[PATCH] arm64/mm: Describe TTBR1_BADDR_4852_OFFSET

[Mark Rutland]

On Wed, Feb 25, 2026 at 06:40:28AM +0000, Anshuman Khandual wrote:
> TTBR1_BADDR_4852_OFFSET is a constant offset which gets added into kernel
> page table physical address for TTBR1_EL1 when kernel is build for 52 bit
> VA but found to be running on 48 bit VA capable system. Although there is
> no explanation on how the macro is computed.
> 
> Describe TTBR1_BADDR_4852_OFFSET computation in detail via deriving from
> all required parameters involved thus improving clarity and readability.
> 
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Ryan Roberts <ryan.roberts at arm.com>
> Cc: linux-arm-kernel at lists.infradead.org
> Cc: linux-kernel at vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual at arm.com>
> ---
> This applies on v7.0-rc1
> 
> Earlier context can be found here.
> 
> https://lore.kernel.org/linux-arm-kernel/aRb8ezhQd0c0jp9G@J2N7QTR9R3/

As with my comments last time, I have no strong feelings on this either
way; I'll leave it to Cataln and Will.

Mark.

>  arch/arm64/include/asm/pgtable-hwdef.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
> index 5e6809a462c7..72f31800c703 100644
> --- a/arch/arm64/include/asm/pgtable-hwdef.h
> +++ b/arch/arm64/include/asm/pgtable-hwdef.h
> @@ -285,9 +285,12 @@
>  #endif
>  
>  #ifdef CONFIG_ARM64_VA_BITS_52
> +#define PTRS_PER_PGD_52_VA (UL(1) << (52 - PGDIR_SHIFT))
> +#define PTRS_PER_PGD_48_VA (UL(1) << (48 - PGDIR_SHIFT))
> +#define PTRS_PER_PGD_EXTRA (PTRS_PER_PGD_52_VA - PTRS_PER_PGD_48_VA)
> +
>  /* Must be at least 64-byte aligned to prevent corruption of the TTBR */
> -#define TTBR1_BADDR_4852_OFFSET	(((UL(1) << (52 - PGDIR_SHIFT)) - \
> -				 (UL(1) << (48 - PGDIR_SHIFT))) * 8)
> +#define TTBR1_BADDR_4852_OFFSET (PTRS_PER_PGD_EXTRA << PTDESC_ORDER)
>  #endif
>  
>  #endif
> -- 
> 2.30.2
>