.. SPDX-License-Identifier: 0BSD

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XZ data compression in Linux
============================

Introduction
============

XZ is a general purpose data compression format with high compression
ratio. The XZ decompressor in Linux is called XZ Embedded. It supports
the LZMA2 filter and optionally also Branch/Call/Jump (BCJ) filters
for executable code. CRC32 is supported for integrity checking. The
home page of XZ Embedded is at <https://tukaani.org/xz/embedded.html>.
There you can find the latest version which includes a few optional
extra features that aren't required in the Linux kernel and information
about using the code outside the Linux kernel.

For userspace, XZ Utils provide a zlib-like compression library
and a gzip-like command line tool. XZ Utils can be downloaded from
<https://tukaani.org/xz/>.

XZ related components in the kernel
===================================

The xz_dec module provides XZ decompressor with single-call (buffer
to buffer) and multi-call (stateful) APIs. The usage of the xz_dec
module is documented in include/linux/xz.h.

For decompressing the kernel image, initramfs, and initrd, there
is a wrapper function in lib/decompress_unxz.c. Its API is the
same as in other decompress_*.c files, which is defined in
include/linux/decompress/generic.h.

For kernel makefiles, three commands are provided for use with
$(call if_changed). They require the xz tool from XZ Utils.

  - $(call if_changed,xzkern) is for compressing the kernel image.
    It runs the script scripts/xz_wrap.sh which uses arch-optimized
    options and a big LZMA2 dictionary.

  - $(call if_changed,xzkern_with_size) is like xzkern above but this
    also appends a four-byte trailer containing the uncompressed size
    of the file, which is needed by the boot code on some archs.

  - Other things can be compressed with $(call if_needed,xzmisc)
    which will use no BCJ filter and 1 MiB LZMA2 dictionary.

Notes on compression options
============================

Since the XZ Embedded supports only streams with no integrity check or
CRC32, make sure that you don't use some other integrity check type
when encoding files that are supposed to be decoded by the kernel. With
liblzma, you need to use either LZMA_CHECK_NONE or LZMA_CHECK_CRC32
when encoding. With the xz command line tool, use --check=none or
--check=crc32 to override the default --check=crc64.

Using CRC32 is strongly recommended unless there is some other layer
which will verify the integrity of the uncompressed data anyway.
Double checking the integrity would probably be waste of CPU cycles.
Note that the headers will always have a CRC32 which will be validated
by the decoder; you can only change the integrity check type (or
disable it) for the actual uncompressed data.

In userspace, LZMA2 is typically used with dictionary sizes of several
megabytes. The decoder needs to have the dictionary in RAM:

  - In multi-call mode the dictionary is allocated as part of the
    decoder state. The reasonable maximum dictionary size for in-kernel
    use will depend on the target hardware: a few megabytes should be
    fine for desktop systems while 64 KiB to 1 MiB might be more
    appropriate on some embedded systems.

  - In single-call mode the output buffer is used as the dictionary
    buffer. That is, the size of the dictionary doesn't affect the
    decompressor memory usage at all. Only the base data structures
    are allocated which take a little less than 30 KiB of memory.
    For the best compression, the dictionary should be at least
    as big as the uncompressed data. A notable example of single-call
    mode is decompressing the kernel itself (except on PowerPC).

The compression presets in XZ Utils may not be optimal when creating
files for the kernel, so don't hesitate to use custom settings to,
for example, set the dictionary size. Also, xz may produce a smaller
file in single-threaded mode so setting that explicitly is recommended.
Example::

    xz --threads=1 --check=crc32 --lzma2=dict=512KiB inputfile