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From: "Ahmed S. Darwish" <>
Subject: [i386] Questions regarding provisional page tables initialization
Date: Sun, 1 Jul 2007 23:38:33 +0300
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 */
	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
	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 ?


Ahmed S. Darwish

             reply index

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 ` 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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