6.5.1

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 aempt to migrate a virtual machine with vMotion, one of the following scenarios applies:
n
The destination host feature set matches the virtual machine’s CPU feature set. CPU compatibility
requirements are met, and migration with vMotion proceeds.
n
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.
N EVC overcomes such incompatibility by providing a "baseline" feature set for all virtual
machines running in a cluster. This baseline feature set hides the dierences among the clustered hosts'
CPUs from the virtual machines.
n
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.
N This type of incompatibility is ignored for migrations among hosts in EVC clusters.
n
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 while 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. This process might cause vMotion
incompatibility if you aempt to migrate the virtual machine back to the original host.
CPU Families and Feature Sets
Processors are grouped into families. Processors within a given family generally have similar feature sets.
Processor vendors dene processor families. You can distinguish dierent processor versions within the
same family by comparing the processors’ model, stepping level, and extended features. Sometimes,
processor vendors have introduced signicant architectural changes within the same processor family, such
as the SSSE3 and SSE4.1 instructions, and NX/XD CPU security features.
By default, vCenter Server identies mismatches on features accessible to applications as incompatible to
guarantee the stability of virtual machines after migrations with vMotion.
Server hardware’s CPU specications usually indicate whether or not the CPUs contain the features that
aect vMotion compatibility.
For more information on identifying Intel processors and their features, see Application Note 485: Intel
®
Processor Identication and the CPUID Instruction, available from Intel. For more information on identifying
AMD processors and their features, see CPUID Specication, available from AMD.
About Enhanced vMotion Compatibility
You can use the Enhanced vMotion Compatibility (EVC) feature to help ensure vMotion compatibility for
the hosts in a cluster. EVC ensures that all hosts in a cluster present the same CPU feature set to virtual
machines, even if the actual CPUs on the hosts dier. Using EVC prevents migrations with vMotion from
failing because of incompatible CPUs.
Congure EVC from the cluster seings dialog box. When you congure EVC, you congure all host
processors in the cluster to present the feature set of a baseline processor. This baseline feature set is called
the EVC mode. EVC uses AMD-V Extended Migration technology (for AMD hosts) and Intel FlexMigration
technology (for Intel hosts) to mask processor features so that hosts can present the feature set of an earlier
generation of processors. The EVC mode must be equivalent to, or a subset of, the feature set of the host
with the smallest feature set in the cluster.
vCenter Server and Host Management
126 VMware, Inc.