Re: [PATCH v3 04/21] arm64: decouple early fixmap init from linear mapping

[Mark Rutland]

On Mon, Jan 11, 2016 at 05:15:13PM +0100, Ard Biesheuvel wrote:
> On 11 January 2016 at 17:09, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > On Mon, Jan 11, 2016 at 02:18:57PM +0100, Ard Biesheuvel wrote:
> >> Since the early fixmap page tables are populated using pages that are
> >> part of the static footprint of the kernel, they are covered by the
> >> initial kernel mapping, and we can refer to them without using __va/__pa
> >> translations, which are tied to the linear mapping.
> >>
> >> Since the fixmap page tables are disjoint from the kernel mapping up
> >> to the top level pgd entry, we can refer to bm_pte[] directly, and there
> >> is no need to walk the page tables and perform __pa()/__va() translations
> >> at each step.
> >>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> >> ---
> >>  arch/arm64/mm/mmu.c | 32 ++++++--------------
> >>  1 file changed, 9 insertions(+), 23 deletions(-)
> >>
> >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> >> index 7711554a94f4..75b5f0dc3bdc 100644
> >> --- a/arch/arm64/mm/mmu.c
> >> +++ b/arch/arm64/mm/mmu.c
> >> @@ -570,38 +570,24 @@ void vmemmap_free(unsigned long start, unsigned long end)
> >>  #endif       /* CONFIG_SPARSEMEM_VMEMMAP */
> >>
> >>  static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
> >> -#if CONFIG_PGTABLE_LEVELS > 2
> >>  static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss;
> >> -#endif
> >> -#if CONFIG_PGTABLE_LEVELS > 3
> >>  static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss;
> >> -#endif
> >>
> >>  static inline pud_t * fixmap_pud(unsigned long addr)
> >>  {
> >> -     pgd_t *pgd = pgd_offset_k(addr);
> >> -
> >> -     BUG_ON(pgd_none(*pgd) || pgd_bad(*pgd));
> >> -
> >> -     return pud_offset(pgd, addr);
> >> +     return (CONFIG_PGTABLE_LEVELS > 3) ? &bm_pud[pud_index(addr)]
> >> +                                        : (pud_t *)pgd_offset_k(addr);
> >
> > If we move patch 6 earlier, we could use pud_offset_kimg here, and avoid
> > the cast, at the cost of passing the pgd into fixmap_pud.
> >
> > Similarly for fixmap_pmd.
> >
> 
> Is that necessarily an improvement? I know it hides the cast, but I
> think having an explicit pgd_t* to pud_t* cast that so obviously
> applies to CONFIG_PGTABLE_LEVELS < 4 only is fine as well.

True; it's not a big thing either way.

> >>  }
> >>
> >> -static inline pmd_t * fixmap_pmd(unsigned long addr)
> >> +static inline pte_t * fixmap_pmd(unsigned long addr)
> >>  {
> >> -     pud_t *pud = fixmap_pud(addr);
> >> -
> >> -     BUG_ON(pud_none(*pud) || pud_bad(*pud));
> >> -
> >> -     return pmd_offset(pud, addr);
> >> +     return (CONFIG_PGTABLE_LEVELS > 2) ? &bm_pmd[pmd_index(addr)]
> >> +                                        : (pmd_t *)pgd_offset_k(addr);
> >>  }
> >
> > I assume the return type change was unintentional?
> >
> 
> Yes. Thanks for spotting that.

With that fixed:

Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>

> > Side note: is there any reason we can't/shouldn't make
> > STRICT_MM_TYPECHECKS a common config option? Or simply have it on by
> > default for arm64?
> >
> 
> I wouldn't mind at all.

I'll dig into that a bit futher then...

Thanks,
Mark.