🚀 go-pugleaf

In article <3B6587F1...@us.ibm.com>,
Greg Pfister <pfi...@us.ibm.com> writes:
|> 
|> Related -- Once upon a time that same friend told me that you
|> actually have to be able to do three levels of virtualization of
|> memory to be general -- the top-level VM system just doing it is
|> not enough; you have to be able to nest a VM system virtualizing
|> what the top-level system is also virtualizing, and a third level
|> below that. I never did understand why the third level was
|> necessary, but this guy was very, very good and knew that code
|> intimately, so he was probably right.

It's much the same reason that you have to compile a new version
of a compiler with itself three times before you can check it!

In order to be sure that a VM will run at any level, it has to
be able to provide a fully-functional virtual machine while itself
using only virtualised functionality.

In actual fact, there are bugs that can creep by the 3-level test
for both compilers and VMs, but they are mind-bogglingly obscure
by comparison with the ones that adding the third level picks up.


Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QG, England.
Email:  nm...@cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679