Re: [i386] Questions regarding provisional page tables initialization

Ahmed S. Darwish
2007-07-02T01:13:49Z

Hi Jeremy,

On Sun, Jul 01, 2007 at 03:19:30PM -0700, Jeremy Fitzhardinge wrote:
> Ahmed S. Darwish wrote:
> >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?
> >
>
> The pte format contains the pfn in the top 20 bits, and flags in the
> lower 12 bits. As the comment says "0x007 = PRESENT+RW+USER".
>

yes, but isn't the displacement here (0x007) a _bytes_ displacement ?. so
effectively, %ecx now contains physical address of pg0 + 7bytes. Is it A
meaningful place/address ?.

> > 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.
> >
>
> As Andreas said, its (PAGE_OFFSET >> 22) << 2.
>

Great!, Thanks a lot.

> > [...]
> > /* 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 ?
> >
>
> I don't follow you. Are you overlooking the 'L' on stosl?
>

Sorry for not making the question clear. my question was that the first entry in
the page table pointed by (%edi) is initialize with %eax = 0x007, a reasonable
value (setting the 3 pte flags). Beginning from entry 2, they got initialized
with a value = "new %eax = old %eax + 8", generating a table of entries
initialized with 7, 15, 31, .. . While this scheme makes the 3 PRESENT, RW
and USER flags set, it makes alot of "pte"s with equivalent "pfn"s. Here comes
my wonder, why initializing pg0 that way ?.

Thanks,

--
Ahmed S. Darwish
HomePage: http://darwish.07.googlepages.com
Blog: http://darwish-07.blogspot.com