linux-kernel - RE: [PATCH 11/13] x86_64: Remove pointless set

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Message-ID: <d5ed7c4dca774e1a858ac19f5fde291f@AcuMS.aculab.com>
Date:   Mon, 7 Nov 2022 09:14:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Uros Bizjak <ubizjak@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "jroedel@...e.de" <jroedel@...e.de>
Subject: RE: [PATCH 11/13] x86_64: Remove pointless set_64bit() usage

From: Peter Zijlstra
> Sent: 05 November 2022 15:14
> 
> On Sat, Nov 05, 2022 at 02:29:47PM +0100, Jason A. Donenfeld wrote:
> > On Fri, Nov 04, 2022 at 10:15:08AM -0700, Linus Torvalds wrote:
> > > On Fri, Nov 4, 2022 at 9:01 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > > >
> > > > So cmpxchg_double() does a cmpxchg on a double long value and is
> > > > currently supported by: i386, x86_64, arm64 and s390.
> > > >
> > > > On all those, except i386, two longs are u128.
> > > >
> > > > So how about we introduce u128 and cmpxchg128 -- then it directly
> > > > mirrors the u64 and cmpxchg64 usage we already have. It then also
> > > > naturally imposses the alignment thing.
> > >
> > > Ack, except that we might have some "u128" users that do *not*
> > > necessarily want any alignment thing.
> > >
> > > But maybe we could at least start with an u128 type that is marked as
> > > being fully aligned, and if some other user comes in down the line
> > > that wants relaxed alignment we can call it "u128_unaligned" or
> > > something.
> >
> > Hm, sounds maybe not so nice for another use case: arithmetic code that
> > makes use of u128 for efficient computations, but otherwise has
> > no particular alignment requirements. For example, `typedef __uint128_t
> > u128;` in:
> 
> Natural alignment is... natural. Making it unaligned is quite mad. That
> whole u64 is not naturally aligned on i386 thing Linus referred to is a
> sodding pain in the backside.
> 
> If the code has no alignment requirements, natural alignment is as good
> as any. And if it does have requirements, you can use u128_unaligned.
> 
> Also:
> 
> $ ./align
> 16, 16
> 
> ---
> 
> #include <stdio.h>
> 
> int main(int argx, char **argv)
> {
> 	__int128 a;
> 
> 	printf("%d, %d\n", sizeof(a), __alignof(a));
> 	return 0;
> }

Well, __alignof() doesn't return the required value.
(cf 'long long' on 32bit x86).
But the alignment of __int128 is 16 :-)

	David

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