linux-kernel - Re: [PATCH v1 10/14] DCE/DSE: riscv: make every ex

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Message-ID: <DS7PR12MB57652C7C17103B3FF729BD79CBABA@DS7PR12MB5765.namprd12.prod.outlook.com>
Date:   Sun, 5 Nov 2023 13:13:47 -0800
From:   Fangrui Song <i@...kray.me>
To:     Yuan Tan <tanyuan@...ylab.org>
Cc:     falcon@...ylab.org, arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-riscv@...ts.infradead.org,
        luc.vanoostenryck@...il.com, linux-sparse@...r.kernel.org,
        linux@...ssschuh.net, palmer@...osinc.com,
        paul.walmsley@...ive.com, paulburton@...nel.org,
        paulmck@...nel.org, tim.bird@...y.com, tsbogend@...ha.franken.de,
        w@....eu
Subject: Re: [PATCH v1 10/14] DCE/DSE: riscv: make every ex_table's name unique

On Fri, Nov 3, 2023 at 9:02 AM Yuan Tan <tanyuan@...ylab.org> wrote:
>
> gc-sections require section names to be unique.

This is not precise, since a monolithic section can be garbage
collected as well.

https://maskray.me/blog/2021-02-28-linker-garbage-collection

"Without the options, the compiler tends to produce monolithic text
and data sections. The large sections make the GC coarse-grained and
they will likely be retained as a whole. ld --gc-sections can still
discard some sections, but the effect is likely very poor."


> Signed-off-by: Yuan Tan <tanyuan@...ylab.org>
> Signed-off-by: Zhangjin Wu <falcon@...ylab.org>
> ---
>  arch/riscv/include/asm/asm-extable.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/asm-extable.h b/arch/riscv/include/asm/asm-extable.h
> index 00a96e7a9664..d0be5a838242 100644
> --- a/arch/riscv/include/asm/asm-extable.h
> +++ b/arch/riscv/include/asm/asm-extable.h
> @@ -9,10 +9,12 @@
>
>  #ifdef CONFIG_MMU
>
> +#include <linux/compiler.h>
> +
>  #ifdef __ASSEMBLY__
>
>  #define __ASM_EXTABLE_RAW(insn, fixup, type, data)     \
> -       .pushsection    __ex_table, "a";                \
> +       .pushsection    __SECTION_NAME(__ex_table), "a";                \
>         .balign         4;                              \
>         .long           ((insn) - .);                   \
>         .long           ((fixup) - .);                  \
> @@ -31,7 +33,7 @@
>  #include <asm/gpr-num.h>

__SECTION_NAME is probably added by a previous patch.
An alternative is to use the .section directive's `unique` feature
(binutils>=2.35).

>  #define __ASM_EXTABLE_RAW(insn, fixup, type, data)     \
> -       ".pushsection   __ex_table, \"a\"\n"            \
> +       ".pushsection " __SECTION_NAME(__ex_table) ", \"a\"\n"          \
>         ".balign        4\n"                            \
>         ".long          ((" insn ") - .)\n"             \
>         ".long          ((" fixup ") - .)\n"            \
> --
> 2.34.1
>