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

[Suzuki K. Poulose]

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@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.

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

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