netdev - Re: [PATCH v5 net-next] net: Implement fast csum_partial for x86

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Date:	Mon, 7 Mar 2016 17:39:04 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tom Herbert <tom@...bertland.com>
Cc:	Alexander Duyck <alexander.duyck@...il.com>,
	David Laight <David.Laight@...lab.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
	"kernel-team@...com" <kernel-team@...com>
Subject: Re: [PATCH v5 net-next] net: Implement fast csum_partial for x86_64

On Mon, Mar 7, 2016 at 4:07 PM, Tom Herbert <tom@...bertland.com> wrote:
>
> As I said previously, if alignment really is a factor then we can
> check up front if a buffer crosses a page boundary and call the slow
> path function (original code). I'm seeing a 1 nsec hit to add this
> check.

It shouldn't be a factor, and you shouldn't check for it. My code was
self-aligning, and had at most one unaligned access at the beginnig
(the data of which was then used to align the rest).

Tom had a version that used that. Although now that I look back at it,
it seems to be broken by some confusion about the one-byte alignment
vs 8-byte alignment.

             Linus