For historical reasons, the legacy decompressor code on various
architectures supports 7 different compression types for the compressed
kernel image.
EFI zboot is not a compression library museum, and so the options can be
limited to what is likely to be useful in practice:
- GZIP is tried and tested, and is still one of the fastest at
decompression time, although the compression ratio is not very high;
moreover, Fedora is already shipping EFI zboot kernels for arm64 that
use GZIP, and QEMU implements direct support for it when booting a
kernel without firmware loaded;
- ZSTD has a very high compression ratio (although not the highest), and
is almost as fast as GZIP at decompression time.
Reducing the number of options makes it less of a hassle for other
consumers of the EFI zboot format (such as QEMU today, and kexec in the
future) to support it transparently without having to carry 7 different
decompression libraries.
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
0b2c29fb68