KVM: x86: work around infinite loop in microcode when #AC is delivered

It was found that a guest can DoS a host by triggering an infinite
stream of "alignment check" (#AC) exceptions.  This causes the
microcode to enter an infinite loop where the core never receives
another interrupt.  The host kernel panics pretty quickly due to the
effects (CVE-2015-5307).

Signed-off-by: Eric Northup <digitaleric@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
static void wakeup_handler(void)
{
	struct kvm_vcpu *vcpu;
	int cpu = smp_processor_id();

	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
	list_for_each_entry(vcpu, &per_cpu(blocked_vcpu_on_cpu, cpu),
			blocked_vcpu_list) {
		struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);

		if (pi_test_on(pi_desc) == 1)
			kvm_vcpu_kick(vcpu);
	}
	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
}

static void wakeup_handler(void)
{
	struct kvm_vcpu *vcpu;
	int cpu = smp_processor_id();

	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
	list_for_each_entry(vcpu, &per_cpu(blocked_vcpu_on_cpu, cpu),
			blocked_vcpu_list) {
		struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);

		if (pi_test_on(pi_desc) == 1)
			kvm_vcpu_kick(vcpu);
	}
	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
}
