On Wed, Oct 07, 2015 at 10:26:14AM +0100, Marc Zyngier wrote: > On 07/10/15 09:26, Christoffer Dall wrote: > > Hi Suzuki, > > > > On Tue, Sep 15, 2015 at 04:41:12PM +0100, Suzuki K. Poulose wrote: > >> From: "Suzuki K. Poulose" <suzuki.poulose@xxxxxxx> > >> > >> 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.biesheuvel@xxxxxxxxxx> > >> Cc: Mark Rutland <mark.rutland@xxxxxxx> > >> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> > >> Cc: Will Deacon <will.deacon@xxxxxxx> > >> Signed-off-by: Suzuki K. Poulose <suzuki.poulose@xxxxxxx> > >> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> > >> Tested-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> > >> --- > >> arch/arm64/include/asm/pgtable-hwdef.h | 15 ++++++++++++--- > >> 1 file changed, 12 insertions(+), 3 deletions(-) > >> > >> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h > >> index 24154b0..ce18389 100644 > >> --- a/arch/arm64/include/asm/pgtable-hwdef.h > >> +++ b/arch/arm64/include/asm/pgtable-hwdef.h > >> @@ -16,13 +16,21 @@ > >> #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 > >> + */ > >> +#define ARM64_HW_PGTABLE_LEVELS(va_bits) (((va_bits) - 4) / (PAGE_SHIFT - 3)) > > > > I don't understand the '(va_bits) - 4' here, can you explain it (and add a > > comment to that effect) ? > > I just had a chat with Catalin, who did shed some light on this. > It all has to do with rounding up. What you would like to have here is: > > #define ARM64_HW_PGTABLE_LEVELS(va_bits) DIV_ROUND_UP(va_bits - PAGE_SHIFT, PAGE_SHIFT - 3) > > where (va_bits - PAGE_SHIFT) is the total number of bits we deal > with during a page table walk, and (PAGE_SHIFT - 3) is the number > of bits we deal with per level. > > The clue is in how DIV_ROUND_UP is written: > > #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d)) > > which gives you Suzuki's magic formula. > > I'd vote for the DIV_ROUND_UP(), which will make things a lot more readable. > Thanks for the explanation, I vote for DIV_ROUND_UP too. You can stash this away for a cryptic interview question ;) -Christoffer -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
