Re: [PATCH v4 08/20] irqchip/gic-v4.1: Plumb get/set_irqchip_state SGI callbacks

From: Marc Zyngier <maz@kernel.org>
To: Zenghui Yu <yuzenghui@huawei.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Jason Cooper <jason@lakedaemon.net>,
	kvm@vger.kernel.org, Suzuki K Poulose <suzuki.poulose@arm.com>,
	linux-kernel@vger.kernel.org, Eric Auger <eric.auger@redhat.com>,
	Robert Richter <rrichter@marvell.com>,
	James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 08/20] irqchip/gic-v4.1: Plumb get/set_irqchip_state SGI callbacks
Date: Sun, 01 Mar 2020 19:00:51 +0000	[thread overview]
Message-ID: <54c52057161f925c818446953050c951@kernel.org> (raw)
In-Reply-To: <dd9f1224b3b21ad793862406bd8855ba@kernel.org>

On 2020-02-28 19:37, Marc Zyngier wrote:
> On 2020-02-20 03:11, Zenghui Yu wrote:

>> Do we really need to grab the vpe_lock for those which are belong to
>> the same irqchip with its_vpe_set_affinity()? The IRQ core code should
>> already ensure the mutual exclusion among them, wrong?
> 
> I've been trying to think about that, but jet-lag keeps getting in the 
> way.
> I empirically think that you are right, but I need to go and check the 
> various
> code paths to be sure. Hopefully I'll have a bit more brain space next 
> week.

So I slept on it and came back to my senses. The only case we actually 
need
to deal with is when an affinity change impacts *another* interrupt.

There is only two instances of this issue:

- vLPIs have their *physical* affinity impacted by the affinity of the
   vPE. Their virtual affinity is of course unchanged, but the physical
   one becomes important with direct invalidation. Taking a per-VPE lock
   in such context should address the issue.

- vSGIs have the exact same issue, plus the matter of requiring some
   *extra* one when reading the pending state, which requires a RMW
   on two different registers. This requires an extra per-RD lock.

My original patch was stupidly complex, and the irq_desc lock is
perfectly enough to deal with anything that only affects the interrupt
state itself.

GICv4 + direct invalidation for vLPIs breaks this by bypassing the
serialization initially provided by the ITS, as the RD is completely
out of band. The per-vPE lock brings back this serialization.

I've updated the branch, which seems to run OK on D05. I still need
to run the usual tests on the FVP model though.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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