[PATCH v4 09/15] arm64: mm: Permit contiguous attribute for preliminary mappings

Ard Biesheuvel
2026-04-27T15:35:45Z

From: Ard Biesheuvel <ardb@kernel.org>

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 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@kernel.org>
---
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 4eab40f4aa6f..5e2348b15783 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1000,8 +1000,7 @@ 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,
@@ -1028,8 +1027,7 @@ 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);
@@ -1175,10 +1173,8 @@ 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);
}

--
2.54.0.rc2.544.gc7ae2d5bb8-goog