CPU identification (CPU ID) masks control the CPU features visible to the virtual machine's guest operating system. Masking or hiding CPU features can make a virtual machine widely available to ESXi hosts for migration. 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 disallow migration with vMotion.

For example, masking the AMD No eXecute (NX) and the Intel eXecute Disable (XD) bits prevents the virtual machine from using these features, but provides compatibility that allows you to migrate virtual machines to ESXi hosts that do not include this capability. When the NX/XD bit is visible to the guest operating system, the virtual machine can use this feature, but you can migrate the virtual machine only to hosts on which the feature is enabled.

Note: You rarely need to change the CPU identification mask configuration settings. Almost all changes are made only to the NX/XD bit.

See the vCenter Server and Host Management documentation for detailed information about vMotion compatibility and CPU masks.


  • Verify that you have access to the virtual machine in the vSphere Client inventory list.
  • Power off the virtual machine.


  1. In the vSphere Client inventory, right-click the virtual machine and select Edit Settings.
  2. Click the Options tab and under Advanced, select CPUID Mask.
  3. In the CPU Identification Mask panel, select an NX flag option.
    Option Description
    Hide the NX/XD flag from guest Increases vMotion compatibility.

    Hiding the NX/XD flag increases vMotion compatibility between hosts, but might disable certain CPU security features.

    Expose the NX/XD flag to guest Keeps all CPU security features enabled.
    Keep current Advanced setting values for the NX/XD flag Uses the NX/XD flag settings specified in the CPU Identification Mask dialog box. Enabled only when current settings specify something other than what is specified in the other NX/XD flag options, for example, if the NX/XD flag bit setting varies with processor brand.
  4. (Optional) To edit mask values other than the NX bit or to set NX mask values to states other than “0” or “H”, click Advanced.
    1. Select the relevant tab.
    2. Click a row and edit the mask value.
      To view an explanation of a values symbol, click Legend.
    3. Click OK to apply the changes and return to the Virtual Machine Properties dialog box.
  5. Click OK to save your changes.