Re: [PATCH v2 16/19] ARC: mm: support 3 levels of page tables

[Mike Rapoport]

On Thu, Aug 12, 2021 at 04:37:50PM -0700, Vineet Gupta wrote:
> ARCv2 MMU is software walked and Linux implements 2 levels of paging: pgd/pte.
> Forthcoming hw will have multiple levels, so this change preps mm code
> for same. It is also fun to try multi levels even on soft-walked code to
> ensure generic mm code is robust to handle.
> 
> overview
> ________
> 
> 2 levels {pgd, pte} : pmd is folded but pmd_* macros are valid and operate on pgd
> 3 levels {pgd, pmd, pte}:
>   - pud is folded and pud_* macros point to pgd
>   - pmd_* macros operate on actual pmd
> 
> code changes
> ____________
> 
> 1. #include <asm-generic/pgtable-nopud.h>
> 
> 2. Define CONFIG_PGTABLE_LEVELS 3
> 
> 3a. Define PMD_SHIFT, PMD_SIZE, PMD_MASK, pmd_t
> 3b. Define pmd_val() which actually deals with pmd
>     (pmd_offset(), pmd_index() are provided by generic code)
> 3c. pmd_alloc_one()/pmd_free() also provided by generic code
>     (pmd_populate/pmd_free already exist)
> 
> 4. Define pud_none(), pud_bad() macros based on generic pud_val() which
>    internally pertains to pgd now.
> 4b. define pud_populate() to just setup pgd
> 
> Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxx>
> ---

...

> diff --git a/arch/arc/include/asm/pgtable-levels.h b/arch/arc/include/asm/pgtable-levels.h
> index 8ece75335bb5..1c2f022d4ad0 100644
> --- a/arch/arc/include/asm/pgtable-levels.h
> +++ b/arch/arc/include/asm/pgtable-levels.h
> @@ -10,6 +10,8 @@
>  #ifndef _ASM_ARC_PGTABLE_LEVELS_H
>  #define _ASM_ARC_PGTABLE_LEVELS_H
>  
> +#if CONFIG_PGTABLE_LEVELS == 2
> +
>  /*
>   * 2 level paging setup for software walked MMUv3 (ARC700) and MMUv4 (HS)
>   *
> @@ -37,16 +39,38 @@
>  #define PGDIR_SHIFT		21
>  #endif
>  
> -#define PGDIR_SIZE		BIT(PGDIR_SHIFT)	/* vaddr span, not PDG sz */
> -#define PGDIR_MASK		(~(PGDIR_SIZE - 1))
> +#else

Adding /* CONFIG_PGTABLE_LEVELS == 2 */ would make the whole thing a bit
more readable, I think.

> +
> +/*
> + * A default 3 level paging testing setup in software walked MMU
> + *   MMUv4 (8K page): <4> : <7> : <8> : <13>
> + */
> +#define PGDIR_SHIFT		28
> +#if CONFIG_PGTABLE_LEVELS > 2
> +#define PMD_SHIFT		21
> +#endif
> +
> +#endif

and here as well.
  
> +#define PGDIR_SIZE		BIT(PGDIR_SHIFT)
> +#define PGDIR_MASK		(~(PGDIR_SIZE - 1))
>  #define PTRS_PER_PGD		BIT(32 - PGDIR_SHIFT)
>  
> -#define PTRS_PER_PTE		BIT(PGDIR_SHIFT - PAGE_SHIFT)
> +#if CONFIG_PGTABLE_LEVELS > 2
> +#define PMD_SIZE		BIT(PMD_SHIFT)
> +#define PMD_MASK		(~(PMD_SIZE - 1))
> +#define PTRS_PER_PMD		BIT(PGDIR_SHIFT - PMD_SHIFT)
> +#endif
> +
> +#define PTRS_PER_PTE		BIT(PMD_SHIFT - PAGE_SHIFT)
>  

-- 
Sincerely yours,
Mike.