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

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Date:   Tue, 06 Sep 2022 08:32:13 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     Huacai Chen <chenhuacai@...nel.org>
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

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?

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