• Nick Desaulniers's avatar
    arm64: prevent regressions in compressed kernel image size when upgrading to binutils 2.27 · fd9dde6a
    Nick Desaulniers authored
    Upon upgrading to binutils 2.27, we found that our lz4 and gzip
    compressed kernel images were significantly larger, resulting is 10ms
    boot time regressions.
    
    As noted by Rahul:
    "aarch64 binaries uses RELA relocations, where each relocation entry
    includes an addend value. This is similar to x86_64.  On x86_64, the
    addend values are also stored at the relocation offset for relative
    relocations. This is an optimization: in the case where code does not
    need to be relocated, the loader can simply skip processing relative
    relocations.  In binutils-2.25, both bfd and gold linkers did this for
    x86_64, but only the gold linker did this for aarch64.  The kernel build
    here is using the bfd linker, which stored zeroes at the relocation
    offsets for relative relocations.  Since a set of zeroes compresses
    better than a set of non-zero addend values, this behavior was resulting
    in much better lz4 compression.
    
    The bfd linker in binutils-2.27 is now storing the actual addend values
    at the relocation offsets. The behavior is now consistent with what it
    does for x86_64 and what gold linker does for both architectures.  The
    change happened in this upstream commit:
    https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=1f56df9d0d5ad89806c24e71f296576d82344613
    Since a bunch of zeroes got replaced by non-zero addend values, we see
    the side effect of lz4 compressed image being a bit bigger.
    
    To get the old behavior from the bfd linker, "--no-apply-dynamic-relocs"
    flag can be used:
    $ LDFLAGS="--no-apply-dynamic-relocs" make
    With this flag, the compressed image size is back to what it was with
    binutils-2.25.
    
    If the kernel is using ASLR, there aren't additional runtime costs to
    --no-apply-dynamic-relocs, as the relocations will need to be applied
    again anyway after the kernel is relocated to a random address.
    
    If the kernel is not using ASLR, then presumably the current default
    behavior of the linker is better. Since the static linker performed the
    dynamic relocs, and the kernel is not moved to a different address at
    load time, it can skip applying the relocations all over again."
    
    Some measurements:
    
    $ ld -v
    GNU ld (binutils-2.25-f3d35cf6) 2.25.51.20141117
                        ^
    $ ls -l vmlinux
    -rwxr-x--- 1 ndesaulniers eng 300652760 Oct 26 11:57 vmlinux
    $ ls -l Image.lz4-dtb
    -rw-r----- 1 ndesaulniers eng 16932627 Oct 26 11:57 Image.lz4-dtb
    
    $ ld -v
    GNU ld (binutils-2.27-53dd00a1) 2.27.0.20170315
                        ^
    pre patch:
    $ ls -l vmlinux
    -rwxr-x--- 1 ndesaulniers eng 300376208 Oct 26 11:43 vmlinux
    $ ls -l Image.lz4-dtb
    -rw-r----- 1 ndesaulniers eng 18159474 Oct 26 11:43 Image.lz4-dtb
    
    post patch:
    $ ls -l vmlinux
    -rwxr-x--- 1 ndesaulniers eng 300376208 Oct 26 12:06 vmlinux
    $ ls -l Image.lz4-dtb
    -rw-r----- 1 ndesaulniers eng 16932466 Oct 26 12:06 Image.lz4-dtb
    
    By Siqi's measurement w/ gzip:
    binutils 2.27 with this patch (with --no-apply-dynamic-relocs):
    Image 41535488
    Image.gz 13404067
    
    binutils 2.27 without this patch (without --no-apply-dynamic-relocs):
    Image 41535488
    Image.gz 14125516
    
    Any compression scheme should be able to get better results from the
    longer runs of zeros, not just GZIP and LZ4.
    
    10ms boot time savings isn't anything to get excited about, but users of
    arm64+compression+bfd-2.27 should not have to pay a penalty for no
    runtime improvement.
    Reported-by: default avatarGopinath Elanchezhian <gelanchezhian@google.com>
    Reported-by: default avatarSindhuri Pentyala <spentyala@google.com>
    Reported-by: default avatarWei Wang <wvw@google.com>
    Suggested-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
    Suggested-by: default avatarRahul Chaudhry <rahulchaudhry@google.com>
    Suggested-by: default avatarSiqi Lin <siqilin@google.com>
    Suggested-by: default avatarStephen Hines <srhines@google.com>
    Signed-off-by: default avatarNick Desaulniers <ndesaulniers@google.com>
    Reviewed-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
    [will: added comment to Makefile]
    Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
    fd9dde6a
Makefile 5.35 KB