linux-kernel - Re: [PATCH 0/5] LoongArch: Support new relocation types

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Message-ID: <aced7303ca2a4130b73cfa46d7a25c1f4812a651.camel@xry111.site>
Date:   Thu, 28 Jul 2022 00:46:53 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     loongarch@...ts.linux.dev
Cc:     linux-kernel@...r.kernel.org, WANG Xuerui <kernel@...0n.name>,
        Huacai Chen <chenhuacai@...nel.org>
Subject: Re: [PATCH 0/5] LoongArch: Support new relocation types

Self notes:

With "-mla-global-with-pcrel" in KBUILD_CFLAGS_KERNEL gone (even if I
don't remove it, it will be ineffective with binutils-2.40/gcc-13), the
kernel will use GOT to access the objects in other translation units. 
It may cause a performance regression.

*If* such a performance regression is really observed, we need to change
GCC to use PC relative addressing (instead of GOT) for loading external
objects when -fno-PIC.  Such a change will benefit userspace apps as
well, *if* PC relative addressing is really faster than GOT access.

(Emphasize "if" here: I've not run any benchmarks yet.)

I'll be mostly AFK in the following one or two weeks, apologize for
possible slow respond.

On Thu, 2022-07-28 at 00:24 +0800, Xi Ruoyao wrote:
> The version 2.00 of LoongArch ELF ABI specification introduced new
> relocation types, and the development tree of Binutils and GCC has
> started to use them.  If the kernel is built with the latest snapshot of
> Binutils or GCC, it will fail to load the modules because of unrecognized
> relocation types in modules.
> 
> Add support for GOT and new relocation types for the module loader, so
> the kernel (with modules) can be built with the "normal" code model and
> function properly.
> 
> Tested by building the kernel with Binutils & GCC master branch, and
> running the kernel with 35 in-tree modules loaded.
> 
> Link: https://github.com/loongson/LoongArch-Documentation/pull/57
> Link: https://gcc.gnu.org/r13-1834
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=f09482a
> 
> Xi Ruoyao (5):
>   LoongArch: Add section of GOT for kernel module
>   LoongArch: Support R_LARCH_SOP_PUSH_GPREL relocation type in kernel
>     module
>   LoongArch: Support relocation against _GLOBAL_OFFSET_TABLE_
>   LoongArch: Stop using undocumented assembler options
>   LoongArch: Support modules with new relocation types
> 
>  arch/loongarch/Makefile                 |  5 +-
>  arch/loongarch/include/asm/elf.h        | 37 ++++++++++
>  arch/loongarch/include/asm/module.h     | 23 ++++++
>  arch/loongarch/include/asm/module.lds.h |  1 +
>  arch/loongarch/kernel/head.S            | 10 +--
>  arch/loongarch/kernel/module-sections.c | 72 +++++++++++++++++--
>  arch/loongarch/kernel/module.c          | 94 +++++++++++++++++++++++++
>  7 files changed, 229 insertions(+), 13 deletions(-)
> 

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University