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

From: Or Gerlitz <[email-masked]>
Date: Thu, 30 Oct 2014 18:54:20 +0200

On 10/30/2014 6:39 PM, David Laight wrote:
> From: Or Gerlitz [mailto:[email-masked]]
>> 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;
+}