Re: [RFC V2 08/14] arm64/mm: Route all pgtable writes via pxxval_set()

[Ryan Roberts]

On 13/05/2026 05:45, Anshuman Khandual wrote:
> Currently pxxval_set() is defined as WRITE_ONCE() but this will change for
> D128 pgtable builds, for which WRITE_ONCE() is not sufficient for single
> copy atomicity.
> 
> In future this infrastructure can be used for D128 to maintain single copy
> atomicity semantics with inline asm blocks.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
> Changes in RFC V2:
> 
> - Renamed all ptdesc_ instances as pxxval_ instead
> 
>  arch/arm64/include/asm/pgtable.h | 11 ++++++-----
>  arch/arm64/mm/mmu.c              |  4 ++--
>  2 files changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 72da582e8d12..c71bb829e9f1 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -85,6 +85,7 @@ static inline void arch_leave_lazy_mmu_mode(void)
>  }
>  
>  #define pxxval_get(x)		READ_ONCE(x)
> +#define pxxval_set(x, val)	WRITE_ONCE(x, val)

Same comment as for pxxval_get(): I think it would be clearer if this was an
inline function that took a pointer.

Thanks,
Ryan


>  
>  #define pmdp_get pmdp_get
>  static inline pmd_t pmdp_get(pmd_t *pmdp)
> @@ -385,7 +386,7 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)
>  
>  static inline void __set_pte_nosync(pte_t *ptep, pte_t pte)
>  {
> -	WRITE_ONCE(*ptep, pte);
> +	pxxval_set(*ptep, pte);
>  }
>  
>  static inline void __set_pte_complete(pte_t pte)
> @@ -856,7 +857,7 @@ static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
>  	}
>  #endif /* __PAGETABLE_PMD_FOLDED */
>  
> -	WRITE_ONCE(*pmdp, pmd);
> +	pxxval_set(*pmdp, pmd);
>  
>  	if (pmd_valid(pmd))
>  		queue_pte_barriers();
> @@ -921,7 +922,7 @@ static inline void set_pud(pud_t *pudp, pud_t pud)
>  		return;
>  	}
>  
> -	WRITE_ONCE(*pudp, pud);
> +	pxxval_set(*pudp, pud);
>  
>  	if (pud_valid(pud))
>  		queue_pte_barriers();
> @@ -1003,7 +1004,7 @@ static inline void set_p4d(p4d_t *p4dp, p4d_t p4d)
>  		return;
>  	}
>  
> -	WRITE_ONCE(*p4dp, p4d);
> +	pxxval_set(*p4dp, p4d);
>  	queue_pte_barriers();
>  }
>  
> @@ -1131,7 +1132,7 @@ static inline void set_pgd(pgd_t *pgdp, pgd_t pgd)
>  		return;
>  	}
>  
> -	WRITE_ONCE(*pgdp, pgd);
> +	pxxval_set(*pgdp, pgd);
>  	queue_pte_barriers();
>  }
>  
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 7fbb2ef86cfa..6eb92d8f46be 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -76,7 +76,7 @@ void noinstr set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
>  	 * writable in the kernel mapping.
>  	 */
>  	if (rodata_is_rw) {
> -		WRITE_ONCE(*pgdp, pgd);
> +		pxxval_set(*pgdp, pgd);
>  		dsb(ishst);
>  		isb();
>  		return;
> @@ -84,7 +84,7 @@ void noinstr set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
>  
>  	spin_lock(&swapper_pgdir_lock);
>  	fixmap_pgdp = pgd_set_fixmap(__pa_symbol(pgdp));
> -	WRITE_ONCE(*fixmap_pgdp, pgd);
> +	pxxval_set(*fixmap_pgdp, pgd);
>  	/*
>  	 * We need dsb(ishst) here to ensure the page-table-walker sees
>  	 * our new entry before set_p?d() returns. The fixmap's