linux-kernel - Re: [PATCH] Replace __toc

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Date:   Sat, 6 Mar 2021 22:31:15 -0800
From:   Fāng-ruì Sòng <maskray@...gle.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <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.

On Sat, Mar 6, 2021 at 10:25 PM Segher Boessenkool
<segher@...nel.crashing.org> wrote:
>
> 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

The kernel code probably wants to access .TOC. (the TOC base symbol)
via __toc_start+0x8000.
If the kernel understood TOC base is different from the linker
understood TOC base (.TOC.), there should be a problem.
By using .TOC. in the kernel code, the two concepts are guaranteed to match.