linux-kernel - RE: x86/csum: Remove unnecessary odd handling

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Date: Sun, 7 Jan 2024 11:44:19 +0000
From: David Laight <David.Laight@...LAB.COM>
To: "'H. Peter Anvin'" <hpa@...or.com>, 'Linus Torvalds'
	<torvalds@...ux-foundation.org>
CC: Noah Goldstein <goldstein.w.n@...il.com>, "x86@...nel.org"
	<x86@...nel.org>, "oe-kbuild-all@...ts.linux.dev"
	<oe-kbuild-all@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com"
	<mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: RE: x86/csum: Remove unnecessary odd handling

From: H. Peter Anvin
> Sent: 07 January 2024 01:09
> 
> On January 6, 2024 2:08:48 PM PST, David Laight <David.Laight@...LAB.COM> wrote:
...
> >The best loop for 256+ bytes is an adxc/adxo one.
> >However that requires the run-time patching.
...
> Rather than runtime patching perhaps separate paths...

It will need to detect the cpu type earlier, so a static
branch is probably enough.
Easier than substituting the entire code block.

I think it is silvermont and knight's landing that have
a 4 clock penalty for 64bit adxc (Intel atom family).
That might only be a decode penalty, so doesn't affect
the loop 'that much' (adc is 2 clocks on those cpu).
So probably not actually worth doing a run-time
performance check.

I might 'cook up' a full checksum function later.

	David

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