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Subject: Re: [PATCH v3] arch/riscv: add Zihintpause support
To:     Dao Lu <daolu@rivosinc.com>
Cc:     linux-kernel@vger.kernel.org, Heiko Stuebner <heiko@sntech.de>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Atish Patra <atishp@atishpatra.org>,
        Anup Patel <anup@brainfault.org>, Wei Fu <wefu@redhat.com>,
        Niklas Cassel <niklas.cassel@wdc.com>,
        Alexandre Ghiti <alexandre.ghiti@canonical.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Qinglin Pan <panqinglin2020@iscas.ac.cn>,
        Rob Herring <robh@kernel.org>,
        Tsukasa OI <research_trasio@irq.a4lg.com>,
        Yury Norov <yury.norov@gmail.com>,
        "open list:RISC-V ARCHITECTURE" <linux-riscv@lists.infradead.org>
References: <20220615211415.3111271-1-daolu@rivosinc.com>
From:   Samuel Holland <samuel@sholland.org>
Message-ID: <6b52cc2c-7bc5-5015-b7a6-4c13bb4789f7@sholland.org>
Date:   Thu, 16 Jun 2022 22:49:38 -0500
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On 6/15/22 4:14 PM, Dao Lu 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 unchanged if either the
> system or the toolchain does not support ZiHintPause
> ---
>  arch/riscv/Makefile                     |  4 ++++
>  arch/riscv/include/asm/hwcap.h          |  1 +
>  arch/riscv/include/asm/vdso/processor.h | 19 ++++++++++++++++---
>  arch/riscv/kernel/cpu.c                 |  1 +
>  arch/riscv/kernel/cpufeature.c          |  8 ++++++++
>  5 files changed, 30 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 4e2486881840..f24f4f8c9144 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -53,6 +53,7 @@ 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,
>  };
>  
> diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
> index 134388cbaaa1..314ec17c40d8 100644
> --- a/arch/riscv/include/asm/vdso/processor.h
> +++ b/arch/riscv/include/asm/vdso/processor.h
> @@ -4,15 +4,28 @@
>  
>  #ifndef __ASSEMBLY__
>  
> +#include <linux/jump_label.h>
>  #include <asm/barrier.h>
> +#include <asm/hwcap.h>
>  
> +extern struct static_key_false riscv_pause_available;
>  static inline void cpu_relax(void)
>  {
> +	if (!static_branch_likely(&riscv_pause_available)) {
>  #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
> +#ifdef __riscv_zihintpause
> +	} else {
> +		/*
> +		 * Reduce instruction retirement.
> +		 * This assumes the PC changes.
> +		 */
> +		__asm__ __volatile__ ("pause");
> +#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 a6f62a6d1edd..78c284c487e8 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -30,6 +30,8 @@ static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
>  #ifdef CONFIG_FPU
>  __ro_after_init DEFINE_STATIC_KEY_FALSE(cpu_hwcap_fpu);
>  #endif
> +DEFINE_STATIC_KEY_FALSE(riscv_pause_available);
> +EXPORT_SYMBOL_GPL(riscv_pause_available);

This patch should probably be rebased to use the unified static key
mechanism[1], since that patch has now been merged.

Regards,
Samuel

[1]: https://lore.kernel.org/linux-riscv/20220522153543.2656-2-jszhang@kernel.org/