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Re: [Xen-devel] running Windows (albeit slowly)

To: "Eric S. Johansson" <esj@xxxxxxxxxx>
Subject: Re: [Xen-devel] running Windows (albeit slowly)
From: Steven Hand <Steven.Hand@xxxxxxxxxxxx>
Date: Wed, 24 Nov 2004 13:27:50 +0000
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> forgive me if this has been beaten to death but I've been thinking again 
> about the problem of running Windows as a guest OS under xen.  I 
> understand and accept all the arguments of why you need to modify the 
> core OS for your virtualization process.
> Under the opinion that there is no crime if optimization fails on 
> incorrect code, would it not be appropriate to do all the things 
> necessary to run Windows unmodified albeit poorly slash slowly?
> assuming of course that people want to run Windows.  The only reason I 
> do is because of speech recognition requirements but that's a very 
> different conversation.

To run unmodified arbitrary OS images you need to be able to 

   a) provide 'shadow mode' page tables and segmentation 
   b) trap and virtualize trappable instructions, 
   c) dynamically scan code for untrappable instructions
      and rewrite. 

We do most of (a) already; doing (b) is not hard but not done. 
Doing (c) is tedious, difficult and fragile. 

We're happy for anyone out there to have a go at this, but
we're unlikely to put any effort into it ourselves at this 
time. There may be hardware support in the future which makes
(a) and (b) easier and (c) disappear; or MS may themselves
produce a version of XP which doesn't require (c). Who knows 
if and when either of these will happen tho...



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