KVM: Route irq 0 to vcpu 0 exclusively

Some Linux versions allow the timer interrupt to be processed by more than
one cpu, leading to hangs due to tsc instability.  Work around the issue
by only disaptching the interrupt to vcpu 0.

Problem analyzed (and patch tested) by Sheng Yang.
Signed-off-by: Avi Kivity <avi@qumranet.com>

virt/kvm/ioapic.c

@@ -211,6 +211,10 @@ static void ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
 	case IOAPIC_LOWEST_PRIORITY:
 		vcpu = kvm_get_lowest_prio_vcpu(ioapic->kvm, vector,
 				deliver_bitmask);
+#ifdef CONFIG_X86
+		if (irq == 0)
+			vcpu = ioapic->kvm->vcpus[0];
+#endif
 		if (vcpu != NULL)
 			ioapic_inj_irq(ioapic, vcpu, vector,
 				       trig_mode, delivery_mode);
@@ -220,6 +224,10 @@ static void ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
 				     deliver_bitmask, vector, IOAPIC_LOWEST_PRIORITY);
 		break;
 	case IOAPIC_FIXED:
+#ifdef CONFIG_X86
+		if (irq == 0)
+			deliver_bitmask = 1;
+#endif
 		for (vcpu_id = 0; deliver_bitmask != 0; vcpu_id++) {
 			if (!(deliver_bitmask & (1 << vcpu_id)))
 				continue;