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Date:   Wed, 03 Jun 2020 13:44:58 +0100
From:   Marc Zyngier <maz@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     "Herrenschmidt, Benjamin" <benh@amazon.com>,
        "Saidi, Ali" <alisaidi@amazon.com>, jason@lakedaemon.net,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        "Woodhouse, David" <dwmw@amazon.co.uk>,
        "Zilberman, Zeev" <zeev@amazon.com>,
        "Machulsky, Zorik" <zorik@amazon.com>
Subject: Re: [PATCH] irqchip/gic-v3-its: Don't try to move a disabled irq
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On 2020-06-02 21:54, Thomas Gleixner wrote:
> "Herrenschmidt, Benjamin" <benh@amazon.com> writes:
>> On Sun, 2020-05-31 at 12:09 +0100, Marc Zyngier wrote:
>>> > The semantic of activate/deactivate (which maps to started/shutdown
>>> > in the IRQ code) is that the HW resources for a given interrupt are
>>> > only committed when the interrupt is activated. Trying to perform
>>> > actions involving the HW on an interrupt that isn't active cannot be
>>> > guaranteed to take effect.
>>> >
>>> > I'd rather address it in the core code, by preventing set_affinity (and
>>> > potentially others) to take place when the interrupt is not in the
>>> > STARTED state. Userspace would get an error, which is perfectly
>>> > legitimate, and which it already has to deal with it for plenty of
>>> > other
>>> > reasons.
>> 
>> So I finally found time to dig a bit in there :) Code has changed a 
>> bit
>> since last I looked. But I have memories of the startup code messing
>> around with the affinity, and here it is. In irq_startup() :
>> 
>> 
>> 		switch (__irq_startup_managed(desc, aff, force)) {
>> 		case IRQ_STARTUP_NORMAL:
>> 			ret = __irq_startup(desc);
>> 			irq_setup_affinity(desc);
>> 			break;
>> 		case IRQ_STARTUP_MANAGED:
>> 			irq_do_set_affinity(d, aff, false);
>> 			ret = __irq_startup(desc);

Grump. Nice catch. In hindsight, this is obvious, as managed interrupts
may have been allocated to target CPUs that have been hot-plugged off.

>> 			break;
>> 		case IRQ_STARTUP_ABORT:
>> 			irqd_set_managed_shutdown(d);
>> 			return 0;
>> 
>> So we have two cases here. Normal and managed.
>> 
>> In the managed case, we set the affinity before startup. I feel like 
>> your
>> patch might break that or am I missing something ?
> 
> It will break stuff because the affinity is not stored in case that the
> interrupt is not started.
> 
> I think we can fix this in the core code but that needs more thought.
> __irq_can_set_affinity() is definitely the wrong place.

Indeed. I completely missed the above. Back to square one.

Thanks,

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