KVM: x86: drop KVM_PV_REASON_PAGE_READY case from kvm_handle_page_fault()

KVM guest code in Linux enables APF only when KVM_FEATURE_ASYNC_PF_INT
is supported, this means we will never see KVM_PV_REASON_PAGE_READY
when handling page fault vmexit in KVM.

While on it, make sure we only follow genuine page fault path when
APF reason is zero. If we happen to see something else this means
that the underlying hypervisor is misbehaving. Leave WARN_ON_ONCE()
to catch that.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -4156,6 +4156,7 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
 				u64 fault_address, char *insn, int insn_len)
 {
 	int r = 1;
+	u32 flags = vcpu->arch.apf.host_apf_flags;
 
 #ifndef CONFIG_X86_64
 	/* A 64-bit CR2 should be impossible on 32-bit KVM. */
@@ -4164,28 +4165,22 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
 #endif
 
 	vcpu->arch.l1tf_flush_l1d = true;
-	switch (vcpu->arch.apf.host_apf_flags) {
-	default:
+	if (!flags) {
 		trace_kvm_page_fault(fault_address, error_code);
 
 		if (kvm_event_needs_reinjection(vcpu))
 			kvm_mmu_unprotect_page_virt(vcpu, fault_address);
 		r = kvm_mmu_page_fault(vcpu, fault_address, error_code, insn,
 				insn_len);
-		break;
-	case KVM_PV_REASON_PAGE_NOT_PRESENT:
+	} else if (flags & KVM_PV_REASON_PAGE_NOT_PRESENT) {
 		vcpu->arch.apf.host_apf_flags = 0;
 		local_irq_disable();
 		kvm_async_pf_task_wait_schedule(fault_address);
 		local_irq_enable();
-		break;
-	case KVM_PV_REASON_PAGE_READY:
-		vcpu->arch.apf.host_apf_flags = 0;
-		local_irq_disable();
-		kvm_async_pf_task_wake(fault_address);
-		local_irq_enable();
-		break;
+	} else {
+		WARN_ONCE(1, "Unexpected host async PF flags: %x\n", flags);
 	}
+
 	return r;
 }
 EXPORT_SYMBOL_GPL(kvm_handle_page_fault);