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

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1C9E258D@AcuExch.aculab.com>
Date:	Thu, 30 Oct 2014 17:09:31 +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

> OK, talking to Matan, hecame up with this super-quick (compile tested
> only) alternative, is
> this what you were advocating for?
>   {
> +       sum = csum_partial(saddr, sizeof(saddr->in6_u.u6_addr32), sum);
> +       sum = csum_partial(saddr, sizeof(daddr->in6_u.u6_addr32), sum);

I'm pretty sure your 'saddr' and 'daddr' are adjacent.
Whether you can prove that is another matter.

> +       sum = csum_add(sum, (__force __wsum)htonl(len));
> +       sum = csum_add(sum, (__force __wsum)htons(proto));
> +
> +       return sum;
> +}

On 64 bit systems you probably want to end up with something akin to:
	__u64 a, b, c, d;

	a = saddr[0] + saddr[1];
	b = saddr[2] + saddr[3];
	c = daddr[0] + daddr[1];
	d = daddr[2] + daddr[3];
	a += b;
	c += d;
	a += c;
and then collapse down the 64bit value.
However if you write the above in proper C, gcc will probably
convert it to long dependence chain.

An architecture specific csum_partial() probably manages to DTRT.

	David



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