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SubjectRe: [PATCH v6 2/6] LoongArch: Adjust CFLAGS for CONFIG_AS_HAS_EXPLICIT_RELOCS
Hi, Ruoyao,

On Mon, Aug 29, 2022 at 9:35 PM Xi Ruoyao <xry111@xry111.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_AS_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.
>
> Remove -fplt because it's the default for all known LoongArch C
> compilers.
>
> The behavior with different assemblers and compilers are summarized in
> the following table:
>
> AS has CC has
> explicit reloc explicit reloc Behavior
> ==============================================================
> No No Use la.* macros.
> No change from Linux 6.0.
> --------------------------------------------------------------
> No Yes Disable explicit reloc.
> No change from Linux 6.0.
> --------------------------------------------------------------
> Yes No Not supported.
> --------------------------------------------------------------
> Yes Yes Use explicit relocs.
> No -Wa,-mla* options.
> ==============================================================
>
> Signed-off-by: Xi Ruoyao <xry111@xry111.site>
> ---
> arch/loongarch/Makefile | 19 ++++++++++++++++++-
> arch/loongarch/kernel/head.S | 10 +++++-----
> 2 files changed, 23 insertions(+), 6 deletions(-)
>
> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index 7051a95f7f31..1563747c4fa8 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -40,10 +40,27 @@ endif
>
> cflags-y += -G0 -pipe -msoft-float
> LDFLAGS_vmlinux += -G0 -static -n -nostdlib
> +
> +# When the assembler supports explicit relocation hint, we must use it.
> +# GCC may have -mexplicit-relocs off by default if it was built with an old
> +# assembler, so we force it via an option.
> +#
> +# When the assembler does not supports explicit relocation hint, we can't use
> +# it. Disable it if the compiler supports it.
> +#
> +# If you've seen "unknown reloc hint" message building the kernel and you are
> +# now wondering why "-mexplicit-relocs" is not wrapped with cc-option: the
> +# combination of a "new" assembler and "old" compiler is not supported. Either
> +# upgrade the compiler or downgrade the assembler.
> +ifdef CONFIG_AS_HAS_EXPLICIT_RELOCS
> +cflags-y += -mexplicit-relocs
> +else
> +cflags-y += $(call cc-option,-mno-explicit-relocs)
> 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
Though -fplt is the default, keep it can make code clearer.

Huacai
> +endif
>
> 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
>
>

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