Re: [PATCH v2] arch/riscv: add Zihintpause support

["Heiko Stübner" <heiko@sntech.de>]

Am Dienstag, 24. Mai 2022, 23:19:50 CEST schrieb Dao Lu:
> Implement support for the ZiHintPause extension.
> 
> The PAUSE instruction is a HINT that indicates the current hart’s rate of
> instruction retirement should be temporarily reduced or paused.
> 
> Reviewed-by: Heiko Stuebner <heiko@sntech.de>
> Tested-by: Heiko Stuebner <heiko@sntech.de>
> Signed-off-by: Dao Lu <daolu@rivosinc.com>
> ---
> 
> v1 -> v2:
>  Remove the usage of static branch, use PAUSE if toolchain supports it
> 
>  arch/riscv/Makefile                     | 4 ++++
>  arch/riscv/include/asm/hwcap.h          | 1 +
>  arch/riscv/include/asm/vdso/processor.h | 8 +++++++-
>  arch/riscv/kernel/cpu.c                 | 1 +
>  arch/riscv/kernel/cpufeature.c          | 2 ++
>  5 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 7d81102cffd4..900a8fda1a2d 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -56,6 +56,10 @@ riscv-march-$(CONFIG_RISCV_ISA_C)	:= $(riscv-march-y)c
>  toolchain-need-zicsr-zifencei := $(call cc-option-yn, -march=$(riscv-march-y)_zicsr_zifencei)
>  riscv-march-$(toolchain-need-zicsr-zifencei) := $(riscv-march-y)_zicsr_zifencei
>  
> +# Check if the toolchain supports Zihintpause extension
> +toolchain-supports-zihintpause := $(call cc-option-yn, -march=$(riscv-march-y)_zihintpause)
> +riscv-march-$(toolchain-supports-zihintpause) := $(riscv-march-y)_zihintpause
> +
>  KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
>  KBUILD_AFLAGS += -march=$(riscv-march-y)
>  
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index 0734e42f74f2..caa9ee5459b4 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -52,6 +52,7 @@ extern unsigned long elf_hwcap;
>   */
>  enum riscv_isa_ext_id {
>  	RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,

svpbmt got merged meanwhile, so this patch needs a rebase
onto 5.19-rc.

One more nit below

> +	RISCV_ISA_EXT_ZIHINTPAUSE,
>  	RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
>  };
>  
> diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
> index 134388cbaaa1..4de911a25051 100644
> --- a/arch/riscv/include/asm/vdso/processor.h
> +++ b/arch/riscv/include/asm/vdso/processor.h
> @@ -8,7 +8,13 @@
>  
>  static inline void cpu_relax(void)
>  {
> -#ifdef __riscv_muldiv
> +#ifdef __riscv_zihintpause
> +	/*
> +	 * Reduce instruction retirement.
> +	 * This assumes the PC changes.
> +	 */
> +	__asm__ __volatile__ ("pause");
> +#elif __riscv_muldiv
>  	int dummy;
>  	/* In lieu of a halt instruction, induce a long-latency stall. */
>  	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> index ccb617791e56..89e563e9c4cc 100644
> --- a/arch/riscv/kernel/cpu.c
> +++ b/arch/riscv/kernel/cpu.c
> @@ -88,6 +88,7 @@ int riscv_of_parent_hartid(struct device_node *node)
>   */
>  static struct riscv_isa_ext_data isa_ext_arr[] = {
>  	__RISCV_ISA_EXT_DATA(sscofpmf, RISCV_ISA_EXT_SSCOFPMF),
> +	__RISCV_ISA_EXT_DATA(zihintpause, RISCV_ISA_EXT_ZIHINTPAUSE),
>  	__RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
>  };
>  
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 1b2d42d7f589..37ff06682ae6 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -25,6 +25,7 @@ static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
>  __ro_after_init DEFINE_STATIC_KEY_FALSE(cpu_hwcap_fpu);
>  #endif
>  
> +

this is an unrelated change and also is adding an unneeded extra empty line.


Heiko

>  /**
>   * riscv_isa_extension_base() - Get base extension word
>   *
> @@ -192,6 +193,7 @@ void __init riscv_fill_hwcap(void)
>  				set_bit(*ext - 'a', this_isa);
>  			} else {
>  				SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
> +				SET_ISA_EXT_MAP("zihintpause", RISCV_ISA_EXT_ZIHINTPAUSE);
>  			}
>  #undef SET_ISA_EXT_MAP
>  		}
>