Re: [PATCH v2 0/7] irqchip/apple-aic: Add support for AICv2

From: Marc Zyngier <maz@kernel.org>
To: Hector Martin <marcan@marcan.st>, Mark Rutland <mark.rutland@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Rob Herring <robh+dt@kernel.org>, Sven Peter <sven@svenpeter.dev>,
	Alyssa Rosenzweig <alyssa@rosenzweig.io>,
	Mark Kettenis <mark.kettenis@xs4all.nl>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, devicetree@vger.kernel.org
Subject: Re: [PATCH v2 0/7] irqchip/apple-aic: Add support for AICv2
Date: Thu, 24 Feb 2022 19:06:12 +0000	[thread overview]
Message-ID: <87r17s2ifv.wl-maz@kernel.org> (raw)
In-Reply-To: <YhfN4YtR+Nq5vmVr@lakrids>

On Thu, 24 Feb 2022 18:26:41 +0000,
Mark Rutland <mark.rutland@arm.com> wrote:
> 
> On Thu, Feb 24, 2022 at 10:07:34PM +0900, Hector Martin wrote:
> > Hi folks,
> 
> Hi Hector,
> 
> > In the t6000/t6001 (M1 Pro / Max) SoCs, Apple introduced a new version
> > of their interrupt controller. This is a significant departure from
> > AICv1 and seems designed to better scale to larger chips. This series
> > adds support for it to the existing AIC driver.
> > 
> > Gone are CPU affinities; instead there seems to be some kind of
> > "automagic" dispatch to willing CPU cores, and cores can also opt-out
> > via an IMP-DEF sysreg (!). Right now the bootloader just sets up all
> > cores to accept IRQs, and we ignore all this and let the magic
> > algorithm pick a CPU to accept the IRQ.
> 
> Maybe that's ok for the set of peripherals attached, but in general that
> violates existing expectations regarding affinity, and I fear there'll
> be some subtle brokenness resulting from this automatic target
> selection.
> 
> For example, in the perf events subsystem there are PMU drivers (even
> those for "uncore" or "system" devices which are shared by many/all
> CPUs) which rely on a combination of interrupt affinity and local IRQ
> masking (without any other locking) to provide exclusion between a PMU's
> IRQ handler and any other management operations for that PMU (which are
> all handled from the same CPU).

It will definitely break anything that relies on managed interrupts,
where the kernel expects to allocate interrupts that have a strict
affinity. Drivers using this feature can legitimately expect that they
can keep their state in per-CPU pointers, and that obviously breaks.

This may affect any PCIe device with more than a couple of queues.
Maybe users of this HW do not care (yet), but we'll have to find a way
to tell drivers of the limitation.

	M.

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

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