linux-kernel - Re: [PATCH v10 2/2] riscv: Add runtime constant support

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Message-ID: <20250401192833.GA3645424@ax162>
Date: Tue, 1 Apr 2025 12:28:33 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Pasha Bouzarjomehri <pasha@...osinc.com>,
	Emil Renner Berthing <emil.renner.berthing@...onical.com>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Jason Baron <jbaron@...mai.com>,
	Andrew Jones <ajones@...tanamicro.com>,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v10 2/2] riscv: Add runtime constant support

Hi Charlie,

On Wed, Mar 19, 2025 at 11:35:20AM -0700, Charlie Jenkins wrote:
> Implement the runtime constant infrastructure for riscv. Use this
> infrastructure to generate constants to be used by the d_hash()
> function.
> 
> This is the riscv variant of commit 94a2bc0f611c ("arm64: add 'runtime
> constant' support") and commit e3c92e81711d ("runtime constants: add
> x86 architecture support").
> 
> Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>
...
> diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h
...
> +#define RISCV_RUNTIME_CONST_64_ZBA				\
> +	".option push\n\t"					\
> +	".option arch,+zba\n\t"					\
> +	"slli	%[__tmp],%[__tmp],32\n\t"			\
> +	"add.uw %[__ret],%[__ret],%[__tmp]\n\t"			\
> +	"nop\n\t"						\
> +	"nop\n\t"						\
> +	".option pop\n\t"					\
...
> +#if defined(CONFIG_RISCV_ISA_ZBA) && defined(CONFIG_RISCV_ISA_ZBKB)
...
> +#elif defined(CONFIG_RISCV_ISA_ZBA)
> +#define runtime_const_ptr(sym)						\
> +({									\
> +	typeof(sym) __ret, __tmp;					\
> +	asm_inline(RISCV_RUNTIME_CONST_64_PREAMBLE			\
> +		ALTERNATIVE(						\
> +			RISCV_RUNTIME_CONST_64_BASE,			\
> +			RISCV_RUNTIME_CONST_64_ZBA,			\
> +			0, RISCV_ISA_EXT_ZBA, 1				\
> +		)							\
> +		RISCV_RUNTIME_CONST_64_POSTAMBLE(sym)			\
> +		: [__ret] "=r" (__ret), [__tmp] "=r" (__tmp));		\
> +	__ret;								\
> +})

This breaks the build for clang versions 16 and earlier because they do
not support '.option arch' and it is used in CONFIG_RISCV_ISA_ZBA, which
has no dependencies and it is default on.

  $ make -skj"$(nproc)" ARCH=riscv LLVM=1 mrproper defconfig fs/dcache.o
  fs/dcache.c:117:9: warning: unknown option, expected 'push', 'pop', 'rvc', 'norvc', 'relax' or 'norelax' [-Winline-asm]
          return runtime_const_ptr(dentry_hashtable) +
                 ^
  arch/riscv/include/asm/runtime-const.h:103:4: note: expanded from macro 'runtime_const_ptr'
                          RISCV_RUNTIME_CONST_64_ZBA,                     \
                          ^
  arch/riscv/include/asm/runtime-const.h:57:17: note: expanded from macro 'RISCV_RUNTIME_CONST_64_ZBA'
          ".option push\n\t"                                      \
                         ^
  <inline asm>:32:10: note: instantiated into assembly here
          .option arch,+zba
                  ^
  fs/dcache.c:117:9: error: instruction requires the following: 'Zba' (Address Generation Instructions)
          return runtime_const_ptr(dentry_hashtable) +
                 ^
  arch/riscv/include/asm/runtime-const.h:103:4: note: expanded from macro 'runtime_const_ptr'
                          RISCV_RUNTIME_CONST_64_ZBA,                     \
                          ^
  arch/riscv/include/asm/runtime-const.h:59:30: note: expanded from macro 'RISCV_RUNTIME_CONST_64_ZBA'
          "slli   %[__tmp],%[__tmp],32\n\t"                       \
                                        ^
  <inline asm>:34:2: note: instantiated into assembly here
          add.uw a2,a2,a3
          ^
  ...

  $ rg 'OPTION_ARCH|ZBA' .config
  364:CONFIG_RISCV_ISA_ZBA=y

Should it grow a dependency on AS_HAS_OPTION_ARCH or should there be a
different fix?

Cheers,
Nathan