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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Thu, 27 Sep 2018 17:19:20 +0200
Message-ID: <CAHmME9pBRu4hC5=Ef62R_nWOi3jTegxvxNrR8qDM+VfO=2t8Tg@mail.gmail.com>
Subject: Re: [PATCH net-next v6 07/23] zinc: ChaCha20 ARM and ARM64 implementations
To:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        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
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Hey again Thomas,

On Thu, Sep 27, 2018 at 3:26 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Thomas,
>
> I'm trying to optimize this for crypto performance while still taking
> into account preemption concerns. I'm having a bit of trouble figuring
> out a way to determine numerically what the upper bounds for this
> stuff looks like. I'm sure I could pick a pretty sane number that's
> arguably okay -- and way under the limit -- but I still am interested
> in determining what that limit actually is. I was hoping there'd be a
> debugging option called, "warn if preemption is disabled for too
> long", or something, but I couldn't find anything like that. I'm also
> not quite sure what the latency limits are, to just compute this with
> a formula. Essentially what I'm trying to determine is:
>
> preempt_disable();
> asm volatile(".fill N, 1, 0x90;");
> preempt_enable();
>
> What is the maximum value of N for which the above is okay? What
> technique would you generally use in measuring this?
>
> Thanks,
> Jason

From talking to Peter (now CC'd) on IRC, it sounds like what you're
mostly interested in is clocktime latency on reasonable hardware, with
a goal of around ~20=C2=B5s as a maximum upper bound? I don't expect to get
anywhere near this value at all, but if you can confirm that's a
decent ballpark, it would make for some interesting calculations.

Regards,
Jason