KVM: x86: Skip APICv update if APICv is disable at the module level

Bail from the APICv update paths _before_ taking apicv_update_lock if
APICv is disabled at the module level.  kvm_request_apicv_update() in
particular is invoked from multiple paths that can be reached without
APICv being enabled, e.g. svm_enable_irq_window(), and taking the
rw_sem for write when APICv is disabled may introduce unnecessary
contention and stalls.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211208015236.1616697-25-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -112,6 +112,9 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
 	if (!!auto_eoi_old == !!auto_eoi_new)
 		return;
 
+	if (!enable_apicv)
+		return;
+
 	down_write(&vcpu->kvm->arch.apicv_update_lock);
 
 	if (auto_eoi_new)

arch/x86/kvm/x86.c

@@ -9715,6 +9715,9 @@ void __kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
 
 void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
 {
+	if (!enable_apicv)
+		return;
+
 	down_write(&kvm->arch.apicv_update_lock);
 	__kvm_request_apicv_update(kvm, activate, bit);
 	up_write(&kvm->arch.apicv_update_lock);