KVM: page track: introduce kvm_slot_page_track_{add,remove}_page

These two functions are the user APIs:
- kvm_slot_page_track_add_page(): add the page to the tracking pool
  after that later specified access on that page will be tracked

- kvm_slot_page_track_remove_page(): remove the page from the tracking
  pool, the specified access on the page is not tracked after the last
  user is gone

Both of these are called under the protection both of mmu-lock and
kvm->srcu or kvm->slots_lock
Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_page_track.h

@@ -10,4 +10,11 @@ void kvm_page_track_free_memslot(struct kvm_memory_slot *free,
 				 struct kvm_memory_slot *dont);
 int kvm_page_track_create_memslot(struct kvm_memory_slot *slot,
 				  unsigned long npages);
+
+void kvm_slot_page_track_add_page(struct kvm *kvm,
+				  struct kvm_memory_slot *slot, gfn_t gfn,
+				  enum kvm_page_track_mode mode);
+void kvm_slot_page_track_remove_page(struct kvm *kvm,
+				     struct kvm_memory_slot *slot, gfn_t gfn,
+				     enum kvm_page_track_mode mode);
 #endif

arch/x86/kvm/page_track.c

@@ -50,3 +50,88 @@ int kvm_page_track_create_memslot(struct kvm_memory_slot *slot,
 	kvm_page_track_free_memslot(slot, NULL);
 	return -ENOMEM;
 }
+
+static inline bool page_track_mode_is_valid(enum kvm_page_track_mode mode)
+{
+	if (mode < 0 || mode >= KVM_PAGE_TRACK_MAX)
+		return false;
+
+	return true;
+}
+
+static void update_gfn_track(struct kvm_memory_slot *slot, gfn_t gfn,
+			     enum kvm_page_track_mode mode, short count)
+{
+	int index, val;
+
+	index = gfn_to_index(gfn, slot->base_gfn, PT_PAGE_TABLE_LEVEL);
+
+	val = slot->arch.gfn_track[mode][index];
+
+	if (WARN_ON(val + count < 0 || val + count > USHRT_MAX))
+		return;
+
+	slot->arch.gfn_track[mode][index] += count;
+}
+
+/*
+ * add guest page to the tracking pool so that corresponding access on that
+ * page will be intercepted.
+ *
+ * It should be called under the protection both of mmu-lock and kvm->srcu
+ * or kvm->slots_lock.
+ *
+ * @kvm: the guest instance we are interested in.
+ * @slot: the @gfn belongs to.
+ * @gfn: the guest page.
+ * @mode: tracking mode, currently only write track is supported.
+ */
+void kvm_slot_page_track_add_page(struct kvm *kvm,
+				  struct kvm_memory_slot *slot, gfn_t gfn,
+				  enum kvm_page_track_mode mode)
+{
+
+	if (WARN_ON(!page_track_mode_is_valid(mode)))
+		return;
+
+	update_gfn_track(slot, gfn, mode, 1);
+
+	/*
+	 * new track stops large page mapping for the
+	 * tracked page.
+	 */
+	kvm_mmu_gfn_disallow_lpage(slot, gfn);
+
+	if (mode == KVM_PAGE_TRACK_WRITE)
+		if (kvm_mmu_slot_gfn_write_protect(kvm, slot, gfn))
+			kvm_flush_remote_tlbs(kvm);
+}
+
+/*
+ * remove the guest page from the tracking pool which stops the interception
+ * of corresponding access on that page. It is the opposed operation of
+ * kvm_slot_page_track_add_page().
+ *
+ * It should be called under the protection both of mmu-lock and kvm->srcu
+ * or kvm->slots_lock.
+ *
+ * @kvm: the guest instance we are interested in.
+ * @slot: the @gfn belongs to.
+ * @gfn: the guest page.
+ * @mode: tracking mode, currently only write track is supported.
+ */
+void kvm_slot_page_track_remove_page(struct kvm *kvm,
+				     struct kvm_memory_slot *slot, gfn_t gfn,
+				     enum kvm_page_track_mode mode)
+{
+	if (WARN_ON(!page_track_mode_is_valid(mode)))
+		return;
+
+	update_gfn_track(slot, gfn, mode, -1);
+
+	/*
+	 * allow large page mapping for the tracked page
+	 * after the tracker is gone.
+	 */
+	kvm_mmu_gfn_allow_lpage(slot, gfn);
+}