netdev - RE: [PATCH net-next 7/8] net: Add calaulation of non folded IPV6 pseudo header checksum

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Date:	Thu, 30 Oct 2014 16:39:33 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Or Gerlitz' <ogerlitz@...lanox.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Matan Barak <matanb@...lanox.com>,
	Amir Vadai <amirv@...lanox.com>,
	Saeed Mahameed <saeedm@...lanox.com>,
	Shani Michaeli <shanim@...lanox.com>
Subject: RE: [PATCH net-next 7/8] net: Add calaulation of non folded IPV6
 pseudo header checksum

From: Or Gerlitz [mailto:ogerlitz@...lanox.com]
> On 10/30/2014 6:25 PM, David Laight wrote:
> >> >+static inline __wsum csum_ipv6_magic_nofold(const struct in6_addr *saddr,
> >> >+					    const struct in6_addr *daddr,
> >> >+					    __u32 len, unsigned short proto,
> >> >+					    __wsum sum)
> >> >+{
> >> >+	__wsum res = sum;
> >> >+
> >> >+	res = csum_add(res, (__force __wsum)saddr->in6_u.u6_addr32[0]);
> >> >+	res = csum_add(res, (__force __wsum)saddr->in6_u.u6_addr32[1]);
> >> >+	res = csum_add(res, (__force __wsum)saddr->in6_u.u6_addr32[2]);
> >> >+	res = csum_add(res, (__force __wsum)saddr->in6_u.u6_addr32[3]);
> >> >+	res = csum_add(res, (__force __wsum)daddr->in6_u.u6_addr32[0]);
> >> >+	res = csum_add(res, (__force __wsum)daddr->in6_u.u6_addr32[1]);
> >> >+	res = csum_add(res, (__force __wsum)daddr->in6_u.u6_addr32[2]);
> >> >+	res = csum_add(res, (__force __wsum)daddr->in6_u.u6_addr32[3]);
>
> > That probably generates a very long dependency chain.
> >
> 
> Could you clarify this comment a bit?

csum_add() probably generates a 32bit 'add with carry' instruction.
So the above generates 8 instructions that have to be executed in series
(dependencies on the register and the carry flag).

On a 64bit cpu there are other options, eg adding 32bit values into
several 64bit registers, then adding those together and finally
collapsing the value to 32 then 16 bits.

Maybe __wsum does end up being 64bit (not looked), but gcc won't
generate a 'tree' of additions, it will still generate a dependency chain.

Hopefully the software 'checksum a buffer' function is written to
avoid these problems.

	David



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