linux-kernel - Re: [PATCH] crypto: x86/twofish-3way

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Message-ID: <20171219173533.25evvqns4tlxztzj@gmail.com>
Date:   Tue, 19 Dec 2017 18:35:33 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-crypto@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jussi Kivilinna <jussi.kivilinna@....fi>, x86@...nel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Eric Biggers <ebiggers@...gle.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] crypto: x86/twofish-3way - Fix %rbp usage


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Eric Biggers <ebiggers3@...il.com> wrote:
> 
> > There may be a small overhead caused by replacing 'xchg REG, REG' with
> > the needed sequence 'mov MEM, REG; mov REG, MEM; mov REG, REG' once per
> > round.  But, counterintuitively, when I tested "ctr-twofish-3way" on a
> > Haswell processor, the new version was actually about 2% faster.
> > (Perhaps 'xchg' is not as well optimized as plain moves.)
> 
> XCHG has implicit LOCK semantics on all x86 CPUs, so that's not a surprising 
> result I think.

Correction: I think XCHG only implies LOCK if there's a memory operand involved - 
register-register XCHG should not imply any barriers.

So the result is indeed unintuitive.

Thanks,

	Ingo