linux-kernel - Re: [PATCH 2/8] LoongArch: Only use -Wa,-mla-* options for !CONFIG_CC_HAS_EXPLICIT

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Message-ID: <CAAhV-H5S+9MZi5_LL11hCFePG9GK08ot_LK+mwZjh+5Hoyxutg@mail.gmail.com>
Date:   Sun, 28 Aug 2022 22:15:39 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     Xi Ruoyao <xry111@...111.site>
Cc:     loongarch@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Youling Tang <tangyouling@...ngson.cn>,
        Jinyang He <hejinyang@...ngson.cn>
Subject: Re: [PATCH 2/8] LoongArch: Only use -Wa,-mla-* options for !CONFIG_CC_HAS_EXPLICIT_RELOCS

Hi, Ruoyao,

On Sun, Aug 28, 2022 at 1:55 AM Xi Ruoyao <xry111@...111.site> wrote:
>
> If explicit relocation hints is used by the toolchain, -Wa,-mla-*
> options will be useless for C code.  Only use them for
> !CONFIG_CC_HAS_EXPLICIT_RELOCS.
>
> Replace "la" with "la.pcrel" in head.S to keep the semantic consistent
> with new and old toolchains for the low level startup code.
>
> Signed-off-by: Xi Ruoyao <xry111@...111.site>
> ---
>  arch/loongarch/Makefile      | 15 ++++++++++++++-
>  arch/loongarch/kernel/head.S | 10 +++++-----
>  2 files changed, 19 insertions(+), 6 deletions(-)
>
> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index 415e09bdf31a..bd0ea6623245 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -40,10 +40,23 @@ endif
>
>  cflags-y                       += -G0 -pipe -msoft-float
>  LDFLAGS_vmlinux                        += -G0 -static -n -nostdlib
> +
> +ifdef CONFIG_CC_HAS_EXPLICIT_RELOCS
> +# GCC may have -mexplicit-relocs off by default if it was built with an old
> +# assembler.  But with CONFIG_CC_HAS_EXPLICIT_RELOCS we are sure explicit
> +# relocation hints are supported by the assembler, so force it.
> +cflags-y                       += -mexplicit-relocs
> +else
> +# Likewise, disable -mexplicit-relocs if GCC enables by default but the
> +# assembler does not support it.
> +cflags-y                       += $(call cc-option,-mno-explicit-relocs)
> +# With -mno-explicit-relocs, use assembler options to adjust code generation
> +# for symbol address.
>  KBUILD_AFLAGS_KERNEL           += -Wa,-mla-global-with-pcrel
>  KBUILD_CFLAGS_KERNEL           += -Wa,-mla-global-with-pcrel
>  KBUILD_AFLAGS_MODULE           += -Wa,-mla-global-with-abs
> -KBUILD_CFLAGS_MODULE           += -fplt -Wa,-mla-global-with-abs,-mla-local-with-abs
> +KBUILD_CFLAGS_MODULE           += -Wa,-mla-global-with-abs,-mla-local-with-abs
> +endif
Move all comments out of #ifdefs may make code more clear.


Huacai
>
>  cflags-y += -ffreestanding
>  cflags-y += $(call cc-option, -mno-check-zero-division)
> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index 01bac62a6442..eb3f641d5915 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -55,17 +55,17 @@ SYM_CODE_START(kernel_entry)                        # kernel entry point
>         li.w            t0, 0x00                # FPE=0, SXE=0, ASXE=0, BTE=0
>         csrwr           t0, LOONGARCH_CSR_EUEN
>
> -       la              t0, __bss_start         # clear .bss
> +       la.pcrel        t0, __bss_start         # clear .bss
>         st.d            zero, t0, 0
> -       la              t1, __bss_stop - LONGSIZE
> +       la.pcrel        t1, __bss_stop - LONGSIZE
>  1:
>         addi.d          t0, t0, LONGSIZE
>         st.d            zero, t0, 0
>         bne             t0, t1, 1b
>
> -       la              t0, fw_arg0
> +       la.pcrel        t0, fw_arg0
>         st.d            a0, t0, 0               # firmware arguments
> -       la              t0, fw_arg1
> +       la.pcrel        t0, fw_arg1
>         st.d            a1, t0, 0
>
>         /* KSave3 used for percpu base, initialized as 0 */
> @@ -73,7 +73,7 @@ SYM_CODE_START(kernel_entry)                  # kernel entry point
>         /* GPR21 used for percpu base (runtime), initialized as 0 */
>         move            u0, zero
>
> -       la              tp, init_thread_union
> +       la.pcrel        tp, init_thread_union
>         /* Set the SP after an empty pt_regs.  */
>         PTR_LI          sp, (_THREAD_SIZE - 32 - PT_SIZE)
>         PTR_ADD         sp, sp, tp
> --
> 2.37.0
>