Messages in this thread | | | Date | Sun, 01 Jul 2007 15:19:30 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [i386] Questions regarding provisional page tables initialization |
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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".
> 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.
> [...] > /* 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?
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