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Date:   Wed, 15 Sep 2021 22:24:56 +0800
From:   Jisheng Zhang <jszhang3@mail.ustc.edu.cn>
To:     Greentime Hu <greentime.hu@sifive.com>
Cc:     linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        aou@eecs.berkeley.edu, palmer@dabbelt.com,
        paul.walmsley@sifive.com, vincent.chen@sifive.com
Subject: Re: [RFC PATCH v8 21/21] riscv: Turn has_vector into a static key
 if VECTOR=y
Message-ID: <20210915222456.1de541df@xhacker>
In-Reply-To: <56e2328d133045c79b420ecbcaf37cf9d0b2248c.1631121222.git.greentime.hu@sifive.com>
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        <56e2328d133045c79b420ecbcaf37cf9d0b2248c.1631121222.git.greentime.hu@sifive.com>
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On Thu,  9 Sep 2021 01:45:33 +0800
Greentime Hu <greentime.hu@sifive.com> wrote:

> Just like fpu, we can use static key for has_vector.
> The has_vector check sits at hot code path: switch_to(). Currently,
> has_vector is a bool variable if VECTOR=y, switch_to() checks it each time,
> we can optimize out this check by turning the has_vector into a static key.
> 

has_vector is newly introduced in this patch set so I believe this patch can
be folded into has_vector introducing patch, I.E patch 6


> Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
> ---
>  arch/riscv/include/asm/switch_to.h     | 10 +++++++---
>  arch/riscv/kernel/cpufeature.c         |  4 ++--
>  arch/riscv/kernel/kernel_mode_vector.c |  4 ++--
>  arch/riscv/kernel/process.c            |  8 ++++----
>  arch/riscv/kernel/signal.c             |  6 +++---
>  5 files changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
> index b48c9c974564..576204217e0f 100644
> --- a/arch/riscv/include/asm/switch_to.h
> +++ b/arch/riscv/include/asm/switch_to.h
> @@ -71,7 +71,11 @@ static __always_inline bool has_fpu(void) { return false; }
>  #endif
>  
>  #ifdef CONFIG_VECTOR
> -extern bool has_vector;
> +extern struct static_key_false cpu_hwcap_vector;
> +static __always_inline bool has_vector(void)
> +{
> +	return static_branch_likely(&cpu_hwcap_vector);
> +}
>  extern unsigned long riscv_vsize;
>  extern void __vstate_save(struct __riscv_v_state *save_to, void *datap);
>  extern void __vstate_restore(struct __riscv_v_state *restore_from, void *datap);
> @@ -120,7 +124,7 @@ static inline void __switch_to_vector(struct task_struct *prev,
>  }
>  
>  #else
> -#define has_vector false
> +static __always_inline bool has_vector(void) { return false; }
>  #define riscv_vsize (0)
>  #define vstate_save(task, regs) do { } while (0)
>  #define vstate_restore(task, regs) do { } while (0)
> @@ -136,7 +140,7 @@ do {							\
>  	struct task_struct *__next = (next);		\
>  	if (has_fpu())					\
>  		__switch_to_fpu(__prev, __next);	\
> -	if (has_vector)					\
> +	if (has_vector())					\
>  		__switch_to_vector(__prev, __next);	\
>  	((last) = __switch_to(__prev, __next));		\
>  } while (0)
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index af984f875f60..0139ec20adce 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -23,7 +23,7 @@ __ro_after_init DEFINE_STATIC_KEY_FALSE(cpu_hwcap_fpu);
>  #endif
>  #ifdef CONFIG_VECTOR
>  #include <asm/vector.h>
> -bool has_vector __read_mostly;
> +__ro_after_init DEFINE_STATIC_KEY_FALSE(cpu_hwcap_vector);
>  unsigned long riscv_vsize __read_mostly;
>  #endif
>  
> @@ -157,7 +157,7 @@ void __init riscv_fill_hwcap(void)
>  
>  #ifdef CONFIG_VECTOR
>  	if (elf_hwcap & COMPAT_HWCAP_ISA_V) {
> -		has_vector = true;
> +		static_branch_enable(&cpu_hwcap_vector);
>  		/* There are 32 vector registers with vlenb length. */
>  		rvv_enable();
>  		riscv_vsize = csr_read(CSR_VLENB) * 32;
> diff --git a/arch/riscv/kernel/kernel_mode_vector.c b/arch/riscv/kernel/kernel_mode_vector.c
> index 0d990bd8b8dd..0d08954c30af 100644
> --- a/arch/riscv/kernel/kernel_mode_vector.c
> +++ b/arch/riscv/kernel/kernel_mode_vector.c
> @@ -110,7 +110,7 @@ static void vector_flush_cpu_state(void)
>   */
>  void kernel_rvv_begin(void)
>  {
> -	if (WARN_ON(!has_vector))
> +	if (WARN_ON(!has_vector()))
>  		return;
>  
>  	WARN_ON(!may_use_vector());
> @@ -140,7 +140,7 @@ EXPORT_SYMBOL(kernel_rvv_begin);
>   */
>  void kernel_rvv_end(void)
>  {
> -	if (WARN_ON(!has_vector))
> +	if (WARN_ON(!has_vector()))
>  		return;
>  
>  	/* Invalidate vector regs */
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index 05ff5f934e7e..62540815ba1c 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -96,7 +96,7 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
>  		fstate_restore(current, regs);
>  	}
>  
> -	if (has_vector) {
> +	if (has_vector()) {
>  		struct __riscv_v_state *vstate = &(current->thread.vstate);
>  
>  		/* Enable vector and allocate memory for vector registers. */
> @@ -141,11 +141,11 @@ void flush_thread(void)
>  int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
>  {
>  	fstate_save(src, task_pt_regs(src));
> -	if (has_vector)
> +	if (has_vector())
>  		/* To make sure every dirty vector context is saved. */
>  		vstate_save(src, task_pt_regs(src));
>  	*dst = *src;
> -	if (has_vector) {
> +	if (has_vector()) {
>  		/* Copy vector context to the forked task from parent. */
>  		if ((task_pt_regs(src)->status & SR_VS) != SR_VS_OFF) {
>  			dst->thread.vstate.datap = kzalloc(riscv_vsize, GFP_KERNEL);
> @@ -164,7 +164,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
>  void arch_release_task_struct(struct task_struct *tsk)
>  {
>  	/* Free the vector context of datap. */
> -	if (has_vector)
> +	if (has_vector())
>  		kfree(tsk->thread.vstate.datap);
>  }
>  
> diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
> index d30a3b588156..6a19b4b7b206 100644
> --- a/arch/riscv/kernel/signal.c
> +++ b/arch/riscv/kernel/signal.c
> @@ -192,7 +192,7 @@ static long restore_sigcontext(struct pt_regs *regs,
>  				goto invalid;
>  			goto done;
>  		case RVV_MAGIC:
> -			if (!has_vector)
> +			if (!has_vector())
>  				goto invalid;
>  			if (size != rvv_sc_size)
>  				goto invalid;
> @@ -221,7 +221,7 @@ static size_t cal_rt_frame_size(void)
>  
>  	frame_size = sizeof(*frame);
>  
> -	if (has_vector)
> +	if (has_vector())
>  		total_context_size += rvv_sc_size;
>  	/* Preserved a __riscv_ctx_hdr for END signal context header. */
>  	total_context_size += sizeof(struct __riscv_ctx_hdr);
> @@ -288,7 +288,7 @@ static long setup_sigcontext(struct rt_sigframe __user *frame,
>  	if (has_fpu())
>  		err |= save_fp_state(regs, &sc->sc_fpregs);
>  	/* Save the vector state. */
> -	if (has_vector)
> +	if (has_vector())
>  		err |= save_v_state(regs, &sc_reserved_free_ptr);
>  
>  	/* Put END __riscv_ctx_hdr at the end. */