• Ard Biesheuvel's avatar
    arm64: simplify kernel segment mapping granularity · 97740051
    Ard Biesheuvel authored
    The mapping of the kernel consist of four segments, each of which is mapped
    with different permission attributes and/or lifetimes. To optimize the TLB
    and translation table footprint, we define various opaque constants in the
    linker script that resolve to different aligment values depending on the
    page size and whether CONFIG_DEBUG_ALIGN_RODATA is set.
    
    Considering that
    - a 4 KB granule kernel benefits from a 64 KB segment alignment (due to
      the fact that it allows the use of the contiguous bit),
    - the minimum alignment of the .data segment is THREAD_SIZE already, not
      PAGE_SIZE (i.e., we already have padding between _data and the start of
      the .data payload in many cases),
    - 2 MB is a suitable alignment value on all granule sizes, either for
      mapping directly (level 2 on 4 KB), or via the contiguous bit (level 3 on
      16 KB and 64 KB),
    - anything beyond 2 MB exceeds the minimum alignment mandated by the boot
      protocol, and can only be mapped efficiently if the physical alignment
      happens to be the same,
    
    we can simplify this by standardizing on 64 KB (or 2 MB) explicitly, i.e.,
    regardless of granule size, all segments are aligned either to 64 KB, or to
    2 MB if CONFIG_DEBUG_ALIGN_RODATA=y. This also means we can drop the Kconfig
    dependency of CONFIG_DEBUG_ALIGN_RODATA on CONFIG_ARM64_4K_PAGES.
    Signed-off-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
    Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
    97740051
vmlinux.lds.S 4.68 KB