linux-kernel - Re: [PATCH] Replace __toc

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Message-ID: <20210307062336.GD29191@gate.crashing.org>
Date:   Sun, 7 Mar 2021 00:23:36 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Alexey Kardashevskiy <aik@...abs.ru>
Subject: Re: [PATCH] Replace __toc_start + 0x8000 with .TOC.

Hi!

On Sat, Mar 06, 2021 at 09:14:33PM -0800, Fangrui Song wrote:
> TOC relocations are like GOT relocations on other architectures.
> However, unlike other architectures, GNU ld's ppc64 port defines .TOC.
> relative to the .got output section instead of the linker synthesized
> .got input section. LLD defines .TOC. as the .got input section plus
> 0x8000. When CONFIG_PPC_OF_BOOT_TRAMPOLINE=y,
> arch/powerpc/kernel/prom_init.o is built, and LLD computed .TOC. can be
> different from __toc_start defined by the linker script.
> 
> Simplify kernel_toc_addr with asm label .TOC. so that we can get rid of
> __toc_start.
> 
> With this change, powernv_defconfig with CONFIG_PPC_OF_BOOT_TRAMPOLINE=y
> is bootable with LLD. There is still an untriaged issue with Alexey's
> configuration.

Do you have any explanation why this *does* work, while the original
doesn't?  Some explanation that says *what* is wrong.  To me it doesn't
look like the kernel script is.


Segher