Re: [PATCH 5/7] arm64: Select ARCH_HAS_CPU_RELAX

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Mihai Carabas" <mihai.carabas@oracle.com>,
	"Joao Martins" <joao.m.martins@oracle.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Will Deacon" <will@kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	"Daniel Lezcano" <daniel.lezcano@linaro.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Petr Mladek" <pmladek@suse.com>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	"Sami Tolvanen" <samitolvanen@google.com>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Juerg Haefliger" <juerg.haefliger@canonical.com>,
	"Mickaël Salaün" <mic@digikod.net>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Ankur Arora" <ankur.a.arora@oracle.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	linux-pm@vger.kernel.org
Subject: Re: [PATCH 5/7] arm64: Select ARCH_HAS_CPU_RELAX
Date: Tue, 29 Aug 2023 15:49:44 +0100	[thread overview]
Message-ID: <ZO4FiJpLSFISkK10@shell.armlinux.org.uk> (raw)
In-Reply-To: <20230809134941.GN212435@hirez.programming.kicks-ass.net>

On Wed, Aug 09, 2023 at 03:49:41PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 09, 2023 at 02:39:39PM +0300, Mihai Carabas wrote:
> > From: Joao Martins <joao.m.martins@oracle.com>
> > 
> > cpu_relax() is necessary to allow cpuidle poll-state to be used,
> > so select it from ARM64 kconfig.
> > 
> > Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> > Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
> > ---
> >  arch/arm64/Kconfig | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 87ade6549790..7c47617b5722 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -105,6 +105,7 @@ config ARM64
> >  	select ARCH_WANT_LD_ORPHAN_WARN
> >  	select ARCH_WANTS_NO_INSTR
> >  	select ARCH_WANTS_THP_SWAP if ARM64_4K_PAGES
> > +	select ARCH_HAS_CPU_RELAX
> >  	select ARCH_HAS_UBSAN_SANITIZE_ALL
> >  	select ARM_AMBA
> >  	select ARM_ARCH_TIMER
> 
> Uh what ?! cpu_relax() is assumed present on all archs, no?

I think you have x86 to blame for that!

That symbol is used in drivers/acpi/processor_idle.c to setup stuff
for the cpuidle polling, and also by cpuidle's Makefile to build
poll_state.o

It isn't to do with the presence of cpu_relax() or not.

It probably ought to be renamed to CPUIDLE_CPU_RELAX which would
better describe its modern purpose.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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