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diff --git a/Documentation/xz.txt b/Documentation/xz.txt new file mode 100644 index 000000000000..2cf3e2608de3 --- /dev/null +++ b/Documentation/xz.txt @@ -0,0 +1,121 @@ + +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. |
