Re: [PATCH v3] riscv: Handle zicsr/zifencei issue between gcc and binutils

[Conor Dooley]

Hey,

On Mon, Jul 31, 2023 at 05:59:36PM +0800, Mingzheng Xing wrote:
> Binutils-2.38 and GCC-12.1.0 bumped[0][1] the default ISA spec to the newer
> 20191213 version which moves some instructions from the I extension to the
> Zicsr and Zifencei extensions. So if one of the binutils and GCC exceeds
> that version, we should explicitly specifying Zicsr and Zifencei via -march
> to cope with the new changes. but this only occurs when binutils >= 2.36
> and GCC >= 11.1.0. It's a different story when binutils < 2.36.
> 
> binutils-2.36 supports the Zifencei extension[2] and splits Zifencei and
> Zicsr from I[3]. GCC-11.1.0 is particular[4] because it add support Zicsr
> and Zifencei extension for -march. binutils-2.35 does not support the
> Zifencei extension, and does not need to specify Zicsr and Zifencei when
> working with GCC >= 12.1.0.
> 
> To make our lives easier, let's relax the check to binutils >= 2.36 in
> CONFIG_TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI. For the other two cases,
> where clang < 17 or GCC < 11.1.0, we will deal with them in
> CONFIG_TOOLCHAIN_NEEDS_OLD_ISA_SPEC.
> 
> For more information, please refer to:
> commit 6df2a016c0c8 ("riscv: fix build with binutils 2.38")
> commit e89c2e815e76 ("riscv: Handle zicsr/zifencei issues between clang and binutils")
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=aed44286efa8ae8717a77d94b51ac3614e2ca6dc [0]
> Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=98416dbb0a62579d4a7a4a76bab51b5b52fec2cd [1]
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=5a1b31e1e1cee6e9f1c92abff59cdcfff0dddf30 [2]
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=729a53530e86972d1143553a415db34e6e01d5d2 [3]
> Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=b03be74bad08c382da47e048007a78fa3fb4ef49 [4]
> Link: https://lore.kernel.org/all/20230308220842.1231003-1-conor@xxxxxxxxxx
> Link: https://lore.kernel.org/all/20230223220546.52879-1-conor@xxxxxxxxxx
> Signed-off-by: Mingzheng Xing <xingmingzheng@xxxxxxxxxxx>

You need to actually put the CC: stable@xxxxxxxxxxxxxxx into the commit
message for the stable folks to pick things up.

> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 4c07b9189c86..2704bd91dfb5 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -570,24 +570,31 @@ config TOOLCHAIN_HAS_ZIHINTPAUSE
>  config TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI
>  	def_bool y
>  	# https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=aed44286efa8ae8717a77d94b51ac3614e2ca6dc
> -	depends on AS_IS_GNU && AS_VERSION >= 23800
> -	help
> -	  Newer binutils versions default to ISA spec version 20191213 which
> -	  moves some instructions from the I extension to the Zicsr and Zifencei
> -	  extensions.
> +	# https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=98416dbb0a62579d4a7a4a76bab51b5b52fec2cd
> +	depends on AS_IS_GNU && AS_VERSION >= 23600
> +	help
> +	  Binutils-2.38 and GCC-12.1.0 bumped the default ISA spec to the newer
> +	  20191213 version, which moves some instructions from the I extension to
> +	  the Zicsr and Zifencei extensions. This requires explicitly specifying
> +	  zicsr and zifencei when binutils >= 2.38 or GCC >= 12.1.0,

> but this only
> +	  occurs when binutils >= 2.36 and GCC >= 11.1.0.
> +	 It's a different story when binutils < 2.36.

I would replace this with something like:
	Binutils-2.38 and GCC-12.1.0 bumped the default ISA spec to the newer
	20191213 version, which moves some instructions from the I extension to
	the Zicsr and Zifencei extensions. This requires explicitly specifying
	zicsr and zifencei when binutils >= 2.38 or GCC >= 12.1.0. Zicsr
	and Zifencei are supported in binutils from version 2.36 onwards.
	To make life easier, and avoid forcing toolchains that default to a
	newer ISA spec to version 2.2, relax the check to binutils >= 2.36.
	For clang < 17 or GCC < 11.1.0, for which this is not possible, this is
	dealt with in CONFIG_TOOLCHAIN_NEEDS_OLD_ISA_SPEC.

I tried to take the personification out of it & hopefully made it a
little clearer what dropping the check to 2.36 does for us.

> Also, we have to consider the case of clang paired with binutils.

I think this sentence can go, its covered in the other config text.

I'm sorry for being a bit of a pedant about this, but this has been such
a can of worms that I would like things to remain explained well enough
that the text is sufficient next time a revisit is required.

Thanks,
Conor.

> +	  To make our lives easier, we relax the check to binutils >= 2.36. >  
>  config TOOLCHAIN_NEEDS_OLD_ISA_SPEC
>  	def_bool y
>  	depends on TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI
>  	# https://github.com/llvm/llvm-project/commit/22e199e6afb1263c943c0c0d4498694e15bf8a16
> -	depends on CC_IS_CLANG && CLANG_VERSION < 170000
> -	help
> -	  Certain versions of clang do not support zicsr and zifencei via -march
> -	  but newer versions of binutils require it for the reasons noted in the
> -	  help text of CONFIG_TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI. This
> -	  option causes an older ISA spec compatible with these older versions
> -	  of clang to be passed to GAS, which has the same result as passing zicsr
> -	  and zifencei to -march.
> +	# https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=b03be74bad08c382da47e048007a78fa3fb4ef49
> +	depends on (CC_IS_CLANG && CLANG_VERSION < 170000) || (CC_IS_GCC && GCC_VERSION < 110100)
> +	help
> +	  Certain versions of clang and GCC do not support zicsr and zifencei via
> +	  -march. This option causes an older ISA spec compatible with these older
> +	  versions of clang and GCC to be passed to GAS, which has the same result
> +	  as passing zicsr and zifencei to -march.
>  
>  config FPU
>  	bool "FPU support"
> diff --git a/arch/riscv/kernel/compat_vdso/Makefile b/arch/riscv/kernel/compat_vdso/Makefile
> index 189345773e7e..b86e5e2c3aea 100644
> --- a/arch/riscv/kernel/compat_vdso/Makefile
> +++ b/arch/riscv/kernel/compat_vdso/Makefile
> @@ -11,7 +11,13 @@ compat_vdso-syms += flush_icache
>  COMPAT_CC := $(CC)
>  COMPAT_LD := $(LD)
>  
> -COMPAT_CC_FLAGS := -march=rv32g -mabi=ilp32
> +# binutils 2.35 does not support the zifencei extension, but in the ISA
> +# spec 20191213, G stands for IMAFD_ZICSR_ZIFENCEI.
> +ifdef CONFIG_TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI
> +	COMPAT_CC_FLAGS := -march=rv32g -mabi=ilp32
> +else
> +	COMPAT_CC_FLAGS := -march=rv32imafd -mabi=ilp32
> +endif
>  COMPAT_LD_FLAGS := -melf32lriscv
>  
>  # Disable attributes, as they're useless and break the build.
> -- 
> 2.34.1
>

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