[Qemu-devel] [PULL 2/3] apic: fix loss of IPI due to masked ExtINT

[Paolo Bonzini <pbonzini@redhat.com>]

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