KVM: introduce a table to map slot id to index in memslots array

The operation of getting dirty log is frequent when framebuffer-based
displays are used(for example, Xwindow), so, we introduce a mapping table
to speed up id_to_memslot()
Signed-off-by: NXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

include/linux/kvm_host.h

@@ -239,6 +239,8 @@ struct kvm_irq_routing_table {};
 struct kvm_memslots {
 	u64 generation;
 	struct kvm_memory_slot memslots[KVM_MEM_SLOTS_NUM];
+	/* The mapping table from slot id to the index in memslots[]. */
+	int id_to_index[KVM_MEM_SLOTS_NUM];
 };
 
 struct kvm {
@@ -341,14 +343,13 @@ static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm)
 static inline struct kvm_memory_slot *
 id_to_memslot(struct kvm_memslots *slots, int id)
 {
-	int i;
+	int index = slots->id_to_index[id];
+	struct kvm_memory_slot *slot;
 
-	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
-		if (slots->memslots[i].id == id)
-			return &slots->memslots[i];
+	slot = &slots->memslots[index];
 
-	WARN_ON(1);
-	return NULL;
+	WARN_ON(slot->id != id);
+	return slot;
 }
 
 #define HPA_MSB ((sizeof(hpa_t) * 8) - 1)

virt/kvm/kvm_main.c

@@ -446,7 +446,7 @@ static void kvm_init_memslots_id(struct kvm *kvm)
 	struct kvm_memslots *slots = kvm->memslots;
 
 	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
-		slots->memslots[i].id = i;
+		slots->id_to_index[i] = slots->memslots[i].id = i;
 }
 
 static struct kvm *kvm_create_vm(void)
@@ -674,8 +674,13 @@ static int cmp_memslot(const void *slot1, const void *slot2)
  */
 static void sort_memslots(struct kvm_memslots *slots)
 {
+	int i;
+
 	sort(slots->memslots, KVM_MEM_SLOTS_NUM,
 	      sizeof(struct kvm_memory_slot), cmp_memslot, NULL);
+
+	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
+		slots->id_to_index[slots->memslots[i].id] = i;
 }
 
 void update_memslots(struct kvm_memslots *slots, struct kvm_memory_slot *new)