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

[Christoffer Dall]

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) ?

> +#define ARM64_HW_PGTABLE_LEVEL_SHIFT(level) \
> +		((PAGE_SHIFT - 3) * (level) + 3)
> +

While this change is clearly correct, if you can explain the math here
in a comment as well, that would be helpful.

Thanks,
-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 +40,7 @@
>   * 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(3)
>  #define PUD_SIZE		(_AC(1, UL) << PUD_SHIFT)
>  #define PUD_MASK		(~(PUD_SIZE-1))
>  #define PTRS_PER_PUD		PTRS_PER_PTE
> @@ -42,7 +50,8 @@
>   * 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(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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