Intel 64 and IA-32 Architectures Software Developers Manual Volume 3B, System Programming Guide Part 2

Vol. 3 27-13
VIRTUALIZATION OF SYSTEM RESOURCES
the entire guest memory buffer (which contains the microcode update image) will not
cause a page fault when accessed.
If the VMM loads the microcode update, then the VMM must have access to the
current set of microcode updates. These updates could be part of the VMM image or
could be contained in a separate microcode update image database (for example: a
database file on disk or in memory). Again, maintaining a separate microcode update
image database has the advantage of reducing the number of required VMM or OS
releases as a result of microcode update releases.
The VMM may wish to prevent a guest from loading a microcode update or may wish
to support the microcode update requested by a guest using emulation (without
actually loading the microcode update). To prevent microcode update loading, the
VMM may return a microcode update signature value greater than the value of
IA32_BISO_SIGN_ID MSR. A well behaved guest will not attempt to load an older
microcode update. The VMM may also drop the guest attempts to write to
IA32_BIOS_UPDT_TRIG MSR, preventing the guest from loading any microcode
updates. Later, when the guest queries IA32_BIOS_SIGN_ID MSR, the VMM could
emulate the microcode update signature that the guest expects.
In general, loading a microcode update later will limit guest software’s visibility of
features that may be enhanced by a microcode update.