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Date: Thu, 30 Oct 2014 18:54:20 +0200 From: Or Gerlitz <ogerlitz@...lanox.com> To: David Laight <David.Laight@...LAB.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 On 10/30/2014 6:39 PM, David Laight wrote: > 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. OK, talking to Matan, hecame up with this super-quick (compile tested only) alternative, is this what you were advocating for? Or. diff --git a/include/net/ip6_checksum.h b/include/net/ip6_checksum.h index c45d690..cadffdc 100644 --- a/include/net/ip6_checksum.h +++ b/include/net/ip6_checksum.h @@ -46,6 +46,19 @@ static inline __wsum csum_ipv6_magic_nofold(const struct in6_addr *saddr, __u32 len, unsigned short proto, __wsum sum) { + sum = csum_partial(saddr, sizeof(saddr->in6_u.u6_addr32), sum); + sum = csum_partial(saddr, sizeof(daddr->in6_u.u6_addr32), sum); + sum = csum_add(sum, (__force __wsum)htonl(len)); + sum = csum_add(sum, (__force __wsum)htons(proto)); + + return sum; +} -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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