CPU compatibility masks allow per-virtual machine 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 to allow or not migrations with vMotion.
Default values for the CPU compatibility masks are set by VMware to guarantee the stability of virtual machines after a migration with vMotion.
In some cases, where 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 through the Advanced Settings option for the CPU of the virtual machine. For more control over the visibility of CPU features, you can edit CPU compatibility mask of the virtual machine at the bit level.
Changing the CPU compatibility masks can result in an unsupported configuration. Do not manually change the CPU compatibility masks unless instructed to do so by VMware Support or a VMware Knowledge base article.
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.