netdev - RE: [RFC] csum experts, csum_replace2() is too expensive

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Date:	Mon, 24 Mar 2014 10:30:19 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>
CC:	"andi@...stfloor.org" <andi@...stfloor.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"kaber@...sh.net" <kaber@...sh.net>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"hkchu@...gle.com" <hkchu@...gle.com>,
	"mwdalton@...gle.com" <mwdalton@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC] csum experts, csum_replace2() is too expensive

From: Eric Dumazet
> On Fri, 2014-03-21 at 14:52 -0400, David Miller wrote:
> > From: Eric Dumazet <eric.dumazet@...il.com>
> > Date: Fri, 21 Mar 2014 05:50:50 -0700
> >
> > > It looks like a barrier() would be more appropriate.
> >
> > barrier() == __asm__ __volatile__(:::"memory")
> 
> Indeed, but now you mention it, ip_fast_csum() do not uses volatile
> keyword on x86_64, and has no "m" constraint either.

Adding 'volatile' isn't sufficient to force gcc to write data
into the area being checksummed.
ip_fast_csum() either needs an explicit "m" constraint for the actual
buffer (and target) bytes, or the stronger "memory" constraint.
The 'volatile' is then not needed.

	David