This patch fixes an obscure failure of the QNX kernel on QEMU x86 SMP. In QNX, all hardware interrupts come via the PIC, and are delivered by the cpu 0 LAPIC in ExtINT mode, while IPIs are delivered by the LAPIC in fixed mode.
This bug happens as follows: - cpu 0 masks a particular PIC interrupt - IPI sent to cpu 0 (CPU_INTERRUPT_HARD is set) - before the IPI is accepted, the masked interrupt line is asserted by the device Since the interrupt is masked, apic_deliver_pic_intr will clear CPU_INTERRUPT_HARD. The IPI will still be set in the APIC irr, but since CPU_INTERRUPT_HARD is not set the cpu will not notice. Depending on the scenario this can cause a system hang, i.e. if cpu 0 is expected to unmask the interrupt. In order to fix this, do a full check of the APIC before an EXTINT is acknowledged. This can result in clearing CPU_INTERRUPT_HARD, but can also result in delivering the lost IPI. Reported-by: Richard Bilson <rbil...@qnx.com> Tested-by: Richard Bilson <rbil...@qnx.com> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> --- hw/intc/apic.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/hw/intc/apic.c b/hw/intc/apic.c index 0653409..6ec5861 100644 --- a/hw/intc/apic.c +++ b/hw/intc/apic.c @@ -188,7 +188,7 @@ void apic_deliver_pic_intr(DeviceState *dev, int level) apic_reset_bit(s->irr, lvt & 0xff); /* fall through */ case APIC_DM_EXTINT: - cpu_reset_interrupt(CPU(s->cpu), CPU_INTERRUPT_HARD); + apic_update_irq(s); break; } } @@ -376,6 +376,8 @@ static void apic_update_irq(APICCommonState *s) cpu_interrupt(cpu, CPU_INTERRUPT_POLL); } else if (apic_irq_pending(s) > 0) { cpu_interrupt(cpu, CPU_INTERRUPT_HARD); + } else if (!apic_accept_pic_intr(&s->busdev.qdev) || !pic_get_output(isa_pic)) { + cpu_reset_interrupt(cpu, CPU_INTERRUPT_HARD); } } -- 1.8.3.1