KVM: Optimize gfn lookup in kvm_zap_gfn_range()

Introduce a memslots gfn upper bound operation and use it to optimize
kvm_zap_gfn_range().
This way this handler can do a quick lookup for intersecting gfns and won't
have to do a linear scan of the whole memslot set.
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <ef242146a87a335ee93b441dcf01665cb847c902.1638817641.git.maciej.szmigiero@oracle.com>

arch/x86/kvm/mmu/mmu.c

@@ -5728,19 +5728,22 @@ static bool __kvm_zap_rmaps(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
 {
 	const struct kvm_memory_slot *memslot;
 	struct kvm_memslots *slots;
+	struct kvm_memslot_iter iter;
 	bool flush = false;
 	gfn_t start, end;
-	int i, bkt;
+	int i;
 
 	if (!kvm_memslots_have_rmaps(kvm))
 		return flush;
 
 	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
 		slots = __kvm_memslots(kvm, i);
-		kvm_for_each_memslot(memslot, bkt, slots) {
+
+		kvm_for_each_memslot_in_gfn_range(&iter, slots, gfn_start, gfn_end) {
+			memslot = iter.slot;
 			start = max(gfn_start, memslot->base_gfn);
 			end = min(gfn_end, memslot->base_gfn + memslot->npages);
-			if (start >= end)
+			if (WARN_ON_ONCE(start >= end))
 				continue;
 
 			flush = slot_handle_level_range(kvm, memslot, kvm_zap_rmapp,
@@ -5761,6 +5764,9 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
 	bool flush;
 	int i;
 
+	if (WARN_ON_ONCE(gfn_end <= gfn_start))
+		return;
+
 	write_lock(&kvm->mmu_lock);
 
 	kvm_inc_notifier_count(kvm, gfn_start, gfn_end);

include/linux/kvm_host.h

@@ -846,6 +846,100 @@ struct kvm_memory_slot *id_to_memslot(struct kvm_memslots *slots, int id)
 	return NULL;
 }
 
+/* Iterator used for walking memslots that overlap a gfn range. */
+struct kvm_memslot_iter {
+	struct kvm_memslots *slots;
+	struct rb_node *node;
+	struct kvm_memory_slot *slot;
+};
+
+static inline void kvm_memslot_iter_next(struct kvm_memslot_iter *iter)
+{
+	iter->node = rb_next(iter->node);
+	if (!iter->node)
+		return;
+
+	iter->slot = container_of(iter->node, struct kvm_memory_slot, gfn_node[iter->slots->node_idx]);
+}
+
+static inline void kvm_memslot_iter_start(struct kvm_memslot_iter *iter,
+					  struct kvm_memslots *slots,
+					  gfn_t start)
+{
+	int idx = slots->node_idx;
+	struct rb_node *tmp;
+	struct kvm_memory_slot *slot;
+
+	iter->slots = slots;
+
+	/*
+	 * Find the so called "upper bound" of a key - the first node that has
+	 * its key strictly greater than the searched one (the start gfn in our case).
+	 */
+	iter->node = NULL;
+	for (tmp = slots->gfn_tree.rb_node; tmp; ) {
+		slot = container_of(tmp, struct kvm_memory_slot, gfn_node[idx]);
+		if (start < slot->base_gfn) {
+			iter->node = tmp;
+			tmp = tmp->rb_left;
+		} else {
+			tmp = tmp->rb_right;
+		}
+	}
+
+	/*
+	 * Find the slot with the lowest gfn that can possibly intersect with
+	 * the range, so we'll ideally have slot start <= range start
+	 */
+	if (iter->node) {
+		/*
+		 * A NULL previous node means that the very first slot
+		 * already has a higher start gfn.
+		 * In this case slot start > range start.
+		 */
+		tmp = rb_prev(iter->node);
+		if (tmp)
+			iter->node = tmp;
+	} else {
+		/* a NULL node below means no slots */
+		iter->node = rb_last(&slots->gfn_tree);
+	}
+
+	if (iter->node) {
+		iter->slot = container_of(iter->node, struct kvm_memory_slot, gfn_node[idx]);
+
+		/*
+		 * It is possible in the slot start < range start case that the
+		 * found slot ends before or at range start (slot end <= range start)
+		 * and so it does not overlap the requested range.
+		 *
+		 * In such non-overlapping case the next slot (if it exists) will
+		 * already have slot start > range start, otherwise the logic above
+		 * would have found it instead of the current slot.
+		 */
+		if (iter->slot->base_gfn + iter->slot->npages <= start)
+			kvm_memslot_iter_next(iter);
+	}
+}
+
+static inline bool kvm_memslot_iter_is_valid(struct kvm_memslot_iter *iter, gfn_t end)
+{
+	if (!iter->node)
+		return false;
+
+	/*
+	 * If this slot starts beyond or at the end of the range so does
+	 * every next one
+	 */
+	return iter->slot->base_gfn < end;
+}
+
+/* Iterate over each memslot at least partially intersecting [start, end) range */
+#define kvm_for_each_memslot_in_gfn_range(iter, slots, start, end)	\
+	for (kvm_memslot_iter_start(iter, slots, start);		\
+	     kvm_memslot_iter_is_valid(iter, end);			\
+	     kvm_memslot_iter_next(iter))
+
 /*
  * KVM_SET_USER_MEMORY_REGION ioctl allows the following operations:
  * - create a new memory slot