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Message-ID: <5614EA73.5060807@arm.com> Date: Wed, 7 Oct 2015 10:48:35 +0100 From: "Suzuki K. Poulose" <Suzuki.Poulose@....com> To: Marc Zyngier <marc.zyngier@....com>, Christoffer Dall <christoffer.dall@...aro.org> Cc: linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org, ard.biesheuvel@...aro.org, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu Subject: Re: [PATCH 03/15] arm64: Introduce helpers for page table levels On 07/10/15 10:26, 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@....com> >>> >>> 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@...aro.org> >>> Cc: Mark Rutland <mark.rutland@....com> >>> Cc: Catalin Marinas <catalin.marinas@....com> >>> Cc: Will Deacon <will.deacon@....com> >>> Signed-off-by: Suzuki K. Poulose <suzuki.poulose@....com> >>> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@...aro.org> >>> Tested-by: Ard Biesheuvel <ard.biesheuvel@...aro.org> >>> --- >>> 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. Thanks Marc for pitching in. That explains it better. > > I'd vote for the DIV_ROUND_UP(), which will make things a lot more readable. Sure, I can change that. Suzuki -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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