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

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Message-ID: <CA+55aFzf9enVibvBzbu_wqU4OSm_QYivmJf3vmyH1BxvCLxJog@mail.gmail.com>
Date:	Fri, 26 Feb 2016 12:29:10 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tom Herbert <tom@...bertland.com>
Cc:	David Miller <davem@...emloft.net>,
	Network Development <netdev@...r.kernel.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

Looks ok to me.

I am left wondering if the code should just do that

    add32_with_carry3(sum, result >> 32, result);

in the caller instead - right now pretty much every return point in
do_csum() effectively does that,  with the exception of

 - the 0-length case, which is presumably not really an issue in real
life and could as well just return 0

 - the 8-byte case that does two 32-bit loads instead, but could just
do a single 8-byte load and return it (and then the generic version in
the caller would do a shift).

That would simplifiy the code a bit - it wouldn't need to pass in
"sum" to do_csum at all, and we'd have just a single case of that
special 3-input carry add..

But I'm certainly ok with it as-is. I'm not sure how performance
critical the whole csum routine is, but at least now it doesn't
introduce a lot of new complex asm.

And this version might be reasonable to make generic, so that non-x86
architectures could use the same approach. That's what we ended up
doing for the dcache word-at-a-time code too in the end.

                Linus