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

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

On Fri, 6 May 2011, Tian, Kevin wrote:
> > 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. :-)

So can please someone point me to that particular incarnation of
nonsense and provide a reasonable explanation for this abuse?

What is the point of an interrupt, which is permanently disabled, has
a handler with a BUG() inside and an irqaction assigned ?

What's the purpose of this? Why is the irqaction there in the first
place? To be called by some other weird means than by the irq
handling code?

> > 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? :-)

No we can make the unmask conditional on !irqd_irq_disabled() because
that's not violating any of the semantics. The interrupt would be
masked anyway when it arrives and the handler code sees that it is
lazy disabled. I mean real handler code, not the Xen abomination.

The only valid reason why I'd apply that patch is that it avoids a
potential extra interrupt, but not to prevent screwed up handlers from
exploding.

Thanks,

        tglx

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