From: Al Viro > Sent: 22 July 2020 16:55
> To: David Laight <David.Laight@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; linux-
> arch@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum
>
> On Wed, Jul 22, 2020 at 03:22:45PM +0000, David Laight wrote:
>
> > > And the benefit of that would be...? It wouldn't be any simpler,
> > > it almost certainly would not even be a valid microoptimization
> > > (nevermind that this is an arch-independent code)...
> >
> > It ought to give a minor improvement because it saves the extra
> > csum_fold() when the checksum from a buffer is added to the
> > previous total.
> >
>
> Sigh... _WHAT_ csum_fold()?
>
> static inline __wsum
> csum_block_add(__wsum csum, __wsum csum2, int offset)
> {
> u32 sum = (__force u32)csum2;
>
> /* rotate sum to align it with a 16b boundary */
> if (offset & 1)
> sum = ror32(sum, 8);
>
> return csum_add(csum, (__force __wsum)sum);
> }
>
> David, do you *ever* bother to RTFS? I mean, competent supercilious twits
> are annoying, but at least with those you can generally assume that what
> they say makes sense and has some relation to reality. You, OTOH, keep
> spewing utter bollocks, without ever lowering yourself to checking if your
> guesses have anything to do with the reality. With supercilious twit part
> proudly on the display - you do speak with confidence, and the way you
> dispense the oh-so-valuable advice to everyone around...
Yes, I do look at the code.
I've actually spent a lot of time looking at the x86 checksum code.
I've posted a patch for a version that is about twice as fast as the
current one on a large range of x86 cpus.
Possibly I meant the 32bit reduction inside csum_add()
rather than what csum_fold() does.
Having worked on the internals of SYSV, NetBSD and Linux I probably
forget the exact names for a few things.
The brain can only hold so much information.
David
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