Re: win4lin-like porting of win98 to xen?

[Avery Pennarun <apenwarr@nit.ca>]

On Mon, Oct 11, 2004 at 01:38:10AM +0100, Mark A. Williamson wrote:

> > > That suggests using some kind of Xen-compatible kernel that can provide
> > > the DOS syscalls Win 9x needs to run.
> >
> > This certainly won't work. Despite their reliance on DOS for some
> > drivers, Win9x are not strictly layered on DOS; they are 32-bit
> > protected-mode OSes that also rely heavily on direct hardware access.
> 
> Hmmm.  Some tweaks to shadow page tables could be used to trap attempts to
> load the page table base register, so the memory management code would
> work.  Of course IO port accesses would be prevented by Xen completely but
> this could be worked round.

Clearly something must work, because win4lin runs fine (and very fast!)
under Linux, and I'm v*ery* sure it's not allowed to randomly probe I/O
ports there.

> > Duplicating the approach used by Win4Lin is also not a feasible amount of
> > work; it's almost a rewrite of the lower layers of Win9x.
> 
> What bits of Win 9x does Win4Lin actually replace?

At least: video driver, input drivers, sound drivers, network drivers,
parallel/serial port drivers, filesystem.  It doesn't emulate a block device
because it replaces the filesystem entirely.

> From what I've read, the kernel patch adds a DOS syscall interface that
> somehow helps Windows (apps) to run.  Presumably they don't reimplement
> all functionality of the win9x kernel-layer (the thought makes me
> shiver!).  I've never looked at the Win4Lin patch but it would be
> interesting to know exactly where it hooks into Windows Land.

The kernel patch, while mildly interesting, is definitely not the hardest
part of win4lin.

> For win9x under Xen I think that getting Win4Lin running under XenLinux
> would be the most straightforward way of doing things.

Yes.

> Is the kernel patch GPL?  If so, the commercial bit must be the userland
> software and the device drivers, right?  Given that, the quickest way to a
> free Win4Lin would be to reuse their kernel patch and rewrite the tools /
> drivers...  Still a nontrivial piece of work.

A free win4lin (ie. rewriting all those drivers) would be a *massive* amount
of work.  Fun, but a lot of work.  And it gets more and more obsolete each
day.  Lots of stuff still runs under win98, but it's certainly not a target
platform for modern developers.

Have fun,

Avery


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