[i386] Questions regarding provisional page tables initialization

Ahmed S. Darwish
2007-07-01T20:39:46Z

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