KVM: x86: Cache total page count to avoid traversing the memslot array

There is no point in recalculating from scratch the total number of pages
in all memslots each time a memslot is created or deleted.

Just cache the value and update it accordingly on each such operation so
the code doesn't need to traverse the whole memslot array each time.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

arch/x86/include/asm/kvm_host.h

@@ -1025,6 +1025,7 @@ struct kvm_x86_msr_filter {
 #define APICV_INHIBIT_REASON_X2APIC	5
 
 struct kvm_arch {
+	u64 n_memslots_pages;
 	unsigned long n_used_mmu_pages;
 	unsigned long n_requested_mmu_pages;
 	unsigned long n_max_mmu_pages;
@@ -1552,7 +1553,6 @@ void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
 				   const struct kvm_memory_slot *memslot);
 void kvm_mmu_zap_all(struct kvm *kvm);
 void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, u64 gen);
-unsigned long kvm_mmu_calculate_default_mmu_pages(struct kvm *kvm);
 void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned long kvm_nr_mmu_pages);
 
 int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3);

arch/x86/kvm/mmu/mmu.c

@@ -6046,30 +6046,6 @@ int kvm_mmu_module_init(void)
 	return ret;
 }
 
-/*
- * Calculate mmu pages needed for kvm.
- */
-unsigned long kvm_mmu_calculate_default_mmu_pages(struct kvm *kvm)
-{
-	unsigned long nr_mmu_pages;
-	unsigned long nr_pages = 0;
-	struct kvm_memslots *slots;
-	struct kvm_memory_slot *memslot;
-	int i;
-
-	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
-		slots = __kvm_memslots(kvm, i);
-
-		kvm_for_each_memslot(memslot, slots)
-			nr_pages += memslot->npages;
-	}
-
-	nr_mmu_pages = nr_pages * KVM_PERMILLE_MMU_PAGES / 1000;
-	nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
-
-	return nr_mmu_pages;
-}
-
 void kvm_mmu_destroy(struct kvm_vcpu *vcpu)
 {
 	kvm_mmu_unload(vcpu);

arch/x86/kvm/x86.c

@@ -11588,9 +11588,21 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 				const struct kvm_memory_slot *new,
 				enum kvm_mr_change change)
 {
-	if (!kvm->arch.n_requested_mmu_pages)
-		kvm_mmu_change_mmu_pages(kvm,
-				kvm_mmu_calculate_default_mmu_pages(kvm));
+	if (change == KVM_MR_CREATE)
+		kvm->arch.n_memslots_pages += new->npages;
+	else if (change == KVM_MR_DELETE) {
+		WARN_ON(kvm->arch.n_memslots_pages < old->npages);
+		kvm->arch.n_memslots_pages -= old->npages;
+	}
+
+	if (!kvm->arch.n_requested_mmu_pages) {
+		unsigned long nr_mmu_pages;
+
+		nr_mmu_pages = kvm->arch.n_memslots_pages *
+			       KVM_PERMILLE_MMU_PAGES / 1000;
+		nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES);
+		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
+	}
 
 	kvm_mmu_slot_apply_flags(kvm, old, new, change);