Re: [Xen-users] 64-bit guest in 32-bit Xen/dom0 - ever?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > .. and maybe supporting  32-bit kernels on 64-bit Xen are rather more
> > likely. ..
>
> I haven't been working/testing under 64-bit mode at all, but this is not
> yet possible? I thought it was a feature discussion brought about it
> months ago.

The interface for 64-bit Xen is rather different, because it's tailored to the 
characteristics of the x86_64 architecture (for instance, the fact that it's 
not possible to use segmentation for protection in the same way complicates 
the task of protecting Xen from kernels and apps...).

The most promising way of dealing with this is probably to implement a "shim" 
that will adapt hypercalls by the 32-bit kernel to work correctly with a 
64-bit Xen.  The domain builder would be able to insert this shim when 
building a legacy 32-bit domain to enable it to run correctly on Xen64.  
There'll likely also need to be some Xen support.

It certainly seems doable, but there's not yet been enough motivation to 
enable mixed platforms for paravirtualised guests.  If you really need 32-bit 
(or PAE) guests on 64-bit, it should be possible to run them unmodified in a 
VT-x / SVM virtual machine.

HTH,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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