KVM: x86/mmu: Skip calling mmu->sync_spte() when the spte is 0

Sync the spte only when the spte is set and avoid the indirect branch.
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Link: https://lore.kernel.org/r/20230216235321.735214-5-jiangshanlai@gmail.com
[sean: add wrapper instead of open coding each check]
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/mmu.c

@@ -1933,6 +1933,14 @@ static bool kvm_sync_page_check(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 	return true;
 }
 
+static int kvm_sync_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int i)
+{
+	if (!sp->spt[i])
+		return 0;
+
+	return vcpu->arch.mmu->sync_spte(vcpu, sp, i);
+}
+
 static int __kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 {
 	int flush = 0;
@@ -1942,7 +1950,7 @@ static int __kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 		return -1;
 
 	for (i = 0; i < SPTE_ENT_PER_PAGE; i++) {
-		int ret = vcpu->arch.mmu->sync_spte(vcpu, sp, i);
+		int ret = kvm_sync_spte(vcpu, sp, i);
 
 		if (ret < -1)
 			return -1;
@@ -5766,7 +5774,7 @@ static void __kvm_mmu_invalidate_addr(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu
 		struct kvm_mmu_page *sp = sptep_to_sp(iterator.sptep);
 
 		if (sp->unsync) {
-			int ret = mmu->sync_spte(vcpu, sp, iterator.index);
+			int ret = kvm_sync_spte(vcpu, sp, iterator.index);
 
 			if (ret < 0)
 				mmu_page_zap_pte(vcpu->kvm, sp, iterator.sptep, NULL);

arch/x86/kvm/mmu/paging_tmpl.h

@@ -892,7 +892,7 @@ static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
 	gpa_t pte_gpa;
 	gfn_t gfn;
 
-	if (!sp->spt[i])
+	if (WARN_ON_ONCE(!sp->spt[i]))
 		return 0;
 
 	first_pte_gpa = FNAME(get_level1_sp_gpa)(sp);