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Message-ID: <20230525131043.GT83892@hirez.programming.kicks-ass.net>
Date: Thu, 25 May 2023 15:10:43 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
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Subject: Re: [PATCH v3 08/11] slub: Replace cmpxchg_double()
On Thu, May 25, 2023 at 12:52:06PM +0200, Arnd Bergmann wrote:
> On Thu, May 25, 2023, at 12:29, Peter Zijlstra wrote:
> > On Wed, May 24, 2023 at 11:32:47AM +0200, Peter Zijlstra wrote:
> >> On Mon, May 15, 2023 at 09:57:07AM +0200, Peter Zijlstra wrote:
> >
> > This then also means I need to look at this_cpu_cmpxchg128 and
> > this_cpu_cmoxchg64 behaviour when we dont have the CPUID feature.
> >
> > Because current verions seem to assume the instruction is present.
>
> As far as I could tell when reviewing your series, this_cpu_cmpxchg64()
> is always available on all architectures. Depending on compile-time
> feature detection this would be either a native instruction that
> is guaranteed to work, or the irq-disabled version. On x86, this
> is handled at runtime with alternative_io().
>
> this_cpu_cmpxchg128() clearly needed the system_has_cmpxchg128()
> check, same as system_has_cmpxchg_double() today.
So, having just dug through all that, on x86:
this_cpu_cmpxchg64() is:
X86_CMPXCHG64=n -> fallback, irrespective of CX8
X86_CMPXCHG64=y -> cmpxchg8b
X86_64 -> cmpxchg
I've changed it to be similar between 32bit and 64bit such that both:
cmpxchg#b when CX#, otherwise this_cpu_cmpxchg#b_emu
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