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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 1 Dec 2020 23:27:52 +0100
Message-ID: <CAMj1kXFrLiHfv1S1AM=5pc1J9gWwZVuoGvmFoTT0-+oREoojTA@mail.gmail.com>
Subject: Re: [PATCH v2] crypto: aesni - add ccm(aes) algorithm implementation
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Ben Greear <greearb@candelatech.com>,
        Steve deRosier <derosier@cal-sierra.com>
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On Tue, 1 Dec 2020 at 23:16, Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> On Tue, Dec 01, 2020 at 11:12:32PM +0100, Ard Biesheuvel wrote:
> >
> > What do you mean by just one direction? Ben just confirmed a
>
> The TX direction generally executes in process context, which
> would benefit from an accelerated sync implementation.  The RX
> side on the other hand is always in softirq context.
>

Yes, and in both cases, irq_fpu_usable() will return true, unless RX
and TX are running on the same core.

> > substantial speedup for his use case, which requires the use of
> > software encryption even on hardware that could support doing it in
> > hardware, and that software encryption currently only supports the
> > synchronous interface.
>
> The problem is that the degradation would come at the worst time,
> when the system is loaded.  IOW when you get an interrupt during
> your TX path and get RX traffic that's when you'll take the fallback
> path.
>

I can see how in the general case, this is something you would prefer
to avoid. However, on SMP x86_64 systems that implement AES-NI (which
runs at ~1 cycle per byte), I don't see this as a real problem for
this driver.

What we could do is expose both versions, where the async version has
a slightly higher priority, so that all users that do support the
async interface will get it, and the wifi stack can use the sync
interface instead.