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

Dao Lu
2022-07-05T16:57:31Z

ping

On Mon, Jun 20, 2022 at 1:15 PM Dao Lu <daolu@rivosinc.com> 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@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
> 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
> }
> --
> 2.25.1
>