Re: [PATCH] irqchip/gic-v3-its: Don't try to move a disabled irq

[Marc Zyngier]

On 2020-06-02 21:54, Thomas Gleixner wrote:
> "Herrenschmidt, Benjamin" <benh@xxxxxxxxxx> 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...