netdev - Re: [PATCH net-next v6 07/23] zinc: ChaCha20 ARM and ARM64 implementations

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Date:   Thu, 27 Sep 2018 15:26:18 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Samuel Neves <sneves@....uc.pt>,
        Andrew Lutomirski <luto@...nel.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net-next v6 07/23] zinc: ChaCha20 ARM and ARM64 implementations

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