linux-kernel - Re: [RFC PATCH 1/9] kernel: add support for patchable function pointers

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Date:   Fri, 5 Oct 2018 21:13:08 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Eric Biggers <ebiggers@...nel.org>,
        Samuel Neves <sneves@....uc.pt>, Arnd Bergmann <arnd@...db.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        David Miller <davem@...emloft.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, benh@...nel.crashing.org,
        paulus@...ba.org, Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        Kees Cook <keescook@...omium.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, richard@....at,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH 1/9] kernel: add support for patchable function pointers



> On 5 Oct 2018, at 20:28, Jason A. Donenfeld <Jason@...c4.com> wrote:
> 
> Hey Andy,
> 
>> On Fri, Oct 5, 2018 at 7:44 PM Andy Lutomirski <luto@...nel.org> wrote:
>> I *think* the only change to Zinc per se would be that the calls like
>> chacha20_simd() would be static calls or patchable functions or
>> whatever we want to call them.  And there could be a debugfs to
>> override the default selection.
> 
> Yea, right, exactly. It turns out this is really easy to do with the
> way it's structured now. I'd actually experimented considerably with
> using the static keys a while back, but couldn't find any performance
> difference on any platform at all (four ARM microarchitectures, three
> MIPS, various random intel, an old powerpc), so went with the simplest
> solution. But we can certainly play with more elaborate patching
> mechanisms later on and see how those turn out. Also, even with the
> simple bools as we have now, it's quite easy to make all the
> parameters toggle-able.
> 
>> Ard, I don't think that sticking this in udev rules makes sense.  The
>> kernel has bascially complete information as to what the right choice
>> is, and that will change over time as the implementation gets tuned,
>> and the udev rules will never get updated in sync.
> 
> Yes, I agree with this.
> 
> 

I am not referring to udev rules. I just mean the current way that udev autoloads modules based on CPU features.