VMotion stops working after upgrading to vSphere 4.0

A CPU of the host is incompatible error appears and VMotion stops working after upgrading to vSphere 4.0

Just experienced this issue and found the resolution on VMware’s Kb 1011294 >> http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1011294 posted here for reference and if anyone else runs into the problem.

Symptoms
VMotion fails after upgrading from ESX 3.x to ESX 4.0
You receive an error similar to:

Unable to migrate from to : The CPU of the host is incompatible with the CPU feature requirements of the virtual machine; problem detected at CPUID level.

Host CPU is incompatible with the virtual machine’s requirements at cupid level 0x1 register’ecx’
Host bits: 0000:0100:0000:1000:0010:0010:0000:0001
Required: 1000:0100:0000:100x:xxx0:0x1x:xxx0:x001
Mismatch detected for these features:
*General incompatibilities; refer to KB article 1993 for possible solutions.

This issue occurs after upgrading the virtual hardware in the virtual machines
A new virtual machine created on vSphere 4.0 migrates successfully
Resolution
The upgraded virtual machines may have some CPU masks applied which are causing the migration difficulties.

To ensure that VMotion is successful:
Power down the virtual machine.
Click the link to Edit Settings of the virtual machine.
Click the Options tab.
Select CPUID Mask under Advanced.
Click Advanced.
Click Reset All to Default.
Click OK.
Click OK again.
Power on the virtual machine and migrate.
Note: If the issue still exists after trying the steps in this article, file a support request with VMware Support and note this KB article ID in the problem description. For more information, see How to submit a Support Request. For further contact options, see http://www.vmware.com/support/contacts/.