linux-kernel - Re: [PATCH crypto-stable] crypto: arch/lib - limit simd usage to PAGE

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Message-ID: <CAMj1kXE-UZxw0C3WRVh7RfuWE0BNDT4bt4qJa1SyOH3K-qBBcQ@mail.gmail.com>
Date:   Wed, 22 Apr 2020 09:39:19 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH crypto-stable] crypto: arch/lib - limit simd usage to
 PAGE_SIZE chunks

On Wed, 22 Apr 2020 at 09:32, Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> On Tue, Apr 21, 2020 at 10:04 PM Eric Biggers <ebiggers@...nel.org> wrote:
> > Seems this should just be a 'while' loop?
> >
> >         while (bytes) {
> >                 unsigned int todo = min_t(unsigned int, PAGE_SIZE, bytes);
> >
> >                 kernel_neon_begin();
> >                 chacha_doneon(state, dst, src, todo, nrounds);
> >                 kernel_neon_end();
> >
> >                 bytes -= todo;
> >                 src += todo;
> >                 dst += todo;
> >         }
>
> The for(;;) is how it's done elsewhere in the kernel (that this patch
> doesn't touch), because then we can break out of the loop before
> having to increment src and dst unnecessarily. Likely a pointless
> optimization as probably the compiler can figure out how to avoid
> that. But maybe it can't. If you have a strong preference, I can
> reactor everything to use `while (bytes)`, but if you don't care,
> let's keep this as-is. Opinion?
>

Since we're bikeshedding, I'd prefer 'do { } while (bytes);' here,
given that bytes is guaranteed to be non-zero before we enter the
loop. But in any case, I'd prefer avoiding for(;;) or while(1) where
we can.