linux-kernel - Re: [PATCH 2/2] riscv: fix detection of toolchain Zihintpause support

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Date:   Thu, 13 Oct 2022 13:30:16 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Conor Dooley <conor@...nel.org>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Dao Lu <daolu@...osinc.com>, Heiko Stuebner <heiko@...ech.de>,
        Guo Ren <guoren@...nel.org>, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 2/2] riscv: fix detection of toolchain Zihintpause support

On Thu, Oct 06, 2022 at 06:35:21PM +0100, Conor Dooley wrote:
> From: Conor Dooley <conor.dooley@...rochip.com>
> 
> It is not sufficient to check if a toolchain supports a particular
> extension without checking if the linker supports that extension
> too. For example, Clang 15 supports Zihintpause but GNU bintutils
> 2.35.2 does not, leading build errors like so:
> 
> riscv64-linux-gnu-ld: -march=rv64i2p0_m2p0_a2p0_c2p0_zihintpause2p0: Invalid or unknown z ISA extension: 'zihintpause'
> 
> Add a TOOLCHAIN_HAS_ZIHINTPAUSE which checks if each of the compiler,
> assembler and linker support the extension. Replace the ifdef in the
> vdso with one depending on this new symbol.
> 
> Fixes: 8eb060e10185 ("arch/riscv: add Zihintpause support")
> Signed-off-by: Conor Dooley <conor.dooley@...rochip.com>
> ---
> Palmer:
> The VDSO change will conflict with Samuel's one, resolution should be
> trivial.. I only made that change as you warned me about checking for
> the __riscv_foo stuff if I made the march string depend on the Kconfig
> entry rather than on the Makefile's cc-option check.

The versions look correct to me. I see the LLVM zihintpause commit [1]
in llvmorg-15.0.0 and I see the binutils zihintpause commit [2] in
binutils-2_36.

[1]: https://github.com/llvm/llvm-project/commit/005fd8aa702edbc532763038365575da96e5787d
[2]: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=aa881ecde48c7a0224b92e2cfa43b37ee9ec9fa2

Similar comment as patch 1, I think we can just drop the cc-option
checks. Regardless:

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  arch/riscv/Kconfig                      | 7 +++++++
>  arch/riscv/Makefile                     | 3 +--
>  arch/riscv/include/asm/vdso/processor.h | 2 +-
>  3 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 6da36553158b..d7c53896e24f 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -425,6 +425,13 @@ config RISCV_ISA_ZICBOM
>  
>  	   If you don't know what to do here, say Y.
>  
> +config TOOLCHAIN_HAS_ZIHINTPAUSE
> +	bool
> +	default y
> +	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zihintpause)
> +	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zihintpause)
> +	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23600
> +
>  config FPU
>  	bool "FPU support"
>  	default y
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 3607d38edb4f..6651517f3962 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -60,8 +60,7 @@ riscv-march-$(toolchain-need-zicsr-zifencei) := $(riscv-march-y)_zicsr_zifencei
>  riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZICBOM) := $(riscv-march-y)_zicbom
>  
>  # Check if the toolchain supports Zihintpause extension
> -toolchain-supports-zihintpause := $(call cc-option-yn, -march=$(riscv-march-y)_zihintpause)
> -riscv-march-$(toolchain-supports-zihintpause) := $(riscv-march-y)_zihintpause
> +riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE) := $(riscv-march-y)_zihintpause
>  
>  KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
>  KBUILD_AFLAGS += -march=$(riscv-march-y)
> diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
> index 1e4f8b4aef79..fa70cfe507aa 100644
> --- a/arch/riscv/include/asm/vdso/processor.h
> +++ b/arch/riscv/include/asm/vdso/processor.h
> @@ -21,7 +21,7 @@ static inline void cpu_relax(void)
>  		 * Reduce instruction retirement.
>  		 * This assumes the PC changes.
>  		 */
> -#ifdef __riscv_zihintpause
> +#ifdef CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE
>  		__asm__ __volatile__ ("pause");
>  #else
>  		/* Encoding of the pause instruction */
> -- 
> 2.37.3
> 
>