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When we boot from open firmware (OF) using PPC_OF_BOOT_TRAMPOLINE, aka. prom_init, we run parts of the kernel at an address other than the link address. That happens because OF loads the kernel above zero (OF is at zero) and we run prom_init before copying the kernel down to zero. Currently that works even for non-relocatable kernels, because we do various fixups to the prom_init code to make it run where it's loaded. However those fixups are not sufficient if the kernel becomes large enough. In that case prom_init()'s final call to __start() can end up generating a plt branch: bl c000000002000018 <00000078.plt_branch.__start> That results in the kernel jumping to the linked address of __start, 0xc000000000000000, when really it needs to jump to the 0xc000000000000000 + the runtime address because the kernel is still running at the load address. We could do further shenanigans to handle that, see Jordan's patch for example: https://lore.kernel.org/linuxppc-dev/20210421021721.1539289-1-jniethe5@gmail.com However it is much simpler to just require a kernel with prom_init() to be built relocatable. The result works in all configurations without further work, and requires less code. This should have no effect on most people, as our defconfigs and essentially all distro configs already have RELOCATABLE enabled. Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://lore.kernel.org/r/20210623130454.2542945-1-mpe@ellerman.id.au
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.