedemann <mriedem@xxxxxxxxxx>]

** Also affects: nova/liberty
   Importance: Undecided
       Status: New

** Also affects: nova/mitaka
   Importance: Undecided
       Status: New

** Changed in: nova
       Status: New => In Progress

** Changed in: nova/mitaka
       Status: New => Confirmed

** Changed in: nova/mitaka
   Importance: Undecided => High

** Changed in: nova/liberty
       Status: New => Confirmed

** Changed in: nova/liberty
   Importance: Undecided => Medium

-- 
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Engineering Team, which is subscribed to OpenStack Compute (nova).
https://bugs.launchpad.net/bugs/1588003

Title:
  Skip host to guest CPU compatibility check for emulated (QEMU "TCG"
  mode) guests  during live migration

Status in OpenStack Compute (nova):
  In Progress
Status in OpenStack Compute (nova) liberty series:
  Confirmed
Status in OpenStack Compute (nova) mitaka series:
  In Progress

Bug description:
  The _compare_cpu() method of Nova's libvirt driver performs guest vCPU 
  model to destination host CPU model comparison (during live migration) 
  even in the case of emulated (QEMU "TCG" mode) guests, where the CPU
  instructions are emulated completely in software, and no hardware
  acceleration, such as KVM is involved.

  From nova/virt/libvirt/driver.py:

     [...]
     5464     def _compare_cpu(self, guest_cpu, host_cpu_str, instance):
     5465         """Check the host is compatible with the requested CPU
     [...]    [...]
     5481         if CONF.libvirt.virt_type not in ['qemu', 'kvm']:
     5482             return
     5483

  Skip the comparison for 'qemu' part above.

  Fix for master branch is here:

      https://review.openstack.org/#/c/323467/ -- 
      libvirt: Skip CPU compatibility check for emulated guests

  
  This bug is for stable branch backports: Mitaka and Liberty.

  [Thanks: Daniel P. Berrange for the pointer.]

  
  Related context and references
  ------------------------------

  (a) This upstream discussion thread where using the custom CPU model 
      ("gate64") is causing live migration CI jobs to fail.

      http://lists.openstack.org/pipermail/openstack-dev/2016-May/095811.html 
      -- "[gate] [nova] live migration, libvirt 1.3, and the gate"

  (b) Gate DevStack change to avoid setting the custom CPU model in 
      nova.conf

      https://review.openstack.org/#/c/320925/4 -- don't set libvirt 
      cpu_model

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