linux-kernel - Re: [PATCH v7 0/5] LoongArch: Support toolchain with new relocation types

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Message-ID: <CAAhV-H56TQeU54JjvNQg2qZ6L1aSbzbaM2a=VQr9ZcAn4uthyg@mail.gmail.com>
Date:   Tue, 6 Sep 2022 09:52:43 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     Xi Ruoyao <xry111@...111.site>
Cc:     Jinyang He <hejinyang@...ngson.cn>,
        WANG Xuerui <kernel@...0n.name>, loongarch@...ts.linux.dev,
        LKML <linux-kernel@...r.kernel.org>,
        Youling Tang <tangyouling@...ngson.cn>
Subject: Re: [PATCH v7 0/5] LoongArch: Support toolchain with new relocation types

Hi, Ruoyao,

On Tue, Sep 6, 2022 at 8:32 AM Xi Ruoyao <xry111@...111.site> wrote:
>
> On Thu, 2022-09-01 at 10:17 +0800, Huacai Chen wrote:
>
> > Now all global variable accesses are via got, I think the performance
> > may be much worse than before when we didn't use explicit-relocs.
> > I don't know whether "a new code model" or your "(1)(2)(3)" is easier
> > to implement, but I think it is better to solve the performance issue
> > before 6.1-rc1.
>
> Hi Huacai,
>
> We've added a GCC option for this at https://gcc.gnu.org/r13-2433.  On
> the kernel side we need a one-line change:
>
> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index 92c4a52c4c3e..69b39ba3a09d 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -54,6 +54,7 @@ LDFLAGS_vmlinux                       += -G0 -static -n -nostdlib
>  # upgrade the compiler or downgrade the assembler.
>  ifdef CONFIG_AS_HAS_EXPLICIT_RELOCS
>  cflags-y                       += -mexplicit-relocs
> +KBUILD_CFLAGS_KERNEL           += -mdirect-extern-access
>  else
>  cflags-y                       += $(call cc-option,-mno-explicit-relocs)
>  KBUILD_AFLAGS_KERNEL           += -Wa,-mla-global-with-pcrel
>
> And we also need a one-line change in the EFI stub patch (under review):
>
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index 8931ed24379e..8c1225b92492 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -27,7 +27,7 @@ cflags-$(CONFIG_ARM)          := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
>  cflags-$(CONFIG_RISCV)         := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
>                                    -fpic
>  cflags-$(CONFIG_LOONGARCH)     := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
> -                                  -fpic
> +                                  -fpie
>
>  cflags-$(CONFIG_EFI_GENERIC_STUB) += -I$(srctree)/scripts/dtc/libfdt
>
> (Some explanation: -fpic does not only mean "generate position-
> independent code", but "generate position-independent code *suitable for
> use in a shared library*".  On LoongArch -mdirect-extern-access cannot
> work for a shared library so the "-fpic -mdirect-extern-access"
> combination is rejected deliberately.)
>
> Not sure how to submit these changes properly...  Do you prefer me to
> send V8 of this series or a single patch on top of your tree on GitHub?
Don't need V8, I will squash it into the previous patch myself. But
can we keep efistub as is?

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