Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   David Laight <David.Laight@ACULAB.COM>
To:     'Peter Zijlstra' <peterz@infradead.org>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
CC:     Linus Torvalds <torvalds@linux-foundation.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Uros Bizjak <ubizjak@gmail.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "willy@infradead.org" <willy@infradead.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "aarcange@redhat.com" <aarcange@redhat.com>,
        "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
        "jroedel@suse.de" <jroedel@suse.de>
Subject: RE: [PATCH 11/13] x86_64: Remove pointless set_64bit() usage
Thread-Topic: [PATCH 11/13] x86_64: Remove pointless set_64bit() usage
Thread-Index: AQHY8Slas2UAHB8/vU20d4zgnfqWVa4zL9Vw
Date:   Mon, 7 Nov 2022 09:14:24 +0000
Message-ID: <d5ed7c4dca774e1a858ac19f5fde291f@AcuMS.aculab.com>
References: <20221022111403.531902164@infradead.org>
 <20221022114425.168036718@infradead.org>
 <Y2QR/BRHjjYUNszh@dev-arch.thelio-3990X>
 <CAFULd4bkn3i0ds1ywhxAZBQH+1O-zbPWscUqjoEcv4xvnxOnSw@mail.gmail.com>
 <Y2QYHZsZNs33NXZB@dev-arch.thelio-3990X>
 <CAHk-=wjCBOwSWec+=h08q3Gbr4UjSfX46GrQjzHZLFokziS7nA@mail.gmail.com>
 <Y2U3WdU61FvYlpUh@hirez.programming.kicks-ass.net>
 <CAHk-=wggJFQJmWtvsFVt69hzRXW3zD5+9q-1Laz=NoZQ8Fy9Ag@mail.gmail.com>
 <Y2ZlS3SHeAPOkVmN@zx2c4.com>
 <Y2Z9yzmtYtmYi5rx@hirez.programming.kicks-ass.net>
In-Reply-To: <Y2Z9yzmtYtmYi5rx@hirez.programming.kicks-ass.net>
Accept-Language: en-GB, en-US
MIME-Version: 1.0
Content-Language: en-US
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Precedence: bulk

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@infradead.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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)