[PATCH] riscv: Use -mno-relax when using lld linker

[Khem Raj]

lld does not implement the RISCV relaxation optimizations like GNU ld
therefore disable it when building with LLVM=1, Also pass it to
assembler when using external GNU assembler ( LLVM_IAS != 1 ), this
ensures that relevant assembler option is also enabled along. if these
options are not used then we see following relocations in objects

0000000000000000 R_RISCV_ALIGN *ABS*+0x0000000000000002

These are then rejected by lld
ld.lld: error: capability.c:(.fixup+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax but the .o is already compiled with -mno-relax

Signed-off-by: Khem Raj <raj....@gmail.com>
Cc: Paul Walmsley <paul.w...@sifive.com>
Cc: Palmer Dabbelt <pal...@dabbelt.com>
Cc: Albert Ou <a...@eecs.berkeley.edu>
Cc: Nathan Chancellor <nat...@kernel.org>
Cc: Nick Desaulniers <ndesau...@google.com>
---
arch/riscv/Makefile | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 3eb9590a0775..519f133e0d53 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -38,6 +38,15 @@ else
KBUILD_LDFLAGS += -melf32lriscv
endif

+ifeq ($(LLVM),1)
+ KBUILD_CFLAGS += -mno-relax
+ KBUILD_AFLAGS += -mno-relax
+ifneq ($(LLVM_IAS),1)
+ KBUILD_CFLAGS += -Wa,-mno-relax
+ KBUILD_AFLAGS += -Wa,-mno-relax
+endif
+endif
+
# ISA string setting
riscv-march-$(CONFIG_ARCH_RV32I) := rv32ima
riscv-march-$(CONFIG_ARCH_RV64I) := rv64ima
--
2.31.1

[Nathan Chancellor]

Hi Khem,

On 5/14/2021 1:56 PM, Khem Raj wrote:
> lld does not implement the RISCV relaxation optimizations like GNU ld
> therefore disable it when building with LLVM=1, Also pass it to
> assembler when using external GNU assembler ( LLVM_IAS != 1 ), this
> ensures that relevant assembler option is also enabled along. if these
> options are not used then we see following relocations in objects
>
> 0000000000000000 R_RISCV_ALIGN *ABS*+0x0000000000000002
>
> These are then rejected by lld
> ld.lld: error: capability.c:(.fixup+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax but the .o is already compiled with -mno-relax
>
> Signed-off-by: Khem Raj <raj....@gmail.com>
> Cc: Paul Walmsley <paul.w...@sifive.com>
> Cc: Palmer Dabbelt <pal...@dabbelt.com>
> Cc: Albert Ou <a...@eecs.berkeley.edu>
> Cc: Nathan Chancellor <nat...@kernel.org>
> Cc: Nick Desaulniers <ndesau...@google.com>

Thank you for the patch! I can build a ARCH=riscv defconfig kernel with
LLVM=1 now. LLVM_IAS=1 still needs work but we have outstanding issues
for that:

https://github.com/ClangBuiltLinux/linux/issues/1023
https://github.com/ClangBuiltLinux/linux/issues/1143

Reviewed-by: Nathan Chancellor <nat...@kernel.org>

One comment below though.

> ---
> arch/riscv/Makefile | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 3eb9590a0775..519f133e0d53 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -38,6 +38,15 @@ else
> KBUILD_LDFLAGS += -melf32lriscv
> endif
>
> +ifeq ($(LLVM),1)

ifeq ($(CONFIG_LD_IS_LLD),y)

is a better choice so that LD=riscv64-linux-gnu-ld LLVM=1 can still work.

[Fāng-ruì Sòng]

On Fri, May 14, 2021 at 1:57 PM Khem Raj <raj....@gmail.com> wrote:
>
> lld does not implement the RISCV relaxation optimizations like GNU ld
> therefore disable it when building with LLVM=1, Also pass it to
> assembler when using external GNU assembler ( LLVM_IAS != 1 ), this
> ensures that relevant assembler option is also enabled along. if these
> options are not used then we see following relocations in objects
>
> 0000000000000000 R_RISCV_ALIGN *ABS*+0x0000000000000002
>
> These are then rejected by lld
> ld.lld: error: capability.c:(.fixup+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax but the .o is already compiled with -mno-relax

This means -mno-relax should emit .option norelax in the assembly.
I'll try fixing this in Clang 13.

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--
宋方睿

[Khem Raj]

lld does not implement the RISCV relaxation optimizations like GNU ld

therefore disable it when building with lld, Also pass it to

assembler when using external GNU assembler ( LLVM_IAS != 1 ), this
ensures that relevant assembler option is also enabled along. if these
options are not used then we see following relocations in objects

0000000000000000 R_RISCV_ALIGN *ABS*+0x0000000000000002

These are then rejected by lld
ld.lld: error: capability.c:(.fixup+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax but the .o is already compiled with -mno-relax

Signed-off-by: Khem Raj <raj....@gmail.com>
Cc: Paul Walmsley <paul.w...@sifive.com>
Cc: Palmer Dabbelt <pal...@dabbelt.com>
Cc: Albert Ou <a...@eecs.berkeley.edu>
Cc: Nathan Chancellor <nat...@kernel.org>
Cc: Nick Desaulniers <ndesau...@google.com>
---

v2: Use CONFIG_LD_IS_LLD instead of LLVM check

arch/riscv/Makefile | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile

index 3eb9590a0775..4be020695428 100644

--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -38,6 +38,15 @@ else
KBUILD_LDFLAGS += -melf32lriscv
endif

+ifeq ($(CONFIG_LD_IS_LLD),y)

[Nathan Chancellor]

On 5/14/2021 2:37 PM, Khem Raj wrote:

> lld does not implement the RISCV relaxation optimizations like GNU ld
> therefore disable it when building with lld, Also pass it to
> assembler when using external GNU assembler ( LLVM_IAS != 1 ), this
> ensures that relevant assembler option is also enabled along. if these
> options are not used then we see following relocations in objects
>
> 0000000000000000 R_RISCV_ALIGN *ABS*+0x0000000000000002
>
> These are then rejected by lld
> ld.lld: error: capability.c:(.fixup+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax but the .o is already compiled with -mno-relax
>
> Signed-off-by: Khem Raj <raj....@gmail.com>
> Cc: Paul Walmsley <paul.w...@sifive.com>
> Cc: Palmer Dabbelt <pal...@dabbelt.com>
> Cc: Albert Ou <a...@eecs.berkeley.edu>
> Cc: Nathan Chancellor <nat...@kernel.org>
> Cc: Nick Desaulniers <ndesau...@google.com>

Reviewed-by: Nathan Chancellor <nat...@kernel.org>

[Palmer Dabbelt]

Thanks, this is on fixes.

[Palmer Dabbelt]

Sorry, replied to the v1 but I merged the v2.