On 10/10/2018 14:47, Thomas Gleixner wrote:
On Wed, 10 Oct 2018, David Woodhouse wrote:
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...
Ah, sorry. That of course has to come back like HLT/WFI for any interrupt, but I have no idea what the Xen HV is doing there.
The Xen HV is doing it right. It is blocking the vcpu in do_poll() and any interrupt will unblock it.
Juergen