On Thu, Oct 15, 2015 at 01:37:35PM +0200, Christoffer Dall wrote: > On Wed, Oct 14, 2015 at 12:20:26PM +0100, Suzuki K. Poulose wrote: > > Introduce helpers for finding the number of page table > > levels required for a given VA width, shift for a particular > > page table level. > > > > Convert the existing users to the new helpers. More users > > to follow. > > > > Cc: Ard Biesheuvel <ard.biesheu...@linaro.org> > > Cc: Mark Rutland <mark.rutl...@arm.com> > > Cc: Catalin Marinas <catalin.mari...@arm.com> > > Cc: Will Deacon <will.dea...@arm.com> > > Cc: Marc Zyngier <marc.zyng...@arm.com> > > Signed-off-by: Suzuki K. Poulose <suzuki.poul...@arm.com> > > > > --- > > Changes since V2: > > - Add comments around the macros > > - Change ARM64_HW_PGTABLE_LEVEL_SHIFT to accept pagetable level as > > described by ARM ARM > > --- > > arch/arm64/include/asm/pgtable-hwdef.h | 25 ++++++++++++++++++++++--- > > 1 file changed, 22 insertions(+), 3 deletions(-) > > > > diff --git a/arch/arm64/include/asm/pgtable-hwdef.h > > b/arch/arm64/include/asm/pgtable-hwdef.h > > index 95c1ec0..c6194ab 100644 > > --- a/arch/arm64/include/asm/pgtable-hwdef.h > > +++ b/arch/arm64/include/asm/pgtable-hwdef.h > > @@ -16,13 +16,31 @@ > > #ifndef __ASM_PGTABLE_HWDEF_H > > #define __ASM_PGTABLE_HWDEF_H > > > > +/* > > + * Number of page-table levels required to address 'va_bits' wide > > + * address, without section mapping. We resolve the top (va_bits - > > PAGE_SHIFT) > > + * bits with (PAGE_SHIFT - 3) bits at each page table level. Hence: > > + * > > + * levels = DIV_ROUND_UP((va_bits - PAGE_SHIFT), (PAGE_SHIFT - 3)) > > + * > > + * We cannot include linux/kernel.h which defines DIV_ROUND_UP here > > + * due to build issues. So we use the following magic formula. > > + */ > > +#define ARM64_HW_PGTABLE_LEVELS(va_bits) (((va_bits) - 4) / (PAGE_SHIFT - > > 3)) > > + > > +/* > > + * Size mapped by an entry at level n > > + * We map PAGE_SHIFT - 3 at all levels, except the PAGE_SHIFT bits at the > > last level > > + */ > > +#define ARM64_HW_PGTABLE_LEVEL_SHIFT(n) ((PAGE_SHIFT - 3) * (4 - (n)) + > > 3) > > I feel like I'm partially failing the interview question again, in that > I don't fully understand the '+ 3' in the end?
The last level handles PAGE_SHIFT bits (the bits from the VA that are the same in the PA). We only accounted for (PAGE_SHIFT - 3) bits at each level when multiplying, so we add those 3 missing bits back at the end. Thanks, Mark. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
