KVM: x86: fix apic_accept_events vs check_nested_events

vmx_apic_init_signal_blocked is buggy in that it returns true
even in VMX non-root mode.  In non-root mode, however, INITs
are not latched, they just cause a vmexit.  Previously,
KVM was waiting for them to be processed when kvm_apic_accept_events
and in the meanwhile it ate the SIPIs that the processor received.

However, in order to implement the wait-for-SIPI activity state,
KVM will have to process KVM_APIC_SIPI in vmx_check_nested_events,
and it will not be possible anymore to disregard SIPIs in non-root
mode as the code is currently doing.

By calling kvm_x86_ops.nested_ops->check_events, we can force a vmexit
(with the side-effect of latching INITs) before incorrectly injecting
an INIT or SIPI in a guest, and therefore vmx_apic_init_signal_blocked
can do the right thing.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -2843,14 +2843,35 @@ void kvm_apic_accept_events(struct kvm_vcpu *vcpu)
 {
 	struct kvm_lapic *apic = vcpu->arch.apic;
 	u8 sipi_vector;
+	int r;
 	unsigned long pe;
 
-	if (!lapic_in_kernel(vcpu) || !apic->pending_events)
+	if (!lapic_in_kernel(vcpu))
+		return;
+
+	/*
+	 * Read pending events before calling the check_events
+	 * callback.
+	 */
+	pe = smp_load_acquire(&apic->pending_events);
+	if (!pe)
+		return;
+
+	if (is_guest_mode(vcpu)) {
+		r = kvm_x86_ops.nested_ops->check_events(vcpu);
+		if (r < 0)
 			return;
+		/*
+		 * If an event has happened and caused a vmexit,
+		 * we know INITs are latched and therefore
+		 * we will not incorrectly deliver an APIC
+		 * event instead of a vmexit.
+		 */
+	}
 
 	/*
 	 * INITs are latched while CPU is in specific states
-	 * (SMM, VMX non-root mode, SVM with GIF=0).
+	 * (SMM, VMX root mode, SVM with GIF=0).
 	 * Because a CPU cannot be in these states immediately
 	 * after it has processed an INIT signal (and thus in
 	 * KVM_MP_STATE_INIT_RECEIVED state), just eat SIPIs
@@ -2858,13 +2879,13 @@ void kvm_apic_accept_events(struct kvm_vcpu *vcpu)
 	 */
 	if (kvm_vcpu_latch_init(vcpu)) {
 		WARN_ON_ONCE(vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED);
-		if (test_bit(KVM_APIC_SIPI, &apic->pending_events))
+		if (test_bit(KVM_APIC_SIPI, &pe))
 			clear_bit(KVM_APIC_SIPI, &apic->pending_events);
 		return;
 	}
 
-	pe = xchg(&apic->pending_events, 0);
 	if (test_bit(KVM_APIC_INIT, &pe)) {
+		clear_bit(KVM_APIC_INIT, &apic->pending_events);
 		kvm_vcpu_reset(vcpu, true);
 		if (kvm_vcpu_is_bsp(apic->vcpu))
 			vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
@@ -2873,6 +2894,7 @@ void kvm_apic_accept_events(struct kvm_vcpu *vcpu)
 	}
 	if (test_bit(KVM_APIC_SIPI, &pe) &&
 	    vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {
+		clear_bit(KVM_APIC_SIPI, &apic->pending_events);
 		/* evaluate pending_events before reading the vector */
 		smp_rmb();
 		sipi_vector = apic->sipi_vector;

arch/x86/kvm/vmx/vmx.c

@@ -7558,7 +7558,7 @@ static void enable_smi_window(struct kvm_vcpu *vcpu)
 
 static bool vmx_apic_init_signal_blocked(struct kvm_vcpu *vcpu)
 {
-	return to_vmx(vcpu)->nested.vmxon;
+	return to_vmx(vcpu)->nested.vmxon && !is_guest_mode(vcpu);
 }
 
 static void vmx_migrate_timers(struct kvm_vcpu *vcpu)