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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Fri, 4 Oct 2019 16:07:53 +0200
Message-ID: <CAKv+Gu-VZT4caxAs5S88k+CVNaZh=K2ZDnj+gUArG959FEaMpQ@mail.gmail.com>
Subject: Re: [PATCH v2 20/20] crypto: lib/chacha20poly1305 - reimplement
 crypt_from_sg() routine
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>
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On Fri, 4 Oct 2019 at 16:03, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> On Wed, Oct 02, 2019 at 04:17:13PM +0200, Ard Biesheuvel wrote:
> > Reimplement the library routines to perform chacha20poly1305 en/decryption
> > on scatterlists, without [ab]using the [deprecated] blkcipher interface,
> > which is rather heavyweight and does things we don't really need.
> >
> > Instead, we use the sg_miter API in a novel and clever way, to iterate
> > over the scatterlist in-place (i.e., source == destination, which is the
> > only way this library is expected to be used). That way, we don't have to
> > iterate over two scatterlists in parallel.
>
> Nice idea. Probably this will result in a real speedup, as I suspect
> those extra prior kmaps weren't free. Looking forward to benching it.

They weren't only non-free, they were also using kmap_atomic()
unconditionally, which means that these routines were running with
preemption disabled even with non-SIMD crypto on non-highmem
architectures.