linux-kernel - RE: [PATCH v4 05/11] arm64: csum: Disable KASAN for do

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Date:   Fri, 24 Apr 2020 09:41:30 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Robin Murphy' <robin.murphy@....com>,
        Will Deacon <will@...nel.org>,
        "Mark Rutland" <mark.rutland@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "kernel-team@...roid.com" <kernel-team@...roid.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: RE: [PATCH v4 05/11] arm64: csum: Disable KASAN for do_csum()

From: Robin Murphy
> Sent: 22 April 2020 12:02
..
> Sure - I have a nagging feeling that it could still do better WRT
> pipelining the loads anyway, so I'm happy to come back and reconsider
> the local codegen later. It certainly doesn't deserve to stand in the
> way of cross-arch rework.

How fast does that loop actually run?
To my mind it seems to do a lot of operations on each 64bit value.
I'd have thought that a loop based on:
	sum64 = *ptr;
	sum64_high = *ptr++ >> 32;
and then fixing up the result would be faster.

The x86-64 code is also bad!
On intel cpu prior to haswell a simple:
	sum_64 += *ptr32++;
is faster than the current code.
(Although you can do a lot better even on ivy bridge.)

	David

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