Commit 5224c88d authored Nov 11, 2014 by Paolo Bonzini

apic: fix incorrect handling of ExtINT interrupts wrt processor priority



This fixes another failure with ExtINT, demonstrated by QNX.  The failure
mode is as follows:
- IPI sent to cpu 0 (bit set in APIC irr)
- IPI accepted by cpu 0 (bit cleared in irr, set in isr)
- IPI sent to cpu 0 (bit set in both irr and isr)
- PIC interrupt sent to cpu 0

The PIC interrupt causes CPU_INTERRUPT_HARD to be set, but
apic_irq_pending observes that the highest pending APIC interrupt priority
(the IPI) is the same as the processor priority (since the IPI is still
being handled), so apic_get_interrupt returns a spurious interrupt rather
than the pending PIC interrupt. The result is an endless sequence of
spurious interrupts, since nothing will clear CPU_INTERRUPT_HARD.

Instead, ExtINT interrupts should have ignored the processor priority.
Calling apic_check_pic early in apic_get_interrupt ensures that
apic_deliver_pic_intr is called instead of delivering the spurious
interrupt.  apic_deliver_pic_intr then clears CPU_INTERRUPT_HARD if needed.

Reported-by: Richard Bilson <rbilson@qnx.com>
Tested-by: Richard Bilson <rbilson@qnx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent 8092cb71

hw/intc/apic.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -571,7 +571,10 @@ int apic_get_interrupt(DeviceState *dev)
		apic_sync_vapic(s, SYNC_FROM_VAPIC);
		intno = apic_irq_pending(s);

		if (intno == 0) {
		/* if there is an interrupt from the 8259, let the caller handle
		* that first since ExtINT interrupts ignore the priority.
		*/
		if (intno == 0 \|\| apic_check_pic(s)) {
		apic_sync_vapic(s, SYNC_TO_VAPIC);
		return -1;
		} else if (intno < 0) {
		@@ -582,9 +585,6 @@ int apic_get_interrupt(DeviceState *dev)
		apic_set_bit(s->isr, intno);
		apic_sync_vapic(s, SYNC_TO_VAPIC);

		/* re-inject if there is still a pending PIC interrupt */
		apic_check_pic(s);

		apic_update_irq(s);

		return intno;