linux-kernel - Re: Proposal: HAVE_SEPARATE_IRQ

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Date:   Thu, 10 Nov 2016 00:34:54 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mips@...ux-mips.org,
        linux-mm@...ck.org,
        WireGuard mailing list <wireguard@...ts.zx2c4.com>,
        k@...ka.home.kg
Subject: Re: Proposal: HAVE_SEPARATE_IRQ_STACK?

Hey Thomas,

On Wed, Nov 9, 2016 at 10:40 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> That preempt_disable() prevents merily preemption as the name says, but it
> wont prevent softirq handlers from running on return from interrupt. So
> what's the point?

Oh, interesting. Okay, then in that case the proposed define wouldn't
be useful for my purposes. What clever tricks do I have at my
disposal, then?

>> If not, do you have a better solution for me (which doesn't
>> involve using kmalloc or choosing a different crypto primitive)?
>
> What's wrong with using kmalloc?

It's cumbersome and potentially slow. This is crypto code, where speed
matters a lot. Avoiding allocations is usually the lowest hanging
fruit among optimizations. To give you some idea, here's a somewhat
horrible solution using kmalloc I hacked together: [1]. I'm not to
happy with what it looks like, code-wise, and there's around a 16%
slowdown, which isn't nice either.

[1] https://git.zx2c4.com/WireGuard/commit/?h=jd/curve25519-kmalloc