Re: [PATCH] irqchip/gic-v3: Workaround for GIC-700 erratum 2941627

From: Marc Zyngier <maz@kernel.org>
To: Lorenzo Pieralisi <lpieralisi@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
	Hanks Chen <hanks.chen@mediatek.com>,
	Cheng-Yuh.Wu@mediatek.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] irqchip/gic-v3: Workaround for GIC-700 erratum 2941627
Date: Tue, 04 Jul 2023 16:23:10 +0100	[thread overview]
Message-ID: <86sfa3wvj5.wl-maz@kernel.org> (raw)
In-Reply-To: <ZKQ3O5yL7WuRga0N@lpieralisi>

On Tue, 04 Jul 2023 16:14:03 +0100,
Lorenzo Pieralisi <lpieralisi@kernel.org> wrote:
> 
> On Tue, Jul 04, 2023 at 03:44:50PM +0100, Marc Zyngier wrote:
> > Lorenzo,
> > 
> > On Tue, 04 Jul 2023 13:34:36 +0100,
> > Lorenzo Pieralisi <lpieralisi@kernel.org> wrote:
> > > 
> > > +static bool gic_enable_quirk_arm64_2941627(void *data)
> > > +{
> > > +	/*
> > > +	 * If CPUidle is not enabled the erratum runtime
> > > +	 * conditions can't be hit, since that requires:
> > > +	 *
> > > +	 * - A core entering a deep power state with
> > > +	 *   the associated GIC redistributor asleep
> > > +	 *   and an IRQ active and pending targeted at it
> > > +	 * - A different core handling the IRQ and
> > > +	 *   related GIC operations at the same time
> > > +	 */
> > > +	if (!IS_ENABLED(CONFIG_CPU_IDLE))
> > > +		return false;
> > 
> > Could this still hit on a system that traps WFI to EL3 and uses this
> > as a way to enter a low-power mode?
> 
> That's a valid point, I have not thought about that. If there are set-ups
> where this is allowed (and I think it *is* architecturally allowed with
> EL3 saving/restoring context and entering a deep power state - if you
> asked I suspect you have something concrete in mind :)) this "optimization"
> must be removed since we can still hit the bug; that's what I shall do
> for v2.

I do not have a concrete example of anyone doing that, but the fact
that it is possible by the letter of the architecture makes me think
that *someone* out there must be inventive enough to do it.

So I'd rather take the safe option and *always* enable the workaround.
And then someone else can rock up and explain why they don't need it.

Thanks,

	M.

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

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