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    SubjectRE: [PATCH v2 2/2] x86: don't unmask disabled irqs when migrating them
    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



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