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 14:30:10 +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

> 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.

Looking at some instruction timings ror32() and bswap32()
seem to need one of the same execution ports.
However on Intel cpus bswap64() takes 2 clocks but the ror64()
instructions only take 1.

AMD cpus are more symmetric and run all variant in 1 clock.

So ror32() is probably preferable in case anyone copies the
code into somewhere with 64bit checksum value.

	David

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