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

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Message-ID: <1467236646.6369.5.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Wed, 29 Jun 2016 23:44:06 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
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, 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.