linux-kernel - Re: [PATCH] riscv: uaccess: Relax the threshold for fast path

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Message-ID: <20798b97-fa19-46fa-abce-54f7931c9210@ghiti.fr>
Date: Fri, 3 May 2024 14:14:34 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Xiao Wang <xiao.w.wang@...el.com>, paul.walmsley@...ive.com,
 palmer@...belt.com, aou@...s.berkeley.edu
Cc: jerry.shih@...ive.com, nick.knight@...ive.com, ajones@...tanamicro.com,
 bjorn@...osinc.com, andy.chiu@...ive.com, viro@...iv.linux.org.uk,
 cleger@...osinc.com, alexghiti@...osinc.com, haicheng.li@...el.com,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: uaccess: Relax the threshold for fast path

Hi Xiao,

On 13/03/2024 10:19, Xiao Wang wrote:
> The bytes copy for unaligned head would cover at most SZREG-1 bytes, so
> it's better to set the threshold as >= (SZREG-1 + word_copy stride size)
> which equals to 9*SZREG-1.
>
> Signed-off-by: Xiao Wang <xiao.w.wang@...el.com>
> ---
>   arch/riscv/lib/uaccess.S | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
> index bc22c078aba8..2e665f8f8fcc 100644
> --- a/arch/riscv/lib/uaccess.S
> +++ b/arch/riscv/lib/uaccess.S
> @@ -44,7 +44,7 @@ SYM_FUNC_START(fallback_scalar_usercopy)
>   	 * Use byte copy only if too small.
>   	 * SZREG holds 4 for RV32 and 8 for RV64
>   	 */
> -	li	a3, 9*SZREG /* size must be larger than size in word_copy */
> +	li	a3, 9*SZREG-1 /* size must >= (word_copy stride + SZREG-1) */
>   	bltu	a2, a3, .Lbyte_copy_tail
>   
>   	/*


This looks good to me:

Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>

Thanks,

Alex