KVM: Clear pv eoi pending bit only when it is set

merge pv_eoi_get_pending and pv_eoi_clr_pending into a single
function pv_eoi_test_and_clear_pending, which returns and clear
the value of the pending bit.

This makes it possible to clear the pending bit only if the guest
had set it, and otherwise skip the call to pv_eoi_put_user().
This can save up to 300 nsec on AMD EPYC processors.

Suggested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Message-Id: <1636026974-50555-2-git-send-email-lirongqing@baidu.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -673,15 +673,6 @@ static inline bool pv_eoi_enabled(struct kvm_vcpu *vcpu)
 	return vcpu->arch.pv_eoi.msr_val & KVM_MSR_ENABLED;
 }
 
-static bool pv_eoi_get_pending(struct kvm_vcpu *vcpu)
-{
-	u8 val;
-	if (pv_eoi_get_user(vcpu, &val) < 0)
-		return false;
-
-	return val & KVM_PV_EOI_ENABLED;
-}
-
 static void pv_eoi_set_pending(struct kvm_vcpu *vcpu)
 {
 	if (pv_eoi_put_user(vcpu, KVM_PV_EOI_ENABLED) < 0)
@@ -690,12 +681,26 @@ static void pv_eoi_set_pending(struct kvm_vcpu *vcpu)
 	__set_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);
 }
 
-static void pv_eoi_clr_pending(struct kvm_vcpu *vcpu)
+static bool pv_eoi_test_and_clr_pending(struct kvm_vcpu *vcpu)
 {
-	if (pv_eoi_put_user(vcpu, KVM_PV_EOI_DISABLED) < 0)
-		return;
+	u8 val;
+
+	if (pv_eoi_get_user(vcpu, &val) < 0)
+		return false;
+
+	val &= KVM_PV_EOI_ENABLED;
+
+	if (val && pv_eoi_put_user(vcpu, KVM_PV_EOI_DISABLED) < 0)
+		return false;
 
+	/*
+	 * Clear pending bit in any case: it will be set again on vmentry.
+	 * While this might not be ideal from performance point of view,
+	 * this makes sure pv eoi is only enabled when we know it's safe.
+	 */
 	__clear_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);
+
+	return val;
 }
 
 static int apic_has_interrupt_for_ppr(struct kvm_lapic *apic, u32 ppr)
@@ -2671,7 +2676,6 @@ void __kvm_migrate_apic_timer(struct kvm_vcpu *vcpu)
 static void apic_sync_pv_eoi_from_guest(struct kvm_vcpu *vcpu,
 					struct kvm_lapic *apic)
 {
-	bool pending;
 	int vector;
 	/*
 	 * PV EOI state is derived from KVM_APIC_PV_EOI_PENDING in host
@@ -2685,14 +2689,8 @@ static void apic_sync_pv_eoi_from_guest(struct kvm_vcpu *vcpu,
 	 * 	-> host enabled PV EOI, guest executed EOI.
 	 */
 	BUG_ON(!pv_eoi_enabled(vcpu));
-	pending = pv_eoi_get_pending(vcpu);
-	/*
-	 * Clear pending bit in any case: it will be set again on vmentry.
-	 * While this might not be ideal from performance point of view,
-	 * this makes sure pv eoi is only enabled when we know it's safe.
-	 */
-	pv_eoi_clr_pending(vcpu);
-	if (pending)
+
+	if (pv_eoi_test_and_clr_pending(vcpu))
 		return;
 	vector = apic_set_eoi(apic);
 	trace_kvm_pv_eoi(apic, vector);