Re: [PATCH v4] arch/riscv: add Zihintpause support

[Palmer Dabbelt]

On Mon, 20 Jun 2022 13:15:25 PDT (-0700), daolu@xxxxxxxxxxxx wrote:
> 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@xxxxxxxxx>
> Tested-by: Heiko Stuebner <heiko@xxxxxxxxx>
> Signed-off-by: Dao Lu <daolu@xxxxxxxxxxxx>
> ---
>
> v1 -> v2:
>  Remove the usage of static branch, use PAUSE if toolchain supports it
> v2 -> v3:
>  Added the static branch back, cpu_relax() behavior is kept the same for
> systems that do not support ZiHintPause
> v3 -> v4:
>  Adopted the newly added unified static keys for extensions
> ---
>  arch/riscv/Makefile                     |  4 ++++
>  arch/riscv/include/asm/hwcap.h          |  5 +++++
>  arch/riscv/include/asm/vdso/processor.h | 21 ++++++++++++++++++---
>  arch/riscv/kernel/cpu.c                 |  1 +
>  arch/riscv/kernel/cpufeature.c          |  1 +
>  5 files changed, 29 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 34cf8a598617..6ddacc6f44b9 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 e48eebdd2631..dc47019a0b38 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -8,6 +8,7 @@
>  #ifndef _ASM_RISCV_HWCAP_H
>  #define _ASM_RISCV_HWCAP_H
>
> +#include <asm/errno.h>
>  #include <linux/bits.h>
>  #include <uapi/asm/hwcap.h>
>
> @@ -54,6 +55,7 @@ extern unsigned long elf_hwcap;
>  enum riscv_isa_ext_id {
>  	RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
>  	RISCV_ISA_EXT_SVPBMT,
> +	RISCV_ISA_EXT_ZIHINTPAUSE,
>  	RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
>  };
>
> @@ -64,6 +66,7 @@ enum riscv_isa_ext_id {
>   */
>  enum riscv_isa_ext_key {
>  	RISCV_ISA_EXT_KEY_FPU,		/* For 'F' and 'D' */
> +	RISCV_ISA_EXT_KEY_ZIHINTPAUSE,
>  	RISCV_ISA_EXT_KEY_MAX,
>  };
>
> @@ -83,6 +86,8 @@ static __always_inline int riscv_isa_ext2key(int num)
>  		return RISCV_ISA_EXT_KEY_FPU;
>  	case RISCV_ISA_EXT_d:
>  		return RISCV_ISA_EXT_KEY_FPU;
> +	case RISCV_ISA_EXT_ZIHINTPAUSE:
> +		return RISCV_ISA_EXT_KEY_ZIHINTPAUSE;
>  	default:
>  		return -EINVAL;
>  	}
> diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
> index 134388cbaaa1..1e4f8b4aef79 100644
> --- a/arch/riscv/include/asm/vdso/processor.h
> +++ b/arch/riscv/include/asm/vdso/processor.h
> @@ -4,15 +4,30 @@
>
>  #ifndef __ASSEMBLY__
>
> +#include <linux/jump_label.h>
>  #include <asm/barrier.h>
> +#include <asm/hwcap.h>
>
>  static inline void cpu_relax(void)
>  {
> +	if (!static_branch_likely(&riscv_isa_ext_keys[RISCV_ISA_EXT_KEY_ZIHINTPAUSE])) {
>  #ifdef __riscv_muldiv
> -	int dummy;
> -	/* In lieu of a halt instruction, induce a long-latency stall. */
> -	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> +		int dummy;
> +		/* In lieu of a halt instruction, induce a long-latency stall. */
> +		__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
>  #endif
> +	} else {
> +		/*
> +		 * Reduce instruction retirement.
> +		 * This assumes the PC changes.
> +		 */
> +#ifdef __riscv_zihintpause
> +		__asm__ __volatile__ ("pause");
> +#else
> +		/* Encoding of the pause instruction */
> +		__asm__ __volatile__ (".4byte 0x100000F");
> +#endif
> +	}
>  	barrier();
>  }
>
> diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> index fba9e9f46a8c..a123e92b14dd 100644
> --- a/arch/riscv/kernel/cpu.c
> +++ b/arch/riscv/kernel/cpu.c
> @@ -89,6 +89,7 @@ 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(svpbmt, RISCV_ISA_EXT_SVPBMT),
> +	__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 1b3ec44e25f5..708df2c0bc34 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -198,6 +198,7 @@ void __init riscv_fill_hwcap(void)
>  			} else {
>  				SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
>  				SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
> +				SET_ISA_EXT_MAP("zihintpause", RISCV_ISA_EXT_ZIHINTPAUSE);
>  			}
>  #undef SET_ISA_EXT_MAP
>  		}

Thanks, this is on for-next.  It needs a sparse patch, which I put in as a link.