linux-kernel - Re: Questions on one PowerPC assembly instruction from hash

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Message-Id: <200706180857.l5I8vT6H000980@harpo.it.uu.se>
Date:	Mon, 18 Jun 2007 10:57:29 +0200 (MEST)
From:	Mikael Pettersson <mikpe@...uu.se>
To:	gshan@...atel-lucent.com, linux-kernel@...r.kernel.org
Cc:	jinminc@...atel-lucent.com
Subject: Re: Questions on one PowerPC assembly instruction from hash_page

On Mon, 18 Jun 2007 16:33:22 +0800, gshan <gshan@...atel-lucent.com> 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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