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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 22 Mar 2019 10:48:20 -0700
Message-ID: <CAHk-=wg2LJ5qQ0B2y+_6Ue62SBP4h9MLxLvn89bfcP7Cp2ac6A@mail.gmail.com>
Subject: Re: [PATCH 0/17] Add zinc using existing algorithm implementations
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Eric Biggers <ebiggers@kernel.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        linux-fscrypt@vger.kernel.org,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Paul Crowley <paulcrowley@google.com>,
        Greg Kaiser <gkaiser@google.com>,
        Samuel Neves <samuel.c.p.neves@gmail.com>,
        Tomer Ashur <tomer.ashur@esat.kuleuven.be>,
        Martin Willi <martin@strongswan.org>
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On Fri, Mar 22, 2019 at 12:56 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
> - The way WireGuard uses crypto in the kernel is essentially a
> layering violation

What? No.

That's just wrong.

It's only a layering violation if you accept and buy into the crypto/ model.

And Jason obviously doesn't.

And honestly, I'm 1000% with Jason on this. The crypto/ model is hard
to use, inefficient, and completely pointless when you know what your
cipher or hash algorithm is, and your CPU just does it well directly.

> we even have support already for async accelerators that implement it,

Afaik, none of the async accelerator code has ever been worth anything
on real hardware and on any sane and real loads. The cost of going
outside the CPU is *so* expensive that you'll always lose, unless the
algorithm has been explicitly designed to be insanely hard on a
regular CPU.

(Corollary: "insanely hard on a regular CPU" is also easy to do by
making the CPU be weak and bad. Which is not a case we should optimize
for).

The whole "external accelerator" model is odd. It was wrong. It only
makes sense if the accelerator does *everything* (ie it's the network
card), and then you wouldn't use the wireguard thing on the CPU at
all, you'd have all those things on the accelerator (ie a "network
card that does WG").

One of the (best or worst, depending on your hangups) arguments for
external accelerators has been "but I trust the external hardware with
the key, but not my own code", aka the TPM or Disney argument.  I
don't think that's at all relevant to the discussion either.

The whole model of async accelerators is completely bogus. The only
crypto or hash accelerator that is worth it are the ones integrated on
the CPU cores, which have the direct access to caches.

And if the accelerator is some tightly coupled thing that has direct
access to your caches, and doesn't need interrupt overhead or address
translation etc (at which point it can be worth using) then you might
as well just consider it an odd version of the above. You'd want to
poll for the result anyway, because not polling is too expensive.

Just a single interrupt would completely undo all the advantages you
got from using specialized hardware - both power and performance.

And that kind of model would work just fine with zinc.

So an accelerator ends up being useful in two cases:

 - it's entirely external and part of the network card, so that
there's no extra data transfer overhead

 - it's tightly coupled enough (either CPU instructions or some on-die
cache coherent engine) that you can and will just use it synchronously
anyway.

In the first case, you wouldn't run wireguard on the CPU anyway - you
have a network card that just implements the VPN.

And in the second case, the zinc model is the right one.

So no. I don't believe "layering violation" is the issue here at all.

The only main issue as far as I'm concerned is how to deal with the
fact that we have duplicate code and effort.

                      Linus