[PATCH] riscv: uaccess: Relax the threshold for fast path

[PATCH] riscv: uaccess: Relax the threshold for fast path

[Xiao Wang]

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@intel.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
 
 	/*
-- 
2.25.1

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

[Alexandre Ghiti]

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@intel.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@rivosinc.com>

Thanks,

Alex

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

[patchwork-bot+linux-riscv]

Hello:

This patch was applied to riscv/linux.git (for-next)
by Palmer Dabbelt <palmer@rivosinc.com>:

On Wed, 13 Mar 2024 17:19:29 +0800 you 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@intel.com>
> ---
>  arch/riscv/lib/uaccess.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Here is the summary with links:
  - riscv: uaccess: Relax the threshold for fast path
    https://git.kernel.org/riscv/c/e8c7995ea148

You are awesome, thank you!
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