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From: karger@osf.org (Paul A. Karger)
Newsgroups: comp.os.mach
Subject: Re: Mach as a Virtual Machine
Keywords: Mach, Virtual Machine OS
Message-ID: <21336@paperboy.OSF.ORG>
Date: 23 Apr 91 19:26:49 GMT
References: <1991Apr19.180259.18834@cs.cmu.edu>
Sender: news@OSF.ORG
Organization: Open Software Foundation
Lines: 29

While several postings have described Mach as not being a
virtual machine monitor in sense of VM/370, it most certainly
does provide the basic tools for implementing a "hybrid
virtual machine system" as defined in Popek and Goldberg's
paper, "Formal Requirements for Virtualizable Third Generation
Architectures" in the July 1974 issue of CACM.

A hybrid virtual machine system allows more instructions to
be emulated, rather than directly executed.   If you combine
that with many modern architectures that, unlike the 360/370,
do not specify an I/O architecture, you find that Mach could
easily support a VMM and do it more efficiently than a 
traditional VMM.   In particular, if the CPU architecture allows
CPU-model dependent I/O interfaces, as the VAX does, then
you can define a VMM with an I/O interface that is a set
of kernel calls, rather than actual I/O commands on a bus.

For an example of such a VMM, see "A VMM Security Kernel
for the VAX Architecture" in the Proceedings of the
1990 IEEE Symposium on Research in Security and Privacy.
The VMM described there was not based on Mach, but could have
been.  (I was one of the co-authors of the paper, so I'm
quite sure that Mach could easily have supported the system.)

The Mach 3.0 microkernel is not a VMM by itself.  However
with a set of judiciously written servers, it can certainly
support a hybrid VMM.

Paul