Re: [Xen-devel] 2.6.20.3 to be the next kernel?

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Mark Williamson wrote:
> I was wondering how you handle PAE in pv-ops.  Presumably you just toggle 
> based on what the kernel was built for?  It seems like PV-ops ought in 
> principle to be able to make the kernel bimodal.  I'm guessing the 
> appropriate response to my question is probably a simple "patches 
> welcome" ;-)

Indeed.

Actually, its something I gave some thought to, but unfortunately it
isn't that simple.  The main problem is that the types of
pte_t/pmd_t/pgd_t change from 32 to 64 bit, and all the pmd folding
stuff is still done at compile-time.

I was also thinking about always using the PAE forms of the structures,
and actually do the conversion just as we read/write the entries.  But
even then, the various pagetable accessors/traversal functions know how
large the entries are and how many levels the pagetables have, etc.

The only slightly workable approach I thought of was to maintain a
sort-of in-kernel shadow pagetable scheme, which maintains parallel PAE
and non-PAE pagetables.  But that's hardly elegant, and poses all sorts
of its own problems (like propagating the hardware-set access/modified
bits properly, for example).

So, its all a bit of an open question.

    J

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