linux-kernel - Re: [PATCH] arm64: Implement optimised IP checksum helpers

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Date:	Thu, 12 May 2016 11:34:37 -0400
From:	Luke Starrett <luke.starrett@...adcom.com>
To:	Robin Murphy <robin.murphy@....com>, will.deacon@....com,
	catalin.marinas@....com
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH] arm64: Implement optimised IP checksum helpers

Hi Robin,

I pulled this in to a userspace test app expecting that the __uint128_t 
type might cause GCC to emit 'ldp'.  Seems like that was that your 
intent based on your commit note.  Instead I see two 64b loads (ldr Xn), 
and a single 32b load (ldr Wn) for the trailing 4B.  This was with 
Linaro GCC 4.9-2015.06.

Otherwise, the C cycle count looks good enough compared to the asm version.

Thanks,

Luke


On 5/12/2016 10:26 AM, Robin Murphy wrote:
> AArch64 is capable of 128-bit memory accesses without alignment
> restrictions, which makes it both possible and highly practical to slurp
> up a typical 20-byte IP header in just 2 loads. Implement our own
> version of ip_fast_checksum() to take advantage of that, resulting in
> considerably fewer instructions and memory accesses than the generic
> version. We can also get more optimal code generation for csum_fold() by
> defining it a slightly different way round from the generic version, so
> throw that into the mix too.
>
> Suggested-by: Luke Starrett <luke.starrett@...adcom.com>
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
>
> Having spent an evening poring over disassembler output, it turns out
> that there's really no need to drop into asm for this, hurrah! I've
> taken it for a spin on both big and little-endian, with no ill effects.
>
>   arch/arm64/include/asm/checksum.h | 49 +++++++++++++++++++++++++++++++++++++++
>   1 file changed, 49 insertions(+)
>   create mode 100644 arch/arm64/include/asm/checksum.h
>
> diff --git a/arch/arm64/include/asm/checksum.h b/arch/arm64/include/asm/checksum.h
> new file mode 100644
> index 000000000000..7e975ffce837
> --- /dev/null
> +++ b/arch/arm64/include/asm/checksum.h
> @@ -0,0 +1,49 @@
> +/*
> + * Copyright (C) 2016 ARM Ltd.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +#ifndef __ASM_CHECKSUM_H
> +#define __ASM_CHECKSUM_H
> +
> +static inline __sum16 csum_fold(__wsum csum)
> +{
> +	u32 sum = (__force u32)csum;
> +	sum += (sum >> 16) | (sum << 16);
> +	return ~(__force __sum16)(sum >> 16);
> +}
> +#define csum_fold csum_fold
> +
> +static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
> +{
> +	__uint128_t tmp;
> +	u64 sum;
> +
> +	tmp = *(const __uint128_t *)iph;
> +	iph += 16;
> +	ihl -= 4;
> +	tmp += ((tmp >> 64) | (tmp << 64));
> +	sum = tmp >> 64;
> +	do {
> +		sum += *(const u32 *)iph;
> +		iph += 4;
> +	} while (--ihl);
> +
> +	sum += ((sum >> 32) | (sum << 32));
> +	return csum_fold(sum >> 32);
> +}
> +#define ip_fast_csum ip_fast_csum
> +
> +#include <asm-generic/checksum.h>
> +
> +#endif	/* __ASM_CHECKSUM_H */