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

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Message-ID: <46773D08.8070103@alcatel-lucent.com>
Date:	Tue, 19 Jun 2007 10:18:48 +0800
From:	gshan <gshan@...atel-lucent.com>
To:	Mikael Pettersson <mikpe@...uu.se>
CC:	linux-kernel@...r.kernel.org, jinminc@...atel-lucent.com
Subject: Re: Questions on one PowerPC assembly instruction from hash_page

Mikael Pettersson wrote:
> 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
>   
Mikael, Thanks a lot. I understood why the instruction is there. So the 
0-11 bits are used as address index for PMD. swapper_pg_dir is 4KB large 
and there are 1K PMDs inside swapper_pg_dir. So 10 bits (0-9 bits) are 
used as index for PMD, right?
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