KVM: x86: Fix lost interrupt on irr_pending race

apic_find_highest_irr assumes irr_pending is set if any vector in APIC_IRR is
set.  If this assumption is broken and apicv is disabled, the injection of
interrupts may be deferred until another interrupt is delivered to the guest.
Ultimately, if no other interrupt should be injected to that vCPU, the pending
interrupt may be lost.

commit 56cc2406 ("KVM: nVMX: fix "acknowledge interrupt on exit" when APICv
is in use") changed the behavior of apic_clear_irr so irr_pending is cleared
after setting APIC_IRR vector. After this commit, if apic_set_irr and
apic_clear_irr run simultaneously, a race may occur, resulting in APIC_IRR
vector set, and irr_pending cleared. In the following example, assume a single
vector is set in IRR prior to calling apic_clear_irr:

apic_set_irr				apic_clear_irr
------------				--------------
apic->irr_pending = true;
					apic_clear_vector(...);
					vec = apic_search_irr(apic);
					// => vec == -1
apic_set_vector(...);
					apic->irr_pending = (vec != -1);
					// => apic->irr_pending == false

Nonetheless, it appears the race might even occur prior to this commit:

apic_set_irr				apic_clear_irr
------------				--------------
apic->irr_pending = true;
					apic->irr_pending = false;
					apic_clear_vector(...);
					if (apic_search_irr(apic) != -1)
						apic->irr_pending = true;
					// => apic->irr_pending == false
apic_set_vector(...);

Fixing this issue by:
1. Restoring the previous behavior of apic_clear_irr: clear irr_pending, call
   apic_clear_vector, and then if APIC_IRR is non-zero, set irr_pending.
2. On apic_set_irr: first call apic_set_vector, then set irr_pending.
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -341,8 +341,12 @@ EXPORT_SYMBOL_GPL(kvm_apic_update_irr);
 
 static inline void apic_set_irr(int vec, struct kvm_lapic *apic)
 {
-	apic->irr_pending = true;
 	apic_set_vector(vec, apic->regs + APIC_IRR);
+	/*
+	 * irr_pending must be true if any interrupt is pending; set it after
+	 * APIC_IRR to avoid race with apic_clear_irr
+	 */
+	apic->irr_pending = true;
 }
 
 static inline int apic_search_irr(struct kvm_lapic *apic)
@@ -374,13 +378,15 @@ static inline void apic_clear_irr(int vec, struct kvm_lapic *apic)
 
 	vcpu = apic->vcpu;
 
-	apic_clear_vector(vec, apic->regs + APIC_IRR);
-	if (unlikely(kvm_apic_vid_enabled(vcpu->kvm)))
+	if (unlikely(kvm_apic_vid_enabled(vcpu->kvm))) {
 		/* try to update RVI */
+		apic_clear_vector(vec, apic->regs + APIC_IRR);
 		kvm_make_request(KVM_REQ_EVENT, vcpu);
-	else {
-		vec = apic_search_irr(apic);
-		apic->irr_pending = (vec != -1);
+	} else {
+		apic->irr_pending = false;
+		apic_clear_vector(vec, apic->regs + APIC_IRR);
+		if (apic_search_irr(apic) != -1)
+			apic->irr_pending = true;
 	}
 }