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Re: [Xen-devel] 18.104.22.168 to be the next kernel?
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" ;-)
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.
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