linux-kernel - Re: [PATCH v1] arm64: mm: Batch dsb and isb when populating pgtables

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Date: Thu, 28 Mar 2024 09:23:14 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, David Hildenbrand <david@...hat.com>, 
	Donald Dutile <ddutile@...hat.com>, Eric Chanudet <echanude@...hat.com>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] arm64: mm: Batch dsb and isb when populating pgtables

On Wed, 27 Mar 2024 at 21:07, Ryan Roberts <ryan.roberts@....com> wrote:
>
> After removing uneccessary TLBIs, the next bottleneck when creating the
> page tables for the linear map is DSB and ISB, which were previously
> issued per-pte in __set_pte(). Since we are writing multiple ptes in a
> given pte table, we can elide these barriers and insert them once we
> have finished writing to the table.
>

Nice!

> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>  arch/arm64/include/asm/pgtable.h |  7 ++++++-
>  arch/arm64/mm/mmu.c              | 13 ++++++++++++-
>  2 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index bd5d02f3f0a3..81e427b23b3f 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -271,9 +271,14 @@ static inline pte_t pte_mkdevmap(pte_t pte)
>         return set_pte_bit(pte, __pgprot(PTE_DEVMAP | PTE_SPECIAL));
>  }
>
> -static inline void __set_pte(pte_t *ptep, pte_t pte)
> +static inline void ___set_pte(pte_t *ptep, pte_t pte)

IMHO, we should either use WRITE_ONCE() directly in the caller, or
find a better name.

>  {
>         WRITE_ONCE(*ptep, pte);
> +}
> +
> +static inline void __set_pte(pte_t *ptep, pte_t pte)
> +{
> +       ___set_pte(ptep, pte);
>
>         /*
>          * Only if the new pte is valid and kernel, otherwise TLB maintenance
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 1b2a2a2d09b7..c6d5a76732d4 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -301,7 +301,11 @@ static pte_t *init_pte(pte_t *ptep, unsigned long addr, unsigned long end,
>         do {
>                 pte_t old_pte = __ptep_get(ptep);
>
> -               __set_pte(ptep, pfn_pte(__phys_to_pfn(phys), prot));
> +               /*
> +                * Required barriers to make this visible to the table walker
> +                * are deferred to the end of alloc_init_cont_pte().
> +                */
> +               ___set_pte(ptep, pfn_pte(__phys_to_pfn(phys), prot));
>
>                 /*
>                  * After the PTE entry has been populated once, we
> @@ -358,6 +362,13 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>         } while (addr = next, addr != end);
>
>         ops->unmap(TYPE_PTE);
> +
> +       /*
> +        * Ensure all previous pgtable writes are visible to the table walker.
> +        * See init_pte().
> +        */
> +       dsb(ishst);
> +       isb();
>  }
>
>  static pmd_t *init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
> --
> 2.25.1
>