KVM: fix kvm_vcpu_kick vs __vcpu_run race

There is a window open between testing of pending IRQ's
and assignment of guest_mode in __vcpu_run.

Injection of IRQ's can race with __vcpu_run as follows:

CPU0                                CPU1
kvm_x86_ops->run()
vcpu->guest_mode = 0                SET_IRQ_LINE ioctl
..
kvm_x86_ops->inject_pending_irq
kvm_cpu_has_interrupt()

                                    apic_test_and_set_irr()
                                    kvm_vcpu_kick
                                    if (vcpu->guest_mode)
                                        send_ipi()

vcpu->guest_mode = 1

So move guest_mode=1 assignment before ->inject_pending_irq, and make
sure that it won't reorder after it.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/x86.c

@@ -2802,6 +2802,13 @@ static int __vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 		goto out;
 	}
 
+	vcpu->guest_mode = 1;
+	/*
+	 * Make sure that guest_mode assignment won't happen after
+	 * testing the pending IRQ vector bitmap.
+	 */
+	smp_wmb();
+
 	if (vcpu->arch.exception.pending)
 		__queue_exception(vcpu);
 	else if (irqchip_in_kernel(vcpu->kvm))
@@ -2813,7 +2820,6 @@ static int __vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 
 	up_read(&vcpu->kvm->slots_lock);
 
-	vcpu->guest_mode = 1;
 	kvm_guest_enter();
 
 	if (vcpu->requests)
@@ -3970,11 +3976,17 @@ static void vcpu_kick_intr(void *info)
 void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
 {
 	int ipi_pcpu = vcpu->cpu;
+	int cpu = get_cpu();
 
 	if (waitqueue_active(&vcpu->wq)) {
 		wake_up_interruptible(&vcpu->wq);
 		++vcpu->stat.halt_wakeup;
 	}
-	if (vcpu->guest_mode)
+	/*
+	 * We may be called synchronously with irqs disabled in guest mode,
+	 * So need not to call smp_call_function_single() in that case.
+	 */
+	if (vcpu->guest_mode && vcpu->cpu != cpu)
 		smp_call_function_single(ipi_pcpu, vcpu_kick_intr, vcpu, 0, 0);
+	put_cpu();
 }