From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: Re: [PATCH net-next v6 07/23] zinc: ChaCha20 ARM and ARM64 implementations
Date: Wed, 26 Sep 2018 16:02:22 +0200
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To: "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Thomas Gleixner <tglx@linutronix.de>
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(+ Herbert, Thomas)

On Wed, 26 Sep 2018 at 15:33, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Ard,
>
> On Wed, Sep 26, 2018 at 10:59 AM Ard Biesheuvel
> <ard.biesheuvel@linaro.org> wrote:
> > > +static inline bool chacha20_arch(struct chacha20_ctx *state, u8 *dst,
> > > +                                const u8 *src, size_t len,
> > > +                                simd_context_t *simd_context)
> > > +{
> > > +#if defined(CONFIG_KERNEL_MODE_NEON)
> > > +       if (chacha20_use_neon && len >= CHACHA20_BLOCK_SIZE * 3 &&
> > > +           simd_use(simd_context))
> > > +               chacha20_neon(dst, src, len, state->key, state->counter);
> > > +       else
> > > +#endif
> >
> > Better to use IS_ENABLED() here:
> >
> > > +       if (IS_ENABLED(CONFIG_KERNEL_MODE_NEON)) &&
> > > +           chacha20_use_neon && len >= CHACHA20_BLOCK_SIZE * 3 &&
> > > +           simd_use(simd_context))
>
> Good idea. I'll fix that up.
>
> >
> > Also, this still has unbounded worst case scheduling latency, given
> > that the outer library function passes its entire input straight into
> > the NEON routine.
>
> The vast majority of crypto routines in arch/*/crypto/ follow this
> same exact pattern, actually. I realize a few don't -- probably the
> ones you had a hand in :) -- but I think this is up to the caller to
> handle.

Anything that uses the scatterwalk API (AEADs and skciphers) will
handle at most a page at a time. Hashes are different, which is why
some of them have to handle it explicitly.

> I made a change so that in chacha20poly1305.c, it calls
> simd_relax after handling each scatter-gather element, so a
> "construction" will handle this gracefully. But I believe it's up to
> the caller to decide on what sizes of information it wants to pass to
> primitives. Put differently, this also hasn't ever been an issue
> before -- the existing state of the tree indicates this -- and so I
> don't anticipate this will be a real issue now.

The state of the tree does not capture all relevant context or
history. The scheduling latency issue was brought up very recently by
the -rt folks on the mailing lists.

> And if it becomes one,
> this is something we can address *later*, but certainly there's no use
> of adding additional complexity to the initial patchset to do this
> now.
>

You are introducing a very useful SIMD abstraction, but it lets code
run with preemption disabled for unbounded amounts of time, and so now
is the time to ensure we get it right.

Part of the [justified] criticism on the current state of the crypto
API is on its complexity, and so I don't think it makes sense to keep
it simple now and add the complexity later (and the same concern
applies to async support btw).