linux-kernel - RE: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum

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Date:   Thu, 23 Jul 2020 13:54:47 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Al Viro' <viro@...iv.linux.org.uk>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: RE: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead
 of 0 as initial sum

From: Al Viro
> Sent: 22 July 2020 18:39
> I would love to see your patch, anyway, along with the testcases and performance
> comparison.

See attached program.
Compile and run (as root): csum_iov 1

Unpatched (as shipped) 16 vectors of 1 byte take ~430 clocks on my haswell cpu.
With dsl_patch defined they take ~393.

The maximum throughput is ~1.16 clocks/word for 16 vectors of 1k.
For longer vectors the data gets lost from the cache between the iterations.

On an older Ivy Bridge cpu it never goes faster than 2 clocks/word.
(Due to the implementation of ADC.)

The absolute limit is 1 clock/word - limited by the memory write.
I suspect that is achievable on Haswell with much less loop unrolling.

I had to replace the ror32() with __builtin_bswap32().
The kernel object do contain the 'ror' instruction - even though I
didn't find the asm for it.

	David

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View attachment "csum_iov.c" of type "text/plain" (7804 bytes)