Re: [PATCH v3 0/8] Add support for ZSTD-compressed kernel and initramfs

Kees Cook
2020-03-20T18:10:11Z

On Thu, Mar 19, 2020 at 12:59:26PM -0700, Nick Terrell wrote:
> Hi all,
>
> This patch set adds support for a ZSTD-compressed kernel, ramdisk, and
> initramfs in the kernel boot process. ZSTD-compressed ramdisk and initramfs
> are supported on all architectures. The ZSTD-compressed kernel is only
> hooked up to x86 in this patch set.
>
> 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 proprortional 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, but it can be controlled at compression time.
>
> This patch set has been boot tested with buildroot and QEMU based off
> of linux-5.6-rc6.
>
> On i386 and x86_64 I have tested the following configurations:
> * zstd compressed kernel and a separate zstd compressed initramfs
> * zstd compressed kernel and a built-in zstd compressed initramfs
> * gzip compressed kernel and a separate gzip compressed initramfs
> * gzip compressed kernel and a built-in gzip compressed initramfs
>
> On arm and aarch64 I tested the same configurations, except that the kernel is
> always gzip compressed.
>
> Facebook has been using v1 of these patches on x86_64 devices for more than 6
> months. When we switched from a xz compressed initramfs to a zstd compressed
> initramfs decompression time shrunk from 12 seconds to 3 seconds. When we
> switched from a xz compressed kernel to a zstd compressed kernel we saved 2
> seconds of boot time.
>
> Facebook has been using v2 of these patches on aarch64 devices for a few weeks.
> When we switched from an lzma compressed initramfs to a zstd compressed initramfs
> decompression time shrunk from 27 seconds to 8 seconds.
>
> The zstd compressed kernel is smaller than the gzip compressed kernel but larger
> than the xz or lzma compressed kernels, and it decompresses faster than
> everything except lz4. See the table below for the measurement of an x86_64
> kernel ordered by compressed size:
>
> algo size
> xz 6,509,792
> lzma 6,856,576
> zstd 7,399,157
> gzip 8,522,527
> bzip 8,629,603
> lzo 9,808,035
> lz4 10,705,570
> none 32,565,672
>
> v1 -> v2:
> - Rebase
> - usr/Makefile and init/Kconfig were changed so the patches were updated
> - No functional changes except to rebase
> - Split the patches up into smaller chunks
>
> v2 -> v3:
> - Add *.zst to the .gitignore in patch 8
> - Style nits in patch 3
> - Rename the PREBOOT macro to ZSTD_PREBOOT and XXH_PREBOOT in patches
> 1 through 3
>
> Best,
> Nick Terrell
>
> Adam Borowski (1):
> .gitignore: add ZSTD-compressed files
>
> Nick Terrell (7):
> lib: prepare zstd for preboot environment
> lib: prepare xxhash for preboot environment
> lib: add zstd support to decompress
> init: add support for zstd compressed kernel
> usr: add support for zstd compressed initramfs
> x86: bump ZO_z_extra_bytes margin for zstd
> x86: Add support for ZSTD compressed kernel
>
> .gitignore | 1 +
> Documentation/x86/boot.rst | 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 +-
> include/linux/decompress/unzstd.h | 11 +
> init/Kconfig | 15 +-
> lib/Kconfig | 4 +
> lib/Makefile | 1 +
> lib/decompress.c | 5 +
> lib/decompress_unzstd.c | 338 ++++++++++++++++++++++++++++++
> lib/xxhash.c | 21 +-
> lib/zstd/decompress.c | 2 +
> lib/zstd/fse_decompress.c | 9 +-
> lib/zstd/zstd_internal.h | 14 +-
> scripts/Makefile.lib | 15 ++
> usr/Kconfig | 20 ++
> usr/Makefile | 1 +
> 20 files changed, 460 insertions(+), 27 deletions(-)
> create mode 100644 include/linux/decompress/unzstd.h
> create mode 100644 lib/decompress_unzstd.c

This looks good to me! Please consider the series:

Reviewed-by: Kees Cook <keescook@chromium.org>

Thanks!

-Kees

--
Kees Cook