linux-kernel - RE: [PATCH] lib/x86: Optimise csum_partial of buffers that are not multiples of 8 bytes.

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Date:   Mon, 13 Dec 2021 18:40:22 +0000
From:   Alexander Duyck <alexanderduyck@...com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Noah Goldstein' <goldstein.w.n@...il.com>,
        'Eric Dumazet' <edumazet@...gle.com>
CC:     "'tglx@...utronix.de'" <tglx@...utronix.de>,
        "'mingo@...hat.com'" <mingo@...hat.com>,
        'Borislav Petkov' <bp@...en8.de>,
        "'dave.hansen@...ux.intel.com'" <dave.hansen@...ux.intel.com>,
        'X86 ML' <x86@...nel.org>, "'hpa@...or.com'" <hpa@...or.com>,
        "'peterz@...radead.org'" <peterz@...radead.org>,
        'open list' <linux-kernel@...r.kernel.org>,
        'netdev' <netdev@...r.kernel.org>
Subject: RE: [PATCH] lib/x86: Optimise csum_partial of buffers that are not
 multiples of 8 bytes.

> -----Original Message-----
> From: David Laight <David.Laight@...LAB.COM>
> Sent: Monday, December 13, 2021 10:01 AM
> To: 'Noah Goldstein' <goldstein.w.n@...il.com>; 'Eric Dumazet'
> <edumazet@...gle.com>
> Cc: 'tglx@...utronix.de' <tglx@...utronix.de>; 'mingo@...hat.com'
> <mingo@...hat.com>; 'Borislav Petkov' <bp@...en8.de>;
> 'dave.hansen@...ux.intel.com' <dave.hansen@...ux.intel.com>; 'X86 ML'
> <x86@...nel.org>; 'hpa@...or.com' <hpa@...or.com>;
> 'peterz@...radead.org' <peterz@...radead.org>; Alexander Duyck
> <alexanderduyck@...com>; 'open list' <linux-kernel@...r.kernel.org>;
> 'netdev' <netdev@...r.kernel.org>
> Subject: [PATCH] lib/x86: Optimise csum_partial of buffers that are not
> multiples of 8 bytes.
> 
> 
> Add in the trailing bytes first so that there is no need to worry about the sum
> exceeding 64 bits.
> 
> Signed-off-by: David Laight <david.laight@...lab.com>
> ---
> 
> This ought to be faster - because of all the removed 'adc $0'.
> Guessing how fast x86 code will run is hard!
> There are other ways of handing buffers that are shorter than 8 bytes, but I'd
> rather hope they don't happen in any hot paths.
> 
> Note - I've not even compile tested it.
> (But have tested an equivalent change before.)
> 
>  arch/x86/lib/csum-partial_64.c | 55 ++++++++++++----------------------
>  1 file changed, 19 insertions(+), 36 deletions(-)
> 
> diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
> index abf819dd8525..fbcc073fc2b5 100644
> --- a/arch/x86/lib/csum-partial_64.c
> +++ b/arch/x86/lib/csum-partial_64.c
> @@ -37,6 +37,24 @@ __wsum csum_partial(const void *buff, int len,
> __wsum sum)
>  	u64 temp64 = (__force u64)sum;
>  	unsigned result;
> 
> +	if (len & 7) {
> +		if (unlikely(len < 8)) {
> +			/* Avoid falling off the start of the buffer */
> +			if (len & 4) {
> +				temp64 += *(u32 *)buff;
> +				buff += 4;
> +			}
> +			if (len & 2) {
> +				temp64 += *(u16 *)buff;
> +				buff += 2;
> +			}
> +			if (len & 1)
> +				temp64 += *(u8 *)buff;
> +			goto reduce_to32;
> +		}
> +		temp64 += *(u64 *)(buff + len - 8) << (8 - (len & 7)) * 8;
> +	}
> +

I don't think your shift is headed in the right direction. If your starting offset is "buff + len - 8" then your remaining bits should be in the upper bytes of the qword, not the lower bytes shouldn't they? So I would think it should be ">>" not "<<".

>  	while (unlikely(len >= 64)) {
>  		asm("addq 0*8(%[src]),%[res]\n\t"
>  		    "adcq 1*8(%[src]),%[res]\n\t"
> @@ -82,43 +100,8 @@ __wsum csum_partial(const void *buff, int len,
> __wsum sum)
>  			: "memory");
>  		buff += 8;
>  	}
> -	if (len & 7) {
> -#ifdef CONFIG_DCACHE_WORD_ACCESS
> -		unsigned int shift = (8 - (len & 7)) * 8;
> -		unsigned long trail;
> -
> -		trail = (load_unaligned_zeropad(buff) << shift) >> shift;

Your code above should be equivalent to the load_unaligned_zeropad() << shift, so the shift you are performing above is equivalent to the later one.

> 
> -		asm("addq %[trail],%[res]\n\t"
> -		    "adcq $0,%[res]"
> -			: [res] "+r" (temp64)
> -			: [trail] "r" (trail));
> -#else
> -		if (len & 4) {
> -			asm("addq %[val],%[res]\n\t"
> -			    "adcq $0,%[res]"
> -				: [res] "+r" (temp64)
> -				: [val] "r" ((u64)*(u32 *)buff)
> -				: "memory");
> -			buff += 4;
> -		}
> -		if (len & 2) {
> -			asm("addq %[val],%[res]\n\t"
> -			    "adcq $0,%[res]"
> -				: [res] "+r" (temp64)
> -				: [val] "r" ((u64)*(u16 *)buff)
> -				: "memory");
> -			buff += 2;
> -		}
> -		if (len & 1) {
> -			asm("addq %[val],%[res]\n\t"
> -			    "adcq $0,%[res]"
> -				: [res] "+r" (temp64)
> -				: [val] "r" ((u64)*(u8 *)buff)
> -				: "memory");
> -		}
> -#endif
> -	}
> +reduce_to32:
>  	result = add32_with_carry(temp64 >> 32, temp64 & 0xffffffff);
>  	return (__force __wsum)result;
>  }
> --
> 2.17.1
> 
> -
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