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Table Of Contents
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ESX/ESXi version running on the host
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The virtual machine's compatibility setting
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The virtual machine's guest operating system
To improve CPU compatibility between hosts of varying CPU feature sets, some host CPU features can
be hidden from the virtual machine by placing the host in an Enhanced vMotion Compatibility (EVC)
cluster.
Note You can hide Host CPU features from a virtual machine by applying a custom CPU compatibility
mask to the virtual machine, but this is not recommended. VMware, in partnership with CPU and
hardware vendors, is working to maintain vMotion compatibility across the widest range of processors.
For additional information, search the VMware Knowledge Base for the vMotion and CPU Compatibility
FAQ.
CPU Compatibility Scenarios
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 blocks migration, whether
or not the virtual machine itself has access to those features. A mismatch between two hosts' kernel-level
features 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 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 can cause application instability if mismatched after a migration with
vMotion.
Kernel-level features are privileged instructions 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:
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The destination host feature set matches the virtual machine’s CPU feature set. CPU compatibility
requirements are met, and migration with vMotion proceeds.
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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. This baseline feature set hides the differences among the clustered
hosts' CPUs from the virtual machines.
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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.
vCenter Server and Host Management
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