linux-kernel - Re: [PATCH 03/15] arm64: Introduce helpers for page table levels

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Date:	Thu, 8 Oct 2015 16:45:46 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	"Suzuki K. Poulose" <suzuki.poulose@....com>,
	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 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@....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.
> 
> 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
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