Re: i386 and PAE: pud_present()

["Magnus Damm" <magnus.damm@gmail.com>]

On 4/30/06, Dave McCracken <dmccr@us.ibm.com> wrote:
>
> --On Friday, April 28, 2006 10:27:21 +0200 Andi Kleen <ak@suse.de> wrote:
>
> >> Take a look a little further down the page for the comment.
> >>
> >> In i386 + PAE, pud is always present.
> >
> > I think his problem is that the PGD is always present too (in
> > pgtables-nopud.h) Indeed looks strange.
>
> The PGD is always fully populated on i386 if PAE is enabled.  All three of
> the pmd pages are allocated at page table creation time and persist till
> the page table is deleted.

The following code snippet is from some kexec patch of mine. The
function is used to build a new set of page tables which are used when
jumping to the new kernel.

The code should be pretty archtecture independent - the same code
works on x86 and x86_64. And x86/PAE with a workaround.

#ifdef CONFIG_X86_PAE
#undef pud_present
#define pud_present(pud) (pud_val(pud) & _PAGE_PRESENT)
#endif

#define pa_page(page) __pa(page_address(page))

static int create_mapping(struct page *root, struct page **pages,
			  unsigned long va, unsigned long pa)
{
	pgd_t *pgd;
	pud_t *pud;
	pmd_t *pmd;
	pte_t *pte;
	int k = 0;

	pgd = (pgd_t *)page_address(root) + pgd_index(va);
	if (!pgd_present(*pgd))
		set_pgd(pgd, __pgd(pa_page(pages[k++]) | _KERNPG_TABLE));

	pud = pud_offset(pgd, va);
	if (!pud_present(*pud))
		set_pud(pud, __pud(pa_page(pages[k++]) | _KERNPG_TABLE));

	pmd = pmd_offset(pud, va);
	if (!pmd_present(*pmd))
		set_pmd(pmd, __pmd(pa_page(pages[k++]) | _KERNPG_TABLE));

	pte = (pte_t *)page_address(pmd_page(*pmd)) + pte_index(va);
	set_pte(pte, __pte(pa | _PAGE_KERNEL_EXEC));

	return k;
}

Any comments?

/ magnus

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