[Xen-devel] RE: [PATCH v2 2/2] x86: don't unmask disabled irqs when migrating them

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Thomas Gleixner
> Sent: Friday, May 06, 2011 6:00 PM
> 
> On Fri, 6 May 2011, Tian, Kevin wrote:
> > x86: don't unmask disabled irqs when migrating them
> >
> > it doesn't make sense to mask/unmask a disabled irq when migrating it
> > from offlined cpu to another, because it's not expected to handle any
> > instance of it. Current mask/set_affinity/unmask steps may trigger
> > unexpected instance on disabled irq which then simply bug on when
> > there is no handler for it. One failing example is observed in Xen.
> > Xen pvops
> 
> So there is no handler, why the heck is there an irq action?
> 
>         if (!irq_has_action(irq) ....
>               continue;
> 
> Should have caught an uninitialized interrupt. If Xen abuses interrupts that 
> way,
> then it rightfully explodes. And we do not fix it by magic somewhere else.

sorry that my bad description here. there does be a dummy handler registered
on such irqs which simply throws out a BUG_ON when hit. I should just say such 
injection is not expected instead of no handler. :-)

> 
> > guest marks a special type of irqs as disabled, which are simply used
> 
> As I explained before several times, IRQF_DISABLED has absolutely nothing to
> do with it and pvops _CANNOT_ mark an interrupt disabled.

I have to admit that I need more study about whole interrupt sub-system, to 
better
understand your explanation here. Also here again my description is not accurate
enough. I meant that Xen pvops request the special irq with below flags:
        IRQF_DISABLED|IRQF_PERCPU|IRQF_NOBALANCING
and then later explicitly disable it with disable_irq(). As you said that 
IRQF_DISABLED
itself has nothing to do with it, and it's the later disable_irq() which takes 
real 
effect because Xen event chip hooks this callback to mask the irq from the chip 
level.

> 
> >
> >             chip = irq_data_get_irq_chip(data);
> > -           if (!irqd_can_move_in_process_context(data) && chip->irq_mask)
> > +           do_mask = !irqd_irq_disabled(data) &&
> > +                   !irqd_can_move_in_process_context(data) && 
> > chip->irq_mask;
> > +           if (do_mask)
> >                     chip->irq_mask(data);
> 
> This is completely wrong. irqd_irq_disabled() is a status information which 
> does
> not tell you whether the interrupt is actually masked at the hardware level
> because we do lazy interrupt hardware masking. So your change would keep
> the line unmasked at the hardware level for all interrupts which are in the 
> lazy
> disabled state.

Got it.

> 
> The only conditional which is interesting is the unmask path and that's a 
> simple
> optimization and not a correctness problem.
> 

So what's your suggestion based on my updated information? Is there any
interface I may take to differentiate above exception with normal case? 
Basically
in Xen usage we want such irqs permanently disabled at the chip level. Or
could we only do mask/unmask for irqs which are unmasked atm if as you said
it's just an optimization step? :-)
 

Thanks
Kevin


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