netdev - Re: [PATCH v4 net-next] net: Implement fast csum_partial for x86

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Date:	Sat, 27 Feb 2016 00:30:07 -0800
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v4 net-next] net: Implement fast csum_partial for x86_64

> +{
> +       asm("lea 40f(, %[slen], 4), %%r11\n\t"
> +           "clc\n\t"
> +           "jmpq *%%r11\n\t"
> +           "adcq 7*8(%[src]),%[res]\n\t"
> +           "adcq 6*8(%[src]),%[res]\n\t"
> +           "adcq 5*8(%[src]),%[res]\n\t"
> +           "adcq 4*8(%[src]),%[res]\n\t"
> +           "adcq 3*8(%[src]),%[res]\n\t"
> +           "adcq 2*8(%[src]),%[res]\n\t"
> +           "adcq 1*8(%[src]),%[res]\n\t"
> +           "adcq 0*8(%[src]),%[res]\n\t"
> +           "nop\n\t"
> +           "40: adcq $0,%[res]"
> +                   : [res] "=r" (sum)
> +                   : [src] "r" (buff),
> +                     [slen] "r" (-((unsigned long)(len >> 3))), "[res]" (sum)
> +                   : "r11");
> +

With this patch I cannot mix/match different length checksums without
things failing.  In perf the jmpq in the loop above seems to be set to
a fixed value so perhaps it is something in how the compiler is
interpreting the inline assembler.

- Alex