From: "Ahmed S. Darwish" <darwish.07@gmail.com>
To: linux-kernel@vger.kernel.org
Subject: [i386] Questions regarding provisional page tables initialization
Date: Sun, 1 Jul 2007 23:38:33 +0300 [thread overview]
Message-ID: <20070701203833.GA3498@Ahmed> (raw)
Hi list,
AFAIK, in the initializaion phase, kernel builds pages tables with two
mappings, identity and PAGE_OFFSET + C mapping. The provisional _global
directory_ is contained in swapper_pg_dir variable. while the provisional
_page tables_ are stored starting from pg0, right after _end.
There're some stuff that confused me for a full day about the code (head.S)
that accomplishes the above words:
movl $(pg0 - __PAGE_OFFSET), %edi
movl $(swapper_pg_dir - __PAGE_OFFSET), %edx
movl $0x007, %eax /* 0x007 = PRESENT+RW+USER */
10:
leal $0x007(%edi),%ecx /* Create PDE entry */
What does the address of 7 bytes displacement after %edi - the physical address
of pg0 - represent ?. Why not just putting the address of %edi (the address of
pagetable cell to be mapped by swapper_pg_dir) in %ecx without displacement?
page_pde_offset = (__PAGE_OFFSET >> 20)
movl %ecx,(%edx) /* Store identity PDE entry */
movl %ecx,page_pde_offset(%edx) /* Store kernel PDE entry */
Why the pde_offset is PAGE_OFFSET >> 20 instead of PAGE_OFFSET >> 22 ?
* 22 to right shift the whole page_shift (12) and pgdir_shift (10) bits.
[...]
/* Initialize the 1024 _page table_ cells with %eax (0x007) */
movl $1024, %ecx
11:
stosl
addl $0x1000,%eax
loop 11b
The page table entries beginning from pg0 (pointed by %edi) and following pages
are initialized with the series 7 + 8 + 8 + ... for each cell. This series has
the property of setting the PRESENT+RW+USER bits in the whole entries to 1 but it
sets lots of the entries BASE address to 0 too. Why is this done ?
Thanks,
--
Ahmed S. Darwish
HomePage: http://darwish.07.googlepages.com
Blog: http://darwish-07.blogspot.com
next reply other threads:[~2007-07-01 20:39 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-07-01 20:38 Ahmed S. Darwish [this message]
2007-07-01 22:02 ` [i386] Questions regarding provisional page tables initialization Andreas Schwab
2007-07-01 22:19 ` Jeremy Fitzhardinge
2007-07-02 1:13 ` Ahmed S. Darwish
2007-07-02 9:18 ` Andreas Schwab
2007-07-02 10:23 ` Ahmed S. Darwish
2007-07-02 11:34 ` Brian Gerst
2007-07-02 15:43 ` Ahmed S. Darwish
2007-07-02 10:43 ` Andi Kleen
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