Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature

[Laura Abbott <labbott@kernel.org>]

On 3/29/20 10:12 AM, Ard Biesheuvel wrote:
> When CONFIG_DEBUG_ALIGN_RODATA is enabled, kernel segments mapped with
> different permissions (r-x for .text, r-- for .rodata, rw- for .data,
> etc) are rounded up to 2 MiB so they can be mapped more efficiently.
> In particular, it permits the segments to be mapped using level 2
> block entries when using 4k pages, which is expected to result in less
> TLB pressure.
> 
> However, the mappings for the bulk of the kernel will use level 2
> entries anyway, and the misaligned fringes are organized such that they
> can take advantage of the contiguous bit, and use far fewer level 3
> entries than would be needed otherwise.
> 
> This makes the value of this feature dubious at best, and since it is not
> enabled in defconfig or in the distro configs, it does not appear to be
> in wide use either. So let's just remove it.
> 

This was supposed to avoid potential performance inconsistencies with
some memory being in pages vs other in block. It was always a minor
concern so if it's preventing other work:

Acked-by: Laura Abbott <labbott@kernel.org>

> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>   arch/arm64/Kconfig.debug                  | 13 -------------
>   arch/arm64/include/asm/memory.h           | 12 +-----------
>   drivers/firmware/efi/libstub/arm64-stub.c |  8 +++-----
>   3 files changed, 4 insertions(+), 29 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
> index 1c906d932d6b..a1efa246c9ed 100644
> --- a/arch/arm64/Kconfig.debug
> +++ b/arch/arm64/Kconfig.debug
> @@ -52,19 +52,6 @@ config DEBUG_WX
>   
>   	  If in doubt, say "Y".
>   
> -config DEBUG_ALIGN_RODATA
> -	depends on STRICT_KERNEL_RWX
> -	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.
> -
> -	  If in doubt, say N.
> -
>   config DEBUG_EFI
>   	depends on EFI && DEBUG_INFO
>   	bool "UEFI debugging"
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 4d94676e5a8b..3b34f7bde2f2 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -119,22 +119,12 @@
>   
>   /*
>    * Alignment of kernel segments (e.g. .text, .data).
> - */
> -#if defined(CONFIG_DEBUG_ALIGN_RODATA)
> -/*
> - *  4 KB granule:   1 level 2 entry
> - * 16 KB granule: 128 level 3 entries, with contiguous bit
> - * 64 KB granule:  32 level 3 entries, with contiguous bit
> - */
> -#define SEGMENT_ALIGN		SZ_2M
> -#else
> -/*
> + *
>    *  4 KB granule:  16 level 3 entries, with contiguous bit
>    * 16 KB granule:   4 level 3 entries, without contiguous bit
>    * 64 KB granule:   1 level 3 entry
>    */
>   #define SEGMENT_ALIGN		SZ_64K
> -#endif
>   
>   /*
>    * Memory types available.
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
> index db0c1a9c1699..fc9f8ab533a7 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -75,14 +75,12 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
>   
>   	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && phys_seed != 0) {
>   		/*
> -		 * If CONFIG_DEBUG_ALIGN_RODATA is not set, produce a
> -		 * displacement in the interval [0, MIN_KIMG_ALIGN) that
> -		 * doesn't violate this kernel's de-facto alignment
> +		 * Produce a displacement in the interval [0, MIN_KIMG_ALIGN)
> +		 * that doesn't violate this kernel's de-facto alignment
>   		 * constraints.
>   		 */
>   		u32 mask = (MIN_KIMG_ALIGN - 1) & ~(EFI_KIMG_ALIGN - 1);
> -		u32 offset = !IS_ENABLED(CONFIG_DEBUG_ALIGN_RODATA) ?
> -			     (phys_seed >> 32) & mask : TEXT_OFFSET;
> +		u32 offset = (phys_seed >> 32) & mask;
>   
>   		/*
>   		 * With CONFIG_RANDOMIZE_TEXT_OFFSET=y, TEXT_OFFSET may not
> 

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