linux-kernel - Re: [RFC PATCH v8 20/21] riscv: Optimize task switch codes of vector

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Date:   Wed, 15 Sep 2021 22:29:37 +0800
From:   Jisheng Zhang <jszhang3@...l.ustc.edu.cn>
To:     Greentime Hu <greentime.hu@...ive.com>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        aou@...s.berkeley.edu, palmer@...belt.com,
        paul.walmsley@...ive.com, vincent.chen@...ive.com
Subject: Re: [RFC PATCH v8 20/21] riscv: Optimize task switch codes of
 vector

On Thu,  9 Sep 2021 01:45:32 +0800
Greentime Hu <greentime.hu@...ive.com> wrote:

> This patch replacees 2 instructions with 1 instruction to do the same thing
> . rs1=x0 with rd != x0 is a special form of the instruction that sets vl to
> MAXVL.

Similarly, the vector.S is newly introduced in this patch set, so could
this optimization be folded into the __vstate_save and __vstate_restore
introduction patch? Or it's better to keep this optimizaion in commit log?

> 
> Suggested-by: Andrew Waterman <andrew@...ive.com>
> Co-developed-by: Vincent Chen <vincent.chen@...ive.com>
> Signed-off-by: Vincent Chen <vincent.chen@...ive.com>
> Signed-off-by: Greentime Hu <greentime.hu@...ive.com>
> ---
>  arch/riscv/kernel/vector.S | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/riscv/kernel/vector.S b/arch/riscv/kernel/vector.S
> index 4f0c5a166e4e..f7223c81b11a 100644
> --- a/arch/riscv/kernel/vector.S
> +++ b/arch/riscv/kernel/vector.S
> @@ -27,8 +27,7 @@
>  #define x_vl     t2
>  #define x_vcsr   t3
>  #define incr     t4
> -#define m_one    t5
> -#define status   t6
> +#define status   t5
>  
>  ENTRY(__vstate_save)
>  	li      status, SR_VS
> @@ -38,8 +37,7 @@ ENTRY(__vstate_save)
>  	csrr    x_vtype, CSR_VTYPE
>  	csrr    x_vl, CSR_VL
>  	csrr    x_vcsr, CSR_VCSR
> -	li      m_one, -1
> -	vsetvli incr, m_one, e8, m8
> +	vsetvli incr, x0, e8, m8
>  	vse8.v   v0, (datap)
>  	add     datap, datap, incr
>  	vse8.v   v8, (datap)
> @@ -61,8 +59,7 @@ ENTRY(__vstate_restore)
>  	li      status, SR_VS
>  	csrs    CSR_STATUS, status
>  
> -	li      m_one, -1
> -	vsetvli incr, m_one, e8, m8
> +	vsetvli incr, x0, e8, m8
>  	vle8.v   v0, (datap)
>  	add     datap, datap, incr
>  	vle8.v   v8, (datap)