Re: [PATCH v6 08/15] arm64: mm: Permit contiguous attribute for preliminary mappings

[Kevin Brodsky]

On 26/05/2026 19:58, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> There are a few cases where we omit the contiguous hint for mappings
> that start out as read-write and are remapped read-only later, on the
> basis that manipulating live descriptors with the PTE_CONT attribute set
> is unsafe. When support for the contiguous hint was added to the code,
> the ARM ARM was ambiguous about this, and so we erred on the side of
> caution.
>
> In the meantime, this has been clarified [0], and regions that will be
> remapped in their entirety, retaining the contiguous bit on all entries,
> can use the contiguous hint both in the initial mapping as well as the
> one that replaces it. Note that this requires that the logic that may be
> called to remap overlapping regions respects existing valid descriptors
> that have the contiguous bit cleared.
>
> So omit the NO_CONT_MAPPINGS flag in places where it is unneeded.
>
> Thanks to Ryan for the reference.
>
> [0] RJQQTC
>
> For a TLB lookup in a contiguous region mapped by translation table entries that
> have consistent values for the Contiguous bit, but have the OA, attributes, or
> permissions misprogrammed, that TLB lookup is permitted to produce an OA, access
> permissions, and memory attributes that are consistent with any one of the
> programmed translation table values.
>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

Reviewed-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>

> ---
>  arch/arm64/mm/mmu.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 55bb40348a47..04cc579c7a15 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1016,8 +1016,7 @@ void __init create_mapping_noalloc(phys_addr_t phys, unsigned long virt,
>  			&phys, virt);
>  		return;
>  	}
> -	early_create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL,
> -				 NO_CONT_MAPPINGS);
> +	early_create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL, 0);
>  }
>  
>  void __init create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
> @@ -1044,8 +1043,7 @@ static void update_mapping_prot(phys_addr_t phys, unsigned long virt,
>  		return;
>  	}
>  
> -	early_create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL,
> -				 NO_CONT_MAPPINGS);
> +	early_create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL, 0);
>  
>  	/* flush the TLBs after updating live kernel mappings */
>  	flush_tlb_kernel_range(virt, virt + size);
> @@ -1191,10 +1189,8 @@ static void __init map_mem(void)
>  	 * alternative patching has completed). This makes the contents
>  	 * of the region accessible to subsystems such as hibernate,
>  	 * but protects it from inadvertent modification or execution.
> -	 * Note that contiguous mappings cannot be remapped in this way,
> -	 * so we should avoid them here.
>  	 */
> -	__map_memblock(kernel_start, kernel_end, PAGE_KERNEL, NO_CONT_MAPPINGS);
> +	__map_memblock(kernel_start, kernel_end, PAGE_KERNEL, 0);
>  	memblock_clear_nomap(kernel_start, kernel_end - kernel_start);
>  }
>