2024-03-13 09:33:46

by Wang, Xiao W

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

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 <[email protected]>
---
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

2024-05-03 12:14:51

by Alexandre Ghiti

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 <[email protected]>
> ---
> 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 <[email protected]>

Thanks,

Alex