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+
+XZ data compression in Linux
+============================
+
+Introduction
+
+    XZ is a general purpose data compression format with high compression
+    ratio and relatively fast decompression. The primary compression
+    algorithm (filter) is LZMA2. Additional filters can be used to improve
+    compression ratio even further. E.g. Branch/Call/Jump (BCJ) filters
+    improve compression ratio of executable data.
+
+    The XZ decompressor in Linux is called XZ Embedded. It supports
+    the LZMA2 filter and optionally also BCJ filters. CRC32 is supported
+    for integrity checking. The home page of XZ Embedded is at
+    <http://tukaani.org/xz/embedded.html>, where you can find the
+    latest version and also 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
+    <http://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.
+
+    The xz_dec_test module is for testing xz_dec. xz_dec_test is not
+    useful unless you are hacking the XZ decompressor. xz_dec_test
+    allocates a char device major dynamically to which one can write
+    .xz files from userspace. The decompressed output is thrown away.
+    Keep an eye on dmesg to see diagnostics printed by xz_dec_test.
+    See the xz_dec_test source code for the details.
+
+    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.
+
+    scripts/xz_wrap.sh is a wrapper for the xz command line tool found
+    from XZ Utils. The wrapper sets compression options to values suitable
+    for compressing the kernel image.
+
+    For kernel makefiles, two commands are provided for use with
+    $(call if_needed). The kernel image should be compressed with
+    $(call if_needed,xzkern) which will use a BCJ filter and a big LZMA2
+    dictionary. It will also append a four-byte trailer containing the
+    uncompressed size of the file, which is needed by the boot code.
+    Other things should 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.
+
+    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, thus big
+    dictionaries cannot be used for files that are intended to be decoded
+    by the kernel. 1 MiB is probably the maximum reasonable dictionary
+    size for in-kernel use (maybe more is OK for initramfs). The presets
+    in XZ Utils may not be optimal when creating files for the kernel,
+    so don't hesitate to use custom settings. Example:
+
+        xz --check=crc32 --lzma2=dict=512KiB inputfile
+
+    An exception to above dictionary size limitation is when the decoder
+    is used in single-call mode. Decompressing the kernel itself is an
+    example of this situation. In single-call mode, the memory usage
+    doesn't depend on the dictionary size, and it is perfectly fine to
+    use a big dictionary: for maximum compression, the dictionary should
+    be at least as big as the uncompressed data itself.
+
+Future plans
+
+    Creating a limited XZ encoder may be considered if people think it is
+    useful. LZMA2 is slower to compress than e.g. Deflate or LZO even at
+    the fastest settings, so it isn't clear if LZMA2 encoder is wanted
+    into the kernel.
+
+    Support for limited random-access reading is planned for the
+    decompression code. I don't know if it could have any use in the
+    kernel, but I know that it would be useful in some embedded projects
+    outside the Linux kernel.
+
+Conformance to the .xz file format specification
+
+    There are a couple of corner cases where things have been simplified
+    at expense of detecting errors as early as possible. These should not
+    matter in practice all, since they don't cause security issues. But
+    it is good to know this if testing the code e.g. with the test files
+    from XZ Utils.
+
+Reporting bugs
+
+    Before reporting a bug, please check that it's not fixed already
+    at upstream. See <http://tukaani.org/xz/embedded.html> to get the
+    latest code.
+
+    Report bugs to <lasse.collin@tukaani.org> or visit #tukaani on
+    Freenode and talk to Larhzu. I don't actively read LKML or other
+    kernel-related mailing lists, so if there's something I should know,
+    you should email to me personally or use IRC.
+
+    Don't bother Igor Pavlov with questions about the XZ implementation
+    in the kernel or about XZ Utils. While these two implementations
+    include essential code that is directly based on Igor Pavlov's code,
+    these implementations aren't maintained nor supported by him.