CPU compatibility masks allow customization of the CPU features visible to a virtual machine.
vCenter Server compares the CPU features available to a virtual machine with the CPU features of the destination host to determine whether or not to allow migrations with vMotion.
To guarantee the stability of virtual machines after a migration with vMotion, VMware sets the default values for CPU compatibility masks.
When a choice between CPU compatibility or guest operating system features (such as NX/XD) exists, VMware provides check-box options to configure individual virtual machines. You can access the configuration options through the Advanced Settings option for the CPU of the virtual machine. For more control over the visibility of CPU features, you can edit the CPU compatibility mask of the virtual machine at the bit level.
CPU compatibility masks cannot prevent virtual machines from accessing masked CPU features in all circumstances. In some circumstances, applications can detect and use masked features even though they are hidden from the guest operating system. In addition, on any host, applications that use unsupported methods of detecting CPU features rather than using the CPUID instruction can access masked features. Virtual machines running applications that use unsupported CPU detection methods might experience stability problems after migration.