Re: [PATCH v3 01/11] riscv: Implement cmpxchg32/64() using Zacas

[Andrew Jones <ajones@ventanamicro.com>]

On Wed, Jul 17, 2024 at 08:19:47AM GMT, Alexandre Ghiti wrote:
> This adds runtime support for Zacas in cmpxchg operations.
> 
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
>  arch/riscv/Kconfig               | 17 +++++++++++++++++
>  arch/riscv/Makefile              |  3 +++
>  arch/riscv/include/asm/cmpxchg.h | 26 +++++++++++++++++++++++---
>  3 files changed, 43 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 05ccba8ca33a..1caaedec88c7 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -596,6 +596,23 @@ config RISCV_ISA_V_PREEMPTIVE
>  	  preemption. Enabling this config will result in higher memory
>  	  consumption due to the allocation of per-task's kernel Vector context.
>  
> +config TOOLCHAIN_HAS_ZACAS
> +	bool
> +	default y
> +	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zacas)
> +	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zacas)
> +	depends on AS_HAS_OPTION_ARCH
> +
> +config RISCV_ISA_ZACAS
> +	bool "Zacas extension support for atomic CAS"
> +	depends on TOOLCHAIN_HAS_ZACAS
> +	default y
> +	help
> +	  Enable the use of the Zacas ISA-extension to implement kernel atomic
> +	  cmpxchg operations when it is detected at boot.
> +
> +	  If you don't know what to do here, say Y.
> +
>  config TOOLCHAIN_HAS_ZBB
>  	bool
>  	default y
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 06de9d365088..9fd13d7a9cc6 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -85,6 +85,9 @@ endif
>  # Check if the toolchain supports Zihintpause extension
>  riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE) := $(riscv-march-y)_zihintpause
>  
> +# Check if the toolchain supports Zacas
> +riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZACAS) := $(riscv-march-y)_zacas
> +
>  # Remove F,D,V from isa string for all. Keep extensions between "fd" and "v" by
>  # matching non-v and non-multi-letter extensions out with the filter ([^v_]*)
>  KBUILD_CFLAGS += -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)fd([^v_]*)v?/\1\2/')
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index 808b4c78462e..5d38153e2f13 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -9,6 +9,7 @@
>  #include <linux/bug.h>
>  
>  #include <asm/fence.h>
> +#include <asm/alternative.h>
>  
>  #define __arch_xchg_masked(sc_sfx, prepend, append, r, p, n)		\
>  ({									\
> @@ -134,21 +135,40 @@
>  	r = (__typeof__(*(p)))((__retx & __mask) >> __s);		\
>  })
>  
> -#define __arch_cmpxchg(lr_sfx, sc_sfx, prepend, append, r, p, co, o, n)	\
> +#define __arch_cmpxchg(lr_sfx, sc_cas_sfx, prepend, append, r, p, co, o, n)	\

I'd either not bother renaming sc_sfx or also rename it in _arch_cmpxchg.

>  ({									\
> +	__label__ no_zacas, end;					\
>  	register unsigned int __rc;					\
>  									\
> +	if (IS_ENABLED(CONFIG_RISCV_ISA_ZACAS)) {			\
> +		asm goto(ALTERNATIVE("j %[no_zacas]", "nop", 0,		\
> +				     RISCV_ISA_EXT_ZACAS, 1)		\
> +			 : : : : no_zacas);				\
> +									\
> +		__asm__ __volatile__ (					\
> +			prepend						\
> +			"	amocas" sc_cas_sfx " %0, %z2, %1\n"	\
> +			append						\
> +			: "+&r" (r), "+A" (*(p))			\
> +			: "rJ" (n)					\
> +			: "memory");					\
> +		goto end;						\
> +	}								\
> +									\
> +no_zacas:								\
>  	__asm__ __volatile__ (						\
>  		prepend							\
>  		"0:	lr" lr_sfx " %0, %2\n"				\
>  		"	bne  %0, %z3, 1f\n"				\
> -		"	sc" sc_sfx " %1, %z4, %2\n"			\
> +		"	sc" sc_cas_sfx " %1, %z4, %2\n"			\
>  		"	bnez %1, 0b\n"					\
>  		append							\
>  		"1:\n"							\
>  		: "=&r" (r), "=&r" (__rc), "+A" (*(p))			\
>  		: "rJ" (co o), "rJ" (n)					\
>  		: "memory");						\
> +									\
> +end:;									\
>  })
>  
>  #define _arch_cmpxchg(ptr, old, new, sc_sfx, prepend, append)		\
> @@ -156,7 +176,7 @@
>  	__typeof__(ptr) __ptr = (ptr);					\
>  	__typeof__(*(__ptr)) __old = (old);				\
>  	__typeof__(*(__ptr)) __new = (new);				\
> -	__typeof__(*(__ptr)) __ret;					\
> +	__typeof__(*(__ptr)) __ret = (old);				\

Is this just to silence some compiler warnings? Can we point out
whatever the reason is in the commit message?

>  									\
>  	switch (sizeof(*__ptr)) {					\
>  	case 1:								\
> -- 
> 2.39.2
>

Thanks,
drew