Re: win4lin-like porting of win98 to xen?

[David Hopwood <david.nospam.hopwood@blueyonder.co.uk>]

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.

I'm skeptical; at the very least, anyone trying to do this would need a very
good understanding of Win9x internals to know what it is possible to get away
without virtualizing.

>>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?

Some information is at
<http://www.netraverse.com/support/docs/Win4Lin-whitepaper.php>.

> From what I've read, the 
> kernel patch adds a DOS syscall interface that somehow helps Windows (apps) 
> to run.

The Win4Lin Linux kernel patch is just a tiny part of Win4Lin, to allow the
user-mode part to get at GDTs, LDTs, TSSs etc., plus some event-handling stuff
(you can basically see what it's doing by skimming through the function names
in the patch at
<http://www.netraverse.com/member/downloads/files/mki-adapter26_1_3_7.patch>).

The user-mode part emulates a PC BIOS, and provides network, filesystem,
printing and display services.

> 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.

It replaces the network, filesystem, low-level printing and display drivers;
the rest can be fooled into running in an incomplete virtualization.

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

Yes.

> 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.

I'm not sure this really helps much; for understandable reasons they've tried
to do as much as possible in user mode.

(I have no connection to NeTraverse -- the above is just what I've inferred
from the white paper and the kernel patch.)

-- 
David Hopwood <david.nospam.hopwood@blueyonder.co.uk>




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