Virtual machines

Terrence W. Zellers
Thu, 22 May 1997 19:59:55 +0000

.> >> | From: Terrence W. Zellers <>
> >> |  
> >> |    IMO it ***MUST*** be capable of fully virtualizing itself in
> >> | execution, not just emulation.   This means it must have a
> >> paging | prefix algorithm, the ability to intercept any
> >> instructions which

.> >    The x86 architecture is NOT self virtualizable, no, but we're
> >discussing an idealized VM to be emulated.  That can be done, of
> >course, though at greater overhead than true on virtualizable
> >hardware. A fair amount of the FREEVM discussion touched on that.

Jim replied:
> The x86 VM (386 and up) *is* self virtualizable using exactly the
> same techniques as the IBM VM architecture (memory and i/o
> mapping/paging/trapping, exception trapping and emulation, privilege
> limits/exceptions).

Are you certain?   I haven't studied the matter but I've been told
that the 32 bit implementations can virtualize the 16 bit instruction
sets ('86, '186, '286) but that they cannot fully virtualize themselves.

> It is interesting to note that, AFAIK, no commercial OS for the x86
> bothers to support self virtual operation.  They all use it for
> security sandboxes.

We'd be whistling away building fVM if we had a hint of a clue that
it was possible on an x86 platform.  Basically it was the perception
that a requisite intermediate emulation layer (architecture unspecified)
would make the project too big, (and the resulting platform too slow)
to make it worth doing.  If someone who understands what *complete*
self virtualization means can tell me that it *is*  possible (Hmm,
the description of PUSHFD doesn't say anything about throwing an
interrupt - ie a guest can determine his state without the supervisor
being able to fake him out; doesn't look virtualizable to me) I'll 
have to dig into the x86 architecture in detail myself - something I 
have a kind of minor perverse pride in having avoided these many 
years ;-) 

But you folks seem willing to tackle the project despite the 
emulation thang - or more properly because of the prospect of
universality of application.   More power to you!  I'm just 
suggesting a property for your idealized architecture. 

                                                      -- TWZ       
| Terrence W. Zellers  | "Careful what you carry 'cause |
| | the man is wise, you ars still |
|                      | an outlaw in their eyes..."    |