|XZ data compression in Linux
| 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
|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
| 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
| 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.
| 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.
| 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 <firstname.lastname@example.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.