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    SubjectRe: [PATCH 02/17] x86: Add support for ZSTD-compressed kernel

    * Adam Borowski <kilobyte@angband.pl> wrote:

    > From: Nick Terrell <terrelln@fb.com>
    >
    > Integrates the ZSTD decompression code to the x86 pre-boot code.
    >
    > Zstandard requires slightly more memory during the kernel decompression
    > on x86 (192 KB vs 64 KB), and the memory usage is independent of the
    > window size.
    >
    > Zstandard requires memory proportional to the window size used during
    > compression for decompressing the ramdisk image, since streaming mode is
    > used. Newer versions of zstd (1.3.2+) list the window size of a file
    > with `zstd -lv <file>'. The absolute maximum amount of memory required
    > is just over 8 MB.
    >
    > Signed-off-by: Nick Terrell <terrelln@fb.com>
    > ---
    > Documentation/x86/boot.txt | 6 +++---
    > arch/x86/Kconfig | 1 +
    > arch/x86/boot/compressed/Makefile | 5 ++++-
    > arch/x86/boot/compressed/misc.c | 4 ++++
    > arch/x86/boot/header.S | 8 +++++++-
    > arch/x86/include/asm/boot.h | 6 ++++--
    > 6 files changed, 23 insertions(+), 7 deletions(-)

    Acked-by: Ingo Molnar <mingo@kernel.org>

    > diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
    > index 4c881c850125..af2efb256527 100644
    > --- a/arch/x86/boot/header.S
    > +++ b/arch/x86/boot/header.S
    > @@ -526,8 +526,14 @@ pref_address: .quad LOAD_PHYSICAL_ADDR # preferred load addr
    > # the size-dependent part now grows so fast.
    > #
    > # extra_bytes = (uncompressed_size >> 8) + 65536
    > +#
    > +# ZSTD compressed data grows by at most 3 bytes per 128K, and only has a 22
    > +# byte fixed overhead but has a maximum block size of 128K, so it needs a
    > +# larger margin.
    > +#
    > +# extra_bytes = (uncompressed_size >> 8) + 131072
    >
    > -#define ZO_z_extra_bytes ((ZO_z_output_len >> 8) + 65536)
    > +#define ZO_z_extra_bytes ((ZO_z_output_len >> 8) + 131072)

    This change would also affect other decompressors, not just ZSTD,
    correct?

    Might want to split this change out into a separate preparatory patch to
    allow it to be bisected to, or at least mention it in the changelog more
    explicitly?

    Thanks,

    Ingo

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