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Alpha only ever supported gzip for the kernel image, and did so in a way
out of step with every other architecture: aboot, the disk boot loader,
decompresses vmlinux.gz itself with its own bundled inflate code, so
adding a compression format meant writing a new decompressor inside
aboot as well as in the kernel.
Build a proper self-extracting arch/alpha/boot/compressed/vmlinux
instead, modeled on parisc: a small ELF stub with the real, compressed
vmlinux linked in, which decompresses the kernel, relocates it into
place and jumps to its entry point. aboot needs no changes.
The stub has no sanitizer or coverage runtime and must run on any Alpha,
so it builds private copies of the arch/alpha/lib/ memory and division
routines it needs instead of linking lib.a, whose objects carry the
kernel's instrumentation and -mcpu=.
The build now emits arch/alpha/boot/vmlinuz rather than vmlinux.gz,
which was never a name aboot cared about and would be a lie for an ELF
stub: the bootpz path in the same directory still produces real gzip
data as vmlinux.nh.gz. Existing aboot.conf files name the image as it
was installed in /boot, so only the command that copies it out of the
build tree changes.
Unlike parisc and mips, the stub is linked at START_ADDR rather than at
a separate low address; see compressed/vmlinux.lds.S. It has no console
output, since bringing up the HWRPB console callbacks needs more than
the CRB dispatch the kernel proper does much later, so error() just
halts.
The legacy SRM netboot path (bootpz.c) is left alone: it has its own
boot protocol (PALcode reinitialization, self-populated ZERO_PGE,
build-time-computed image sizes) that does not fit this design without a
separate rewrite, and keeps using plain gzip via vmlinux.nh.gz.
Co-developed-by: Magnus Lindholm <linmag7@gmail.com>
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://lore.kernel.org/r/20261017-alpha-kernel-compression-v2-2-c71e6b08dc63@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
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