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SubjectRe: VM modules in kernel?
Mike Harris writes:
> Alan writes:
> >Remember something here. IBM tuned the hardware to this, and to an extent
> >they tuned the software on top of VM. They have a lot of cards to play that
> >Motorola m68K chips did but x86 does not.
> I'm not sure I understand what you're saying here Alan... My
> guess is you're taking VM to mean the kernel virtual memory
> system, when I'm meaning it to be "virtual machine".

You must not have heard of IBM mainframes - one of the mainframe
OSes is CALLED VM (Virtual Machine), and the S/3X0 hardware is fully
virtualisable, so you can run any number of copies of the OS inside
itself (even the Linux s390 port runs under VM).

> >In the Linux case we have another weapon (other than fear suprise and the
> >cuddly penguin) which is that we don't give two hoots[1] if the kernel
> >running on the hypervisor is not a normal x86 kernel. If it provides the
> >APIs and it runs x86 user space its fine.
> The 'hypervisor' is a new term to me.. Are you refering to the
> Transmeta CPU and it's native mode? If I read you correctly
> here, then we could have a native VLIW kernel running on the
> Crusoe, which opens an x86 API to userland? Correct me if I'm
> wrong.

I think the hypervisor is a made-up term, the "supervisor of supervisors
(kernels)". I think what Alan means is that as long as the Linux kernel
APIs are available, and the "CPU" can run x86 code from normal programs,
the kernel itself can be running a totally different machine language.

At one time (long ago) there was talk about an IBM PowerPC 635? which
had a PPC core and a 386 core, and the 386 core was only for x86 program
"emulation", but the OS would run under PPC. I think that Crusoe could
(does?) do this with Linux - with the kernel (maybe libraries too?) running
in native mode, and the user code running either the (presumably slower) x86
mode or the (presumably faster) native VLIW mode.

Whether Crusoe does this now, or in the future, I don't know. Maybe it is
running x86 mode just to test that the x86 mode is very compatible (needed
until Windows goes away) and it will slowly migrate to native VLIW mode as
applications/compilers support it.

Cheers, Andreas
Andreas Dilger \ "If a man ate a pound of pasta and a pound of antipasto,
\ would they cancel out, leaving him still hungry?" -- Dogbert

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