Re: [PATCH 1/1] riscv: __asm_copy_to-from_user: Improve using word copy if size < 9*SZREG

[Guenter Roeck]

On Fri, Jul 30, 2021 at 10:52:44PM +0900, Akira Tsukamoto wrote:
> Reduce the number of slow byte_copy when the size is in between
> 2*SZREG to 9*SZREG by using none unrolled word_copy.
> 
> Without it any size smaller than 9*SZREG will be using slow byte_copy
> instead of none unrolled word_copy.
> 
> Signed-off-by: Akira Tsukamoto <akira.tsukamoto@xxxxxxxxx>

Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>

> ---
>  arch/riscv/lib/uaccess.S | 46 ++++++++++++++++++++++++++++++++++++----
>  1 file changed, 42 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
> index 63bc691cff91..6a80d5517afc 100644
> --- a/arch/riscv/lib/uaccess.S
> +++ b/arch/riscv/lib/uaccess.S
> @@ -34,8 +34,10 @@ ENTRY(__asm_copy_from_user)
>  	/*
>  	 * Use byte copy only if too small.
>  	 * SZREG holds 4 for RV32 and 8 for RV64
> +	 * a3 - 2*SZREG is minimum size for word_copy
> +	 *      1*SZREG for aligning dst + 1*SZREG for word_copy
>  	 */
> -	li	a3, 9*SZREG /* size must be larger than size in word_copy */
> +	li	a3, 2*SZREG
>  	bltu	a2, a3, .Lbyte_copy_tail
>  
>  	/*
> @@ -66,9 +68,40 @@ ENTRY(__asm_copy_from_user)
>  	andi	a3, a1, SZREG-1
>  	bnez	a3, .Lshift_copy
>  
> +.Lcheck_size_bulk:
> +	/*
> +	 * Evaluate the size if possible to use unrolled.
> +	 * The word_copy_unlrolled requires larger than 8*SZREG
> +	 */
> +	li	a3, 8*SZREG
> +	add	a4, a0, a3
> +	bltu	a4, t0, .Lword_copy_unlrolled
> +
>  .Lword_copy:
> -        /*
> -	 * Both src and dst are aligned, unrolled word copy
> +	/*
> +	 * Both src and dst are aligned
> +	 * None unrolled word copy with every 1*SZREG iteration
> +	 *
> +	 * a0 - start of aligned dst
> +	 * a1 - start of aligned src
> +	 * t0 - end of aligned dst
> +	 */
> +	bgeu	a0, t0, .Lbyte_copy_tail /* check if end of copy */
> +	addi	t0, t0, -(SZREG) /* not to over run */
> +1:
> +	REG_L	a5, 0(a1)
> +	addi	a1, a1, SZREG
> +	REG_S	a5, 0(a0)
> +	addi	a0, a0, SZREG
> +	bltu	a0, t0, 1b
> +
> +	addi	t0, t0, SZREG /* revert to original value */
> +	j	.Lbyte_copy_tail
> +
> +.Lword_copy_unlrolled:
> +	/*
> +	 * Both src and dst are aligned
> +	 * Unrolled word copy with every 8*SZREG iteration
>  	 *
>  	 * a0 - start of aligned dst
>  	 * a1 - start of aligned src
> @@ -97,7 +130,12 @@ ENTRY(__asm_copy_from_user)
>  	bltu	a0, t0, 2b
>  
>  	addi	t0, t0, 8*SZREG /* revert to original value */
> -	j	.Lbyte_copy_tail
> +
> +	/*
> +	 * Remaining might large enough for word_copy to reduce slow byte
> +	 * copy
> +	 */
> +	j	.Lcheck_size_bulk
>  
>  .Lshift_copy:
>