On Mon, 18 Jun 2007 16:33:22 +0800, gshan <[email protected]> wrote:
> I can't understand the following instructions from
> arch/ppc/mm/hashtable.S::hash_page. If I got the right design, the
> following instruction is to get the PMD (Page Middle Descritor) because
> Linux for 32-bits PowerPC cut page table into 3 domains: root, PMD, PTE.
> The top bits (22 to 31 bit) is the index for PMD, and the next 10 bits
> (12 to 21 bit) is the index for PTE in the associative PMD. The
> remaining 12 bits (0 to 11 bit) indicated the page size (4KB). However,
> the following instruction polled [8-17] bits instead of [22-31] bits as
> expected. Anybody could give me answer?
>
> r4 is the address that caused the DSI
> r5 is the address of swapper_pg_dir if we are under kernel mode.
>
> rlwimi r5,r4,12,20,29 /* insert top 10 bits of address */
POWER/PowerPC has an insanely broken bit numbering scheme, in
which the most significant bit has number 0 (or is it 1?),
and the least significant bit has number N-1 (or is it N?)
where N is number of bits in a word.
The fact that you refer to the top bits as 22-31 makes me
suspect that you haven't compensated for this quirk.
/Mikael
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