Date: 21 Dec 2016 13:07:22 -0500 From: "George Spelvin" <linux@...encehorizons.net> To: linux@...encehorizons.net, torvalds@...ux-foundation.org Cc: ak@...ux.intel.com, davem@...emloft.net, David.Laight@...lab.com, djb@...yp.to, ebiggers3@...il.com, eric.dumazet@...il.com, hannes@...essinduktion.org, Jason@...c4.com, jeanphilippe.aumasson@...il.com, kernel-hardening@...ts.openwall.com, linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org, luto@...capital.net, netdev@...r.kernel.org, tom@...bertland.com, tytso@....edu, vegard.nossum@...il.com Subject: Re: HalfSipHash Acceptable Usage Linus wrote: >> 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.
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