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Re: [Qemu-devel] Perhaps a Virtual CPU for host?

From: Jim C. Brown
Subject: Re: [Qemu-devel] Perhaps a Virtual CPU for host?
Date: Sat, 17 Jul 2004 21:56:24 -0400
User-agent: Mutt/1.4i

On Sat, Jul 17, 2004 at 08:42:13PM -0500, syeng wrote:
> I'm a lurker in this list and I'm not a developer, so please excuse me if
> this isn't a good idea.
> I've been wondering... Since Qemu is a binary translator, would it be
> reasonable to have Qemu generate code for a virtual cpu instead of a real
> cpu?
> Then the program would run the virtual cpu emulator, which runs the code
> generated by qemu.
> In other words... No more back-ends for PPC, x86, Sparc, etc. etc.  Porting
> to a new system would be easier, too.
> Now, I know it sounds pretty bad.
> BUT, I was thinking that if you chose the right virtual cpu architecture,
> you might actually be able to get pretty decent performance out it.  I've
> heard of cases where virtual cpu's ran benchmarks pretty well because each
> instruction was easily decoded and each instruction did a lot of 'work'
> (CISC vs. RISC.  For a virtual cpu, CISC style cpu's are better, I guess.)
> I don't know how efficient the code is that Qemu generates, but it certainly
> wouldn't be extremely efficient.  Perhaps a carefully chosen virtual cpu
> architecture might make up for some of that, and perhaps run as fast as half
> the speed of a native Qemu port?
> If nothing else, it might provide a generic base for currently unsupported
> host systems.
> Any comments?

It should be fairly simple to modify the current qemu-system-i386 code and
make a qemu-system-a386 version. The a386 (http://a386.nocrew.org/) is a virtual
abstraction of the traditional i386 instruction set implemented in C, but it is
slightly cleaner.

a386 hasn't been updated in a while but its open source, so if you are really
interested this is a good place to start.

Infinite complexity begets infinite beauty.
Infinite precision begets infinite perfection.

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