Re: [Xen-users] 32bit domU in 64 bit dom0

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > [*] I think it's a shame we don't support 32-bit non-PAE
> > guests on 64-bit Xen.
> > I'm told most of the machinery should be there already.  As
> > far as I'm aware,
> > only the Linux port supports PAE, although I may be behind on this
> > information.
>
> The main reason for this is that the 64-bit hypervisor uses 4-level
> pagetables, and 32-bit PAE uses 3-level page-tables. Both 4- and 3-level
> page-tables use 64-bit entries in the table. However, 32-bit non-PAE
> uses 32-bit page-table entries. I don't think it's quite so easy to
> switch to 32-bit non-PAE page-tables when running a 64-bit hypervisor -
> it's probably POSSIBLE, but it makes life a lot harder.

Yep, AFAIK the 32-bit PAE page tables are just hooked onto the host's 4-level 
page table (with Xen being mapped above the guest's accessible memory using 
an entry in the PUD).

Well, the fix would probably be to use shadow page tables - I've heard this 
suggested a few times as the means by which this would be supported if there 
was sufficient motivation (which there isn't ... yet, at least).  Shadow 
pagetables for 32-on-64 already sort-of work since they're supported for HVM.  
Code would need to be added to handle 32-bit PV guests, but beyond that I 
don't think there's much required to make non-PAE guests work. (?)

> Since we're 
> talking about PV-guests in the first place, it's not too much to ask
> that the guest is compiled for PAE mode rather than non-PAE - it would
> be different if it was HVM guest that don't necessarily have easy access
> to source-code...

Well, yes and no.  Some folks may want to migrate legacy guests from a 32-bit 
non-PAE host - that was our default install for a long time.  Also, I suspect 
the NetBSD, Plan9 and FreeBSD guest ports don't all support PAE (if any of 
them do).  non-PAE 32-bit on the 64-bit hypervisor is the one thing keeping 
it from being able to "do anything" and virtualise a superset of the guests 
supported by other hypervisors.  Not critical, though.

Cheers,
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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