Re: How did paging_init ever work with PAE?

[Hugh Dickins <hugh@veritas.com>]

On Tue, 30 Jul 2002, Martin J. Bligh wrote:
> We're crashing in paging_init under certain circumstances, but on
> closer inspection, I'm unsure how this ever could have worked.
> Obviously it does most of the time, so I'm missing something ....
> 
> With PAE on, the code path looks like this:
> 
>         pagetable_init();
>         load_cr3(swapper_pg_dir);
>         if (cpu_has_pae)
>                 set_in_cr4(X86_CR4_PAE);
> 
> Hmmm .... pagetable_init sets up a PGD for PAE use, then we load
> cr3 with this table .... then we turn on PAE mode.
> 
> How are we surviving in this limbo state between the point when
> we reload cr3 and when we turn on PAE? If we take a page fault
> (which we will, since reloading cr3 flushes the tlb) is the PGD 
> somehow dual purpose and works for non-PAE systems as well?

Yes.

> Are we relying on the global bit on entries on the TLB cache which
> we're just praying aren't going to fall out?

No.

See swapper_pg_dir in arch/i386/kernel/head.S: at swapper_pg_dir+0xc00
there's a couple of 32-bit non-PAE entries to map 0xc0000000-0xc07fffff
where the kernel is; whereas pagetable_init sets up four 64-bit PAE
entries at swapper_pg_dir (overwriting the non-PAE identity mappings,
yes, but they won't be needed during changeover from non-PAE to PAE).

Hugh

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