Re: HalfSipHash Acceptable Usage

Linus wrote:

quoted

How much does kernel_fpu_begin()/kernel_fpu_end() cost?

It's now better than it used to be, but it's absolutely disastrous
still. We're talking easily many hundreds of cycles. Under some loads,
thousands.

I think I've been thoroughly dissuaded, but just to clarify one thing
that resembles a misunderstanding:

In contrast, in reality, especially with things like "do it once or
twice per incoming packet", you'll easily hit the absolute worst
cases, where not only does it take a few hundred cycles to save the FP
state, you'll then return to user space in between packets, which
triggers the slow-path return code and reloads the FP state, which is
another few hundred cycles plus.

Everything being discussed is per-TCP-connection overhead, *not* per
packet.  (Twice for outgoing connections, because one is to generate
the ephemeral port number.)

I know you know this, but I don't want anyone spectating to be confused
about it.