linux-kernel - Re: [PATCH 15/15] x86/atomics: Remove !CONFIG_X86

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Message-ID: <aA3pj0yVIizZPRxT@gmail.com>
Date: Sun, 27 Apr 2025 10:23:43 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Brian Gerst <brgerst@...il.com>
Cc: Uros Bizjak <ubizjak@...il.com>, linux-kernel@...r.kernel.org,
	"Ahmed S . Darwish" <darwi@...utronix.de>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	John Ogness <john.ogness@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 15/15] x86/atomics: Remove !CONFIG_X86_CX8 methods


* Brian Gerst <brgerst@...il.com> wrote:

> On Fri, Apr 25, 2025 at 10:30 AM Uros Bizjak <ubizjak@...il.com> wrote:
> >
> >
> >
> > On 25. 04. 25 10:42, Ingo Molnar wrote:
> >
> > > -#endif
> > > +#define arch_cmpxchg64                       __cmpxchg64
> > > +#define arch_cmpxchg64_local         __cmpxchg64_local
> > > +#define arch_try_cmpxchg64           __try_cmpxchg64
> > > +#define arch_try_cmpxchg64_local     __try_cmpxchg64_local
> > >
> > >   #define system_has_cmpxchg64()              boot_cpu_has(X86_FEATURE_CX8)
> >
> > #define system_has_cmpxchg64()          1
> 
> Or just outright removed, since x86 was the only arch that defined it.

No, it cannot be removed, x86-32 *is* still the only 32-bit arch that 
enables it.

I.e. defining it to 1 is the correct solution, and that unconditionally 
enables the SLUB code optimizations. It depended on a runtime 
boot_cpu_has() check before.

Removing it would remove the optimization from x86-32 for no good 
reason.

Thanks,

	Ingo