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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Privilege Levels Below User
Date: Wed, 23 Oct 2024 22:38:40 +0000
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On Sun, 9 Jun 2024 2:23:35 +0000, Lawrence D'Oliveiro wrote:

> On Sat, 8 Jun 2024 17:37:46 +0000, MitchAlsup1 wrote:
>
>> VAX was before common era Hypervisors, do you think VAX could have
>> supported secure mode and hypervisor with their 4 levels ??
>
> “Virtualization” was bandied about in the 1980s more as an idle,
> theoretical concept rather than a practical one.
>
> The question was: was the instruction set defined so that code that was
> designed to run in a privileged mode be run unprivileged, so that any
> attempt to do privileged things would be trapped and emulated by the
> real privileged code? And there was nothing it could do to discover
> it wasn’t running in privileged mode?

My 66000 ISA has this property, and it is used when hypervisors host
hypervisors.

On the other hand, there is only 1 privileged instruction which
provides access to 4 separate control register spaces based on
current Core-Stack level.

> (Obviously performance was not the issue here, but correctness was.)
>
> For example, the VAX had a MOVPSL instruction that allowed read-only
> access to the entire processor status register. Through this,
> nonprivileged user-mode code could discover it was running in user mode,
> which would blow the illusion.

While illustrative, we have entered the realm where processor state
is closer to a cache line in size than a register in size. And the
processor (core) stack of software layers is closer to 4 cache lines
in size.

> The Motorola 680x0 family was I think properly virtualizable in this
> sense. Or maybe the 68020 and 68030 were, but the 68040 was. I think the
> Motorola engineers working on the ’040 asked if any customers were
> interested in preserving the self-virtualization feature, and nobody
> seemed to care.

During 020 development and testing, there was a mode whereby each
instruction executed raised every possible exception--this only found
99% of the virtualization problems.