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

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Date:   Sat, 5 Nov 2022 14:29:47 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
        willy@...radead.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        aarcange@...hat.com, kirill.shutemov@...ux.intel.com,
        jroedel@...e.de
Subject: Re: [PATCH 11/13] x86_64: Remove pointless set_64bit() usage

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:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/crypto/poly1305-donna64.c
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/crypto/curve25519-hacl64.c

I always thought it'd be nice to see that typedef alongside the others
in the shared kernel headers, but figured the requirement for 64-bit and
libgcc for some operations on some architectures made it a bit less
general purpose, so I never proposed it.

Jason