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

[Vineet Gupta]

On 8/15/21 2:27 AM, Mike Rapoport wrote:
> 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.

You meant

+#else /* CONFIG_PGTABLE_LEVELS != 2

> > +
> > +/*
> > + * 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.

I added following to indicate conditional coding for levels related code

+#endif /* CONFIG_PGTABLE_LEVELS */


>    
> > +#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)
> >