netdev - Re: [PATCH v2 net-next] tcp: md5: use kmalloc() backed scratch areas

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Date:	Wed, 29 Jun 2016 15:39:37 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	David Miller <davem@...emloft.net>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] tcp: md5: use kmalloc() backed scratch areas

On Wed, Jun 29, 2016 at 2:44 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2016-06-29 at 09:41 -0700, Andy Lutomirski wrote:
>
>> Overall, it looks like there's overhead of something like 50ns for
>> each ahash invocation vs the shash equivalent.  It's not huge, but
>> it's there.  (This is cache-hot.  I bet it's considerably worse if
>> cache-cold, because ahash will require a lot more code cache lines as
>> well as the extra cache lines involved in the scatterlist and whatever
>> arch stuff is needed to map back and forth between virtual and
>> physical addresses.
>
> I am kind of mystified seeing someone caring about TCP MD5, other than
> just making sure it wont crash the host when it needs to be used ;)
>
> The real useful work would be to use a jump label so that we can avoid
> spending cycles for non TCP MD5 sessions, when a host never had to use
> any MD5 negotiation.
>
>
>

I don't care about TCP MD5 performance at all.  Ease of maintenance is
nice, though, and maybe there are other places in the kernel where
performance does matter.

--Andy