[PATCH 1/2] arm64: mm: Enable CONT_SIZE aligned sections for 64k page kernels.

[Ard Biesheuvel]

Hi Jeremy,

Question below:

On 12 February 2016 at 17:06, Jeremy Linton <jeremy.linton at arm.com> wrote:
> This change allows ALIGN_RODATA for 16k and 64k kernels.
> In the case of 64k kernels it actually aligns to the CONT_SIZE
> rather than the SECTION_SIZE (which is 512M). This makes it generally
> more useful, especially for CONT enabled kernels.
>
> Signed-off-by: Jeremy Linton <jeremy.linton at arm.com>
> ---
>  arch/arm64/Kconfig.debug        | 12 ++++++------
>  arch/arm64/kernel/vmlinux.lds.S | 11 ++++++-----
>  2 files changed, 12 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
> index e13c4bf..65705ee 100644
> --- a/arch/arm64/Kconfig.debug
> +++ b/arch/arm64/Kconfig.debug
> @@ -59,15 +59,15 @@ config DEBUG_RODATA
>            If in doubt, say Y
>
>  config DEBUG_ALIGN_RODATA
> -       depends on DEBUG_RODATA && ARM64_4K_PAGES
> +       depends on DEBUG_RODATA
>         bool "Align linker sections up to SECTION_SIZE"
>         help
>           If this option is enabled, sections that may potentially be marked as
> -         read only or non-executable will be aligned up to the section size of
> -         the kernel. This prevents sections from being split into pages and
> -         avoids a potential TLB penalty. The downside is an increase in
> -         alignment and potentially wasted space. Turn on this option if
> -         performance is more important than memory pressure.
> +         read only or non-executable will be aligned up to the section size
> +         or contiguous hint size of the kernel. This prevents sections from
> +         being split into pages and avoids a potential TLB penalty. The downside
> +         is an increase in alignment and potentially wasted space. Turn on
> +         this option if performance is more important than memory pressure.
>
>           If in doubt, say N
>
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index b78a3c7..ab4e436 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -63,13 +63,14 @@ PECOFF_FILE_ALIGNMENT = 0x200;
>  #endif
>
>  #if defined(CONFIG_DEBUG_ALIGN_RODATA)
> -#define ALIGN_DEBUG_RO                 . = ALIGN(1<<SECTION_SHIFT);
> -#define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
> +#if defined(CONFIG_ARM64_64K_PAGES)
> +#define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(CONT_SIZE);
> +#else
> +#define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(SECTION_SIZE);

Doesn't this align to 32 MB on 16k pages kernels?

> +#endif
>  #elif defined(CONFIG_DEBUG_RODATA)
> -#define ALIGN_DEBUG_RO                 . = ALIGN(1<<PAGE_SHIFT);
> -#define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
> +#define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(PAGE_SIZE);
>  #else
> -#define ALIGN_DEBUG_RO
>  #define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(min);
>  #endif
>
> --
> 2.4.3
>