linux-kernel - Re: [PATCHv3 03/11] arm64: Introduce helpers for page table levels

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Date:	Thu, 15 Oct 2015 13:37:35 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	"Suzuki K. Poulose" <suzuki.poulose@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	catalin.marinas@....com, will.deacon@....com, mark.rutland@....com,
	steve.capper@...aro.org, marc.zyngier@....com,
	ard.biesheuvel@...aro.org
Subject: Re: [PATCHv3 03/11] arm64: Introduce helpers for page table levels

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.biesheuvel@...aro.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Signed-off-by: Suzuki K. Poulose <suzuki.poulose@....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?

Also the comment could be expanded to explain the (4 - (n)), but I
couldn't easily come up with good language for explaining that you have
a maximum of 4 levels and you subtract the 'n' levels that your are not
accounting for...

That said, this still looks functionally correct to me.

-Christoffer


> +
>  #define PTRS_PER_PTE		(1 << (PAGE_SHIFT - 3))
>  
>  /*
>   * PMD_SHIFT determines the size a level 2 page table entry can map.
>   */
>  #if CONFIG_PGTABLE_LEVELS > 2
> -#define PMD_SHIFT		((PAGE_SHIFT - 3) * 2 + 3)
> +#define PMD_SHIFT		ARM64_HW_PGTABLE_LEVEL_SHIFT(2)
>  #define PMD_SIZE		(_AC(1, UL) << PMD_SHIFT)
>  #define PMD_MASK		(~(PMD_SIZE-1))
>  #define PTRS_PER_PMD		PTRS_PER_PTE
> @@ -32,7 +50,7 @@
>   * PUD_SHIFT determines the size a level 1 page table entry can map.
>   */
>  #if CONFIG_PGTABLE_LEVELS > 3
> -#define PUD_SHIFT		((PAGE_SHIFT - 3) * 3 + 3)
> +#define PUD_SHIFT		ARM64_HW_PGTABLE_LEVEL_SHIFT(1)
>  #define PUD_SIZE		(_AC(1, UL) << PUD_SHIFT)
>  #define PUD_MASK		(~(PUD_SIZE-1))
>  #define PTRS_PER_PUD		PTRS_PER_PTE
> @@ -42,7 +60,8 @@
>   * PGDIR_SHIFT determines the size a top-level page table entry can map
>   * (depending on the configuration, this level can be 0, 1 or 2).
>   */
> -#define PGDIR_SHIFT		((PAGE_SHIFT - 3) * CONFIG_PGTABLE_LEVELS + 3)
> +#define PGDIR_SHIFT	\
> +		ARM64_HW_PGTABLE_LEVEL_SHIFT(4 - CONFIG_PGTABLE_LEVELS)
>  #define PGDIR_SIZE		(_AC(1, UL) << PGDIR_SHIFT)
>  #define PGDIR_MASK		(~(PGDIR_SIZE-1))
>  #define PTRS_PER_PGD		(1 << (VA_BITS - PGDIR_SHIFT))
> -- 
> 1.7.9.5
> 
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