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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 27 Jun 2023 15:43:44 -0700
Message-ID: <CAHk-=wgMw2VE8afWp4p6RyJKCvnkhNXx4XUByUOHUWKiAo=WLQ@mail.gmail.com>
Subject: Re: [GIT PULL] x86/misc for 6.5
To:     Arjan van de Ven <arjan@linux.intel.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        Noah Goldstein <goldstein.w.n@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        x86-ml <x86@kernel.org>, lkml <linux-kernel@vger.kernel.org>
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On Tue, 27 Jun 2023 at 15:25, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I don't know what the rules are - and some of the functions you test
> seem actively buggy (ie not handling alignment right etc).

Oh. And you *only* test the 40-byte case. That seems a bit bogus.

If I change the packet size from 40 to 1500, I get

  02: 185.1 / 186.4 cycles (8b414316) Upcoming linux kernel version
  04: 184.9 / 186.5 cycles (8b414316) Specialized to size 40
  06: 107.3 / 117.2 cycles (8b414316) New version
  22: 185.6 / 186.5 cycles (8b414316) Odd-alignment handling removed

which seems unexpectedly bad for the other versions.

But those other functions have that 64-byte unrolling, rather than the
"two 40-byte loops", so maybe it is real, and my version is actually
just that good.

Or maybe it's a sign that my version is some seriously buggy crap, and
it just looks good on the benchmark because it does the wrong thing.

Whatever. Back to the merge window again.

              Linus