vCenter Server's CPU compatibility checks compare the CPU features available on the source host, the subset of features that the virtual machine can access, and the features available on the target host. Without the use of EVC, any mismatch between two hosts' user-level features will block migration, whether or not the virtual machine itself has access to those features. A mismatch between two hosts' kernel-level features, however, blocks migration only when the virtual machine has access to a feature that the target host does not provide.

User-level features are non-privileged instructions that might be used by virtual machine applications. These include SSE3, SSSE3, SSE4.1, SSE4.2, and AES. Because they are user-level instructions that bypass the virtualization layer, these instructions could cause application instability if mismatched after a migration with vMotion.

Kernel-level features are privileged instructions that might be used by the virtual machine operating system. These include the AMD No eXecute (NX) and the Intel eXecute Disable (XD) security features.

When you attempt to migrate a virtual machine with vMotion, one of the following scenarios applies:

  • The destination host feature set matches the virtual machine’s CPU feature set. CPU compatibility requirements are met, and migration with vMotion proceeds.

  • The virtual machine’s CPU feature set contains features not supported by the destination host. CPU compatibility requirements are not met, and migration with vMotion cannot proceed.

    Note:

    EVC overcomes such incompatibility by providing a "baseline" feature set for all virtual machines running in a cluster and that hides the differences among the clustered hosts' CPUs from the virtual machines.

  • The destination host supports the virtual machine’s feature set, plus additional user-level features (such as SSE4.1) not found in the virtual machine’s feature set. CPU compatibility requirements are not met, and migration with vMotion cannot proceed.

    Note:

    This type of incompatibility is ignored for migrations among hosts in EVC clusters.

  • The destination host supports the virtual machine’s feature set, plus additional kernel-level features (such as NX or XD) not found in the virtual machine’s feature set. CPU compatibility requirements are met, and migration with vMotion proceeds. The virtual machine retains its CPU feature set as long as it remains powered on, allowing it to migrate freely back to the original host. However, if the virtual machine is rebooted, it acquires a new feature set from the new host, which might cause vMotion incompatibility if you attempt to migrate the virtual machine back to the original host.