VMware ESX: Enhanced VMotion Compatibility

The classic VMotion problem of recent times is the customer who buys tin from a vendor, only to find out that the processor number e.g. 53xx and 54xx, is actually quite significant. The principle difference which will prevent VMotion is the addition of SSE4.1 to the 54xx range of Intel processors (if you previously only had 53xx Intel processors).

An application using the SSE4.1 feature (or is aware of this feature) when VMotioned to a non-SSE4.1 host would likely blue screen trying to make use of this feature on the older host.

People then recalled the CPU mask feature in VMware - we’ll just mask it out they thought. Unfortunately VMware declared this was an unsupported mask with KB1993 and KB1991.

Note that for production environments, VMware neither supports nor recommends modifying VMotion masks for SSE3, SSE3, or SSE4.1 because of the risk of failure of the application or guest operating system after migration.

The reason soon became clear:

* SSE features can be used by user-level code (applications).
* Mask does not work for user-level code (i.e. applications).
* In user-level code, CPUID is executed directly on hardware and is not intercepted by VMware.
* Thus, VM cannot reliably hide SSE from an application

Good stuff, find out more at the source blog.