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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
