[PATCH 1/1] riscv: prevent pipeline stall in __asm_to/copy_from_user

[David Laight]

From: Akira Tsukamoto
> Sent: 04 June 2021 10:57
> 
> Reducing pipeline stall of read after write (RAW).
> 
> These are the results from combination of the speedup with
> Gary's misalign fix. Speeds up from 680Mbps to 900Mbps.
> 
> Before applying these two patches.

I think the changes should be in separate patches.
Otherwise it is difficult to see what is relevant.
It also looks as if there is a register rename.
Maybe that should be a precursor patch?
...

I think this is the old main copy loop:
>  1:
> -    fixup REG_L, t2, (a1), 10f
> -    fixup REG_S, t2, (a0), 10f
> -    addi a1, a1, SZREG
> -    addi a0, a0, SZREG
> -    bltu a1, t1, 1b
and this is the new one:
>  3:
> +    fixup REG_L a4,       0(a1), 10f
> +    fixup REG_L a5,   SZREG(a1), 10f
> +    fixup REG_L a6, 2*SZREG(a1), 10f
> +    fixup REG_L a7, 3*SZREG(a1), 10f
> +    fixup REG_L t0, 4*SZREG(a1), 10f
> +    fixup REG_L t1, 5*SZREG(a1), 10f
> +    fixup REG_L t2, 6*SZREG(a1), 10f
> +    fixup REG_L t3, 7*SZREG(a1), 10f
> +    fixup REG_S a4,       0(t5), 10f
> +    fixup REG_S a5,   SZREG(t5), 10f
> +    fixup REG_S a6, 2*SZREG(t5), 10f
> +    fixup REG_S a7, 3*SZREG(t5), 10f
> +    fixup REG_S t0, 4*SZREG(t5), 10f
> +    fixup REG_S t1, 5*SZREG(t5), 10f
> +    fixup REG_S t2, 6*SZREG(t5), 10f
> +    fixup REG_S t3, 7*SZREG(t5), 10f
> +    addi a1, a1, 8*SZREG
> +    addi t5, t5, 8*SZREG
> +    bltu a1, a3, 3b

I don't know the architecture, but unless there is a stunning
pipeline delay for memory reads a simple interleaved copy
may be fast enough.
So something like:
	a = src[0];
	do {
		b = src[1];
		src += 2;
		dst[0] = a;
		dst += 2;
		a = src[0];
		dst[-1] = b;
	} while (src != src_end);
	dst[0] = a;

It is probably worth doing benchmarks of the copy loop
in userspace.

	David

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