On Wed, 2018-10-10 at 14:30 +0200, Thomas Gleixner wrote:
> On Wed, 10 Oct 2018, David Woodhouse wrote:
>
> > On Mon, 2018-10-01 at 09:16 +0200, Juergen Gross wrote:
> > > - /* If irq pending already clear it and return. */
> > > + /* Guard against reentry. */
> > > + local_irq_save(flags);
> > > +
> > > + /* If irq pending already clear it. */
> > > if (xen_test_irq_pending(irq)) {
> > > xen_clear_irq_pending(irq);
> > > - return;
> > > + } else if (READ_ONCE(*byte) == val) {
> > > + /* Block until irq becomes pending (or a spurious wakeup) */
> > > + xen_poll_irq(irq);
> > > }
> >
> >
> > Does this still allow other IRQs to wake it from xen_poll_irq()?
> >
> > In the case where process-context code is spinning for a lock without
> > disabling interrupts, we *should* allow interrupts to occur still...
> > does this?
>
> Yes. Look at it like idle HLT or WFI. You have to disable interrupt before
> checking the condition and then the hardware or in this case the hypervisor
> has to bring you back when an interrupt is raised.
>
> If that would not work then the check would be racy, because the interrupt
> could hit and be handled after the check and before going into
> HLT/WFI/hypercall and then the thing is out until the next interrupt comes
> along, which might be never.
Right, but in this case we're calling into the hypervisor to poll for
one *specific* IRQ. Everything you say is true for that specific IRQ.
My question is what happens to *other* IRQs. We want them, but are they
masked? I'm staring at the Xen do_poll() code and haven't quite worked
that out...
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