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

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Message-ID: <CAHmME9q=hMRjBG=SBX8gCC3qx-t1wdEwMOYx952m9HkByjiofA@mail.gmail.com>
Date:   Wed, 22 Apr 2020 01:32:33 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH crypto-stable] crypto: arch/lib - limit simd usage to
 PAGE_SIZE chunks

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?

Jason