Re: [PATCH 2/6] crypto: arm64/crct10dif - Use faster 16x64 bit polynomial multiply

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Oct 28, 2024 at 08:02:10PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> The CRC-T10DIF implementation for arm64 has a version that uses 8x8
> polynomial multiplication, for cores that lack the crypto extensions,
> which cover the 64x64 polynomial multiplication instruction that the
> algorithm was built around.
> 
> This fallback version rather naively adopted the 64x64 polynomial
> multiplication algorithm that I ported from ARM for the GHASH driver,
> which needs 8 PMULL8 instructions to implement one PMULL64. This is
> reasonable, given that each 8-bit vector element needs to be multiplied
> with each element in the other vector, producing 8 vectors with partial
> results that need to be combined to yield the correct result.
> 
> However, most PMULL64 invocations in the CRC-T10DIF code involve
> multiplication by a pair of 16-bit folding coefficients, and so all the
> partial results from higher order bytes will be zero, and there is no
> need to calculate them to begin with.
> 
> Then, the CRC-T10DIF algorithm always XORs the output values of the
> PMULL64 instructions being issued in pairs, and so there is no need to
> faithfully implement each individual PMULL64 instruction, as long as
> XORing the results pairwise produces the expected result.
> 
> Implementing these improvements results in a speedup of 3.3x on low-end
> platforms such as Raspberry Pi 4 (Cortex-A72)
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/crypto/crct10dif-ce-core.S | 71 +++++++++++++++-----
>  1 file changed, 54 insertions(+), 17 deletions(-)

Thanks, this makes sense.

> +SYM_FUNC_START_LOCAL(__pmull_p8_16x64)
> +	ext		t6.16b, t5.16b, t5.16b, #8
> +
> +	pmull		t3.8h, t7.8b, t5.8b
> +	pmull		t4.8h, t7.8b, t6.8b
> +	pmull2		t5.8h, t7.16b, t5.16b
> +	pmull2		t6.8h, t7.16b, t6.16b
> +
> +	ext		t8.16b, t3.16b, t3.16b, #8
> +	eor		t4.16b, t4.16b, t6.16b
> +	ext		t7.16b, t5.16b, t5.16b, #8
> +	ext		t6.16b, t4.16b, t4.16b, #8
> +	eor		t8.8b, t8.8b, t3.8b
> +	eor		t5.8b, t5.8b, t7.8b
> +	eor		t4.8b, t4.8b, t6.8b
> +	ext		t5.16b, t5.16b, t5.16b, #14
> +	ret
> +SYM_FUNC_END(__pmull_p8_16x64)

A few comments in the above function would be really helpful.

- Eric