From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: Re: [PATCH net-next v6 07/23] zinc: ChaCha20 ARM and ARM64 implementations
Date: Wed, 26 Sep 2018 15:32:45 +0200
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Cc: LKML <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        David Miller <davem@davemloft.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Samuel Neves <sneves@dei.uc.pt>,
        Andrew Lutomirski <luto@kernel.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        linux-arm-kernel@lists.infradead.org
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
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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. 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. 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.

Jason