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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Fri, 4 Oct 2019 17:05:05 +0200
Message-ID: <CAKv+Gu_=7Gq+4=TGn7XqJX=F4VoR-YbgsLADYuw9PuGKWDvCxA@mail.gmail.com>
Subject: Re: [PATCH v2 05/20] crypto: mips/chacha - import accelerated 32r2
 code from Zinc
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@vger.kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        David Miller <davem@davemloft.net>,
        Greg KH <gregkh@linuxfoundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Samuel Neves <sneves@dei.uc.pt>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Eric Biggers <ebiggers@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Martin Willi <martin@strongswan.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        =?UTF-8?Q?Ren=C3=A9_van_Dorst?= <opensource@vdorst.com>
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On Fri, 4 Oct 2019 at 16:59, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> On Fri, Oct 4, 2019 at 4:44 PM Ard Biesheuvel <ard.biesheuvel@linaro.org>=
 wrote:
> > The round count is passed via the fifth function parameter, so it is
> > already on the stack. Reloading it for every block doesn't sound like
> > a huge deal to me.
>
> Please benchmark it to indicate that, if it really isn't a big deal. I
> recall finding that memory accesses on common mips32r2 commodity
> router hardware was extremely inefficient. The whole thing is designed
> to minimize memory accesses, which are the primary bottleneck on that
> platform.
>

Reloading a single word from the stack each time we load, xor and
store 64 bytes of data from/to memory is highly unlikely to be
noticeable.

> Seems like this thing might be best deferred for after this all lands.
> IOW, let's get this in with the 20 round original now, and later you
> can submit a change for the 12 round and Ren=C3=A9 and I can spend time
> dusting off our test rigs and seeing which strategy works best. I very
> nearly tossed out a bunch of old router hardware last night when
> cleaning up. Glad I saved it!

I don't agree but I don't care deeply enough to argue about it :-)