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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 27 Jun 2023 13:11:13 -0700
Message-ID: <CAHk-=wghuOwXtMH9u7RAW694BOwf_Q9TeWR2v=GgLy0gjTfu4A@mail.gmail.com>
Subject: Re: [GIT PULL] x86/misc for 6.5
To:     Borislav Petkov <bp@alien8.de>,
        Noah Goldstein <goldstein.w.n@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
Cc:     x86-ml <x86@kernel.org>, lkml <linux-kernel@vger.kernel.org>
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On Tue, 27 Jun 2023 at 04:00, Borislav Petkov <bp@alien8.de> wrote:
>
> - Improve csum_partial()'s performance

Honestly, looking at that patch, my reaction is "why did it get
unrolled in 64-byte chunks, if 40 bytes is the magic value"?

Particularly when there is then that "do a carry op each 32 bytes to
make 32-byte chunks independent and increase ILP". So even the 64-byte
case isn't *actuall* doing a 64-byte unrolling, it's really doing two
32-byte unrollings in parallel.

So you have three "magic" values, and the only one that really matters
is likely the 40-byte one.

Yes, yes, 64 bytes is the usual cacheline size, and is "traditional"
for unrolling. But there's nothing really magical about it here.

End result: wouldn't it have been nice to just do 40-byte chunks, and
make the 64-byte "two overlapping 32-byte chunks" be two of the
40-byte chunks.

Something like the (ENTIRELY UNTESTED!) attached patch?

Again: this is *not* tested. I took a quick look at the generated
assembly, and it looked roughly like what I expected it to look like,
but it may be complete garbage.

I added a couple of "likely()" things just because it made the
generated asm look more natural (ie it followed the order of the
source code there), they are otherwise questionable annotations.

Finally: did I already mention that this is completely untested?

             Linus

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