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Open Kernel Labs Blog
September 07, 2008
Is there a difference between a virtual-machine monitor and a hypervisor?
Yes and no...
The terms both go back to the '60s (when CS terminology was less standardised than today), and traditionally refer to the same thing: the lowest (and most privileged) software layer implementing virtual machines. "Hypervisor" was IBM's term, others used virtual-machine monitor (VMM).
With the recent trend towards a microkernel-ish design of VMMs (see my earlier blog), the architecture of VMMs is changing. While it used to be the norm that all VMM functionality (including device drivers) was part of one monolithic kernel, increasingly components are moved outside the kernel. The Xen "Dom0" device-driver guest OS is a (somewhat coarse) example. In such a context, there is a tendency to distinguish between the privileged (kernel-mode or "hypervsior-mode") core functionality, and the de-privileged (user-mode or non-hypervsior-mode) rest. In that case, the term "hypervisor" is frequently used to refer just to the privileged bit.
Why then not call the privileged-mode part "microkernel" and be done with it? Well, some people do, Xen is increasingly referred to as "microkernel-like" — despite the Xen authors claiming their system is different from a microkernel. In fact, they are right: Xen is way too big and complex to qualify as a microkernel.
So, think of a "hypervisor" as the privileged part of the VMM — the part that should be a microkernel in a clean architecture. Which, of course, it is in OKL4 ;-)
Posted by Gernot Heiser on September 07 at 10:48 PM
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About Gernot Heiser:
Gernot Heiser, Co-founder and Consulting Scientist, never thought he would be in the business world. Prior to NICTA's creation in 2003, Dr Heiser was a full-time faculty member at the University of New South Wales. However, this die-hard academic couldn’t pass up the opportunity to see the commercialization of this research. Gernot still loves teaching, almost as much as he loves good wine and good food. And anyone will tell you that Gernot knows his wine.