netdev - Re: [PATCH net-next 6/8] net: gre: Implement LCO for GRE over IPv4

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Date:	Wed, 20 Jan 2016 23:34:03 +0000
From:	"Rustad, Mark D" <mark.d.rustad@...el.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
CC:	Tom Herbert <tom@...bertland.com>,
	Edward Cree <ecree@...arflare.com>,
	David Laight <David.Laight@...lab.com>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-net-drivers@...arflare.com" <linux-net-drivers@...arflare.com>
Subject: Re: [PATCH net-next 6/8] net: gre: Implement LCO for GRE over IPv4

Alexander Duyck <alexander.duyck@...il.com> wrote:

> There isn't any need to add such an indication, nor do we need to
> start bitflipping the return value from csum_fold in all cases.  I
> think there was just some confusion about UDP checksums vs GRE or TCP
> checksums.

Yeah. I think I finally got there. The naive software methods will never  
generate a true 0 unless everything was zero. Real one's complement  
machines did addition in terms of subtraction so that 1 + -1 would never  
produce a -0, only a normal 0. Of course a simple adder would produce a -0,  
making it impossible to get back to a normal 0.

> I'd say we are better off keeping this simple.  The original patch
> just needs to drop the check for the resultant checksum being 0 since
> that is not needed for GRE.

I'm all in favor of simple. I had just started to worry about a possible  
change in behavior that might have interoperability problems with some  
implementations. I wonder if any implementation ever did the addition by  
subtraction, but also failed to make 0 compare equal to -0? I guess if they  
knew enough to do the former, they should not have blown the latter.

--
Mark Rustad, Networking Division, Intel Corporation

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