Re: [PATCH 0/3] arm64: Fix cpuidle with pseudo-NMI enabled

From: Marc Zyngier <maz@kernel.org>
To: Sudeep Holla <sudeep.holla@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Russell King <linux@armlinux.org.uk>,
	kernel-team@android.com
Subject: Re: [PATCH 0/3] arm64: Fix cpuidle with pseudo-NMI enabled
Date: Fri, 11 Jun 2021 09:24:09 +0100	[thread overview]
Message-ID: <87czssreqe.wl-maz@kernel.org> (raw)
In-Reply-To: <20210610174352.rfrovw4p5asgqar4@bogus>

Hi Sudeep,

On Thu, 10 Jun 2021 18:43:52 +0100,
Sudeep Holla <sudeep.holla@arm.com> wrote:
> 
> On Thu, Jun 10, 2021 at 05:28:23PM +0100, Lorenzo Pieralisi wrote:
> > On Tue, Jun 08, 2021 at 06:27:12PM +0100, Marc Zyngier wrote:
> > > It appears that although cpu_do_idle() is correctly dealing with the
> > > PMR/DAIF duality, the PSCI cpu-suspend code has been left unaware of
> > > it.
> > > 
> > > On a system that uses PSCI for idle (such as the Ampere Altra I have
> > > access to), the kernel dies as soon as it enters idle (interrupts are
> > > off at the GIC CPU interface level). Boo.
> > 
> 
> [...]
> 
> > I wonder whether this arm_cpuidle_{save,restore}_context() should
> > be moved into the gic_cpu_pm_notifier() itself - which would
> > solve also the PSCI suspend issue Sudeep raised - it would be
> > a bit ugly though (CPU PM notifiers are run in S2R and CPUidle
> 
> +1 if possible, I hadn't fully understood the issue to make this
> suggestion. But yes if possible, we must to honour the abstraction even
> though PSCI is the only user 😄.

See my reply to Lorenzo. Once we switch from PMR to DAIF masking, none
of the local interrupt control helpers work anymore. If we start
leaking this change of behaviour at a higher level in the stack, I
have no idea what happens anymore.

Which is why we perform such switching in very localised cases such as
exception, guest entry, and (oh surprise! ;-) cpu_do_idle().

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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