KVM: IOAPIC: only set remote_irr if interrupt was injected

There's a bug in the IOAPIC code for level-triggered interrupts. Its
relatively easy to trigger by sharing (virtio-blk + usbtablet was the
testcase, initially reported by Gerd von Egidy).

The "remote_irr" variable is used to indicate accepted but not yet acked
interrupts. Its cleared from the EOI handler.

Problem is that the EOI handler clears remote_irr unconditionally, even
if it reinjected another pending interrupt.

In that case, kvm_ioapic_set_irq() proceeds to ioapic_service() which
sets remote_irr even if it failed to inject (since the IRR was high due
to EOI reinjection).

Since the TMR bit has been cleared by the first EOI, the second one
fails to clear remote_irr.

End result is interrupt line dead.

Fix it by setting remote_irr only if a new pending interrupt has been
generated (and the TMR bit for vector in question set).
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

virt/kvm/ioapic.c

@@ -45,7 +45,7 @@
 #else
 #define ioapic_debug(fmt, arg...)
 #endif
-static void ioapic_deliver(struct kvm_ioapic *vioapic, int irq);
+static int ioapic_deliver(struct kvm_ioapic *vioapic, int irq);
 
 static unsigned long ioapic_read_indirect(struct kvm_ioapic *ioapic,
 					  unsigned long addr,
@@ -89,8 +89,8 @@ static void ioapic_service(struct kvm_ioapic *ioapic, unsigned int idx)
 	pent = &ioapic->redirtbl[idx];
 
 	if (!pent->fields.mask) {
-		ioapic_deliver(ioapic, idx);
-		if (pent->fields.trig_mode == IOAPIC_LEVEL_TRIG)
+		int injected = ioapic_deliver(ioapic, idx);
+		if (injected && pent->fields.trig_mode == IOAPIC_LEVEL_TRIG)
 			pent->fields.remote_irr = 1;
 	}
 	if (!pent->fields.trig_mode)
@@ -133,7 +133,7 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
 	}
 }
 
-static void ioapic_inj_irq(struct kvm_ioapic *ioapic,
+static int ioapic_inj_irq(struct kvm_ioapic *ioapic,
 			   struct kvm_vcpu *vcpu,
 			   u8 vector, u8 trig_mode, u8 delivery_mode)
 {
@@ -143,7 +143,7 @@ static void ioapic_inj_irq(struct kvm_ioapic *ioapic,
 	ASSERT((delivery_mode == IOAPIC_FIXED) ||
 	       (delivery_mode == IOAPIC_LOWEST_PRIORITY));
 
-	kvm_apic_set_irq(vcpu, vector, trig_mode);
+	return kvm_apic_set_irq(vcpu, vector, trig_mode);
 }
 
 static u32 ioapic_get_delivery_bitmask(struct kvm_ioapic *ioapic, u8 dest,
@@ -186,7 +186,7 @@ static u32 ioapic_get_delivery_bitmask(struct kvm_ioapic *ioapic, u8 dest,
 	return mask;
 }
 
-static void ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
+static int ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
 {
 	u8 dest = ioapic->redirtbl[irq].fields.dest_id;
 	u8 dest_mode = ioapic->redirtbl[irq].fields.dest_mode;
@@ -195,7 +195,7 @@ static void ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
 	u8 trig_mode = ioapic->redirtbl[irq].fields.trig_mode;
 	u32 deliver_bitmask;
 	struct kvm_vcpu *vcpu;
-	int vcpu_id;
+	int vcpu_id, r = 0;
 
 	ioapic_debug("dest=%x dest_mode=%x delivery_mode=%x "
 		     "vector=%x trig_mode=%x\n",
@@ -204,7 +204,7 @@ static void ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
 	deliver_bitmask = ioapic_get_delivery_bitmask(ioapic, dest, dest_mode);
 	if (!deliver_bitmask) {
 		ioapic_debug("no target on destination\n");
-		return;
+		return 0;
 	}
 
 	switch (delivery_mode) {
@@ -216,7 +216,7 @@ static void ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
 			vcpu = ioapic->kvm->vcpus[0];
 #endif
 		if (vcpu != NULL)
-			ioapic_inj_irq(ioapic, vcpu, vector,
+			r = ioapic_inj_irq(ioapic, vcpu, vector,
 				       trig_mode, delivery_mode);
 		else
 			ioapic_debug("null lowest prio vcpu: "
@@ -234,7 +234,7 @@ static void ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
 			deliver_bitmask &= ~(1 << vcpu_id);
 			vcpu = ioapic->kvm->vcpus[vcpu_id];
 			if (vcpu) {
-				ioapic_inj_irq(ioapic, vcpu, vector,
+				r = ioapic_inj_irq(ioapic, vcpu, vector,
 					       trig_mode, delivery_mode);
 			}
 		}
@@ -246,6 +246,7 @@ static void ioapic_deliver(struct kvm_ioapic *ioapic, int irq)
 		       delivery_mode);
 		break;
 	}
+	return r;
 }
 
 void kvm_ioapic_set_irq(struct kvm_ioapic *ioapic, int irq, int level)