Why unable to complete live-migration from recently added compute nodes to older compute nodes despite of having same CPU model ?

Issue

• Instances among newly added nodes are getting migrated without any issue. Instances from existing nodes to newly added nodes are also getting migrated successfully. Issue is coming while migrating instance from newly added node to existing [old] compute node.
• CPU are exactly the same as is the libvirt versions but still the migration is getting failed.
• Following error seen on terminal during the migration failure.

# nova live-migration 3326a533-ca2b-4a3d-9345-d4e9f04ed312 old-compute1
ERROR (BadRequest): Unacceptable CPU info: CPU doesn't have compatibility.

• Call trace seen in /var/log/nova/nova-compute.log file :

2016-10-18 13:44:31.668 15362 ERROR nova.virt.libvirt.driver [req-046b2b05-71e1-4693-b18d-c104a8658320 b334764d47cb4ce6bf5ef86eebfcc54e 
3244c329ec6444828cf82e46d927ff3a - - -] CPU doesn't have compatibility.


Refer to http://libvirt.org/html/libvirt-libvirt.html#virCPUCompareResult
2016-10-18 13:44:31.682 15362 ERROR oslo_messaging.rpc.dispatcher [req-046b2b05-71e1-4693-b18d-c104a8658320 b334764d47cb4ce6bf5ef86eebfcc54e 3244c329ec6444828cf82e46d927ff3a - - -] Exception during message handling: Unacceptable CPU info: CPU doesn't have compatibility.

Refer to http://libvirt.org/html/libvirt-libvirt.html#virCPUCompareResult
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher Traceback (most recent call last):
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 142, in _dispatch_and_reply
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     executor_callback))
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 186, in _dispatch
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     executor_callback)
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 130, in _do_dispatch
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     result = func(ctxt, **new_args)
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 6754, in check_can_live_migrate_destination
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     ctxt, instance, block_migration, disk_over_commit)
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/nova/exception.py", line 88, in wrapped
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     payload)
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 85, in __exit__
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     six.reraise(self.type_, self.value, self.tb)
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 357, in decorated_function
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     return function(self, context, *args, **kwargs)
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/nova/compute/manager.py", line 5206, in check_can_live_migrate_destination
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     block_migration, disk_over_commit)
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 5183, in check_can_live_migrate_destination
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     self._compare_cpu(None, source_cpu_info)
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/driver.py", line 5390, in _compare_cpu
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher     raise exception.InvalidCPUInfo(reason=m % {'ret': ret, 'u': u})
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher InvalidCPUInfo: Unacceptable CPU info: CPU doesn't have compatibility.
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher 
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher 0
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher 
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher Refer to http://libvirt.org/html/libvirt-libvirt.html#virCPUCompareResult
2016-10-18 13:44:31.682 15362 TRACE oslo_messaging.rpc.dispatcher 
2016-10-18 13:44:31.682 15362 ERROR oslo_messaging._drivers.common [req-046b2b05-71e1-4693-b18d-c104a8658320 b334764d47cb4ce6bf5ef86eebfcc54e 3244c329ec6444828cf82e46d927ff3a - - -] Returning exception Unacceptable CPU info: CPU doesn't have compatibility.