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

["Jason A. Donenfeld" <Jason@zx2c4.com>]

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