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SubjectRe: Virtual Machines, JVM in kernel, hot-swapped kernel
Tomasz Rola <rtomek@cis.com.pl> wrote:
> On Sat, 22 Aug 1998, Brandon S. Allbery KF8NH wrote:
> > In message <Pine.LNX.3.96.980821010156.25221B-100000@localhost>, Tomasz Rola
> > writes:
> > +-----
> > | I've been looking for some online materials about virtual machines on IBM
> > | System 360 and found almost nothing (I know, it was long long ago...). In
> > +--->8
> >
> > S\360 didn't have virtual machines. S\370 did.

Even the 370 didn't until about the 370/155.

> > Basically, the S\370's CPU was capable of completely virtualizing itself.
> > This is actually a fairly rare capability: x86, for example, can only
> > virtualize a subset of itself (80286 real mode).
> >
> > Given a CPU that can completely virtualize itself, software can be written
>
> Do you know how this was done? Speaking in terms of registers etc.
> Although I'm not a hardware guy I can understand a little - and sometimes
> it really helps :-).

I do, but I am reluctant to dump that much material about another
operating system onto this list. Ask me off this list.

> > to present multiple virtual CPUs. In IBM's VM/SP the CP (control program)
> > did this. Each virtual CPU could run a different OS: CMS (think CP/M on a
> > mainframe), MVS, Amdahl UTS (Unix clone), etc. CP also allowed the virtual
> > machines to communicate with each other via simulated card readers/card
> > punches and the RSCS software.
>
> So, correct me if I am wrong, there was also some software that drove
> everything else - the CP. And the CP provided services to everything run
> above it? So it was really CP that made virtuallisation capabilities of
> CPU usable.

Yes.

> Were all those other OSes, like CMS, UTS etc, capable of running on the
> same hardware without CP program?

Sometimes. I understand that CMS used to be able to run standalone,
but by the time I used it, it was dependent on CP. I think UTS can
run stand alone, as can MVS.

In fact, you could sometimes run VM (CP) under VM. Each new version
of VM was generally developed running under the previous version.
You could, for example, run VM/SP version 4 under version 4, and
you could run version 5 under version 4. You could even run
version 4 under version 5. But, if I remember right, there was
some minor problem that would keep you from running version 5
directly under version 5. You could, if you really wanted,
run version 5 under version 4 while that instance of version
4 ran under version 5.

> I mean, were they specially written to
> be used ontop of CP or they were able to run on real hardware?

CMS evolved into something dependent on CP. In fact, CP was dependent
on CMS in a sense; To change the configuration of CP (e.g. to add
a user account) you had to do edit some files under CMS.

> Was it
> possible to run anything without the CP? I suppose that UTS without CP
> could have made simple Unix machine of S/370 - no virtual machines
> posiible in such case.

That is correct.

> The concept of communication going between different virtual machines is
> very cool for me. I haven't thought about it.

It seemed rather mundane, most of the time, in practice.

This is an interresting subject, and to some degree, perhaps
interresting to the kernel developement community; But, I
would caution against getting too carried away with it on
this list. The subject is certainly large enough to dominate
a list in its on right. I'll answer questions from my
memory to anyone who sends them to me.

--
---
L. Adrian Griffis - KE6CSX - adrian@idir.net

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