KVM: Optimize overlapping memslots check

Do a quick lookup for possibly overlapping gfns when creating or moving
a memslot instead of performing a linear scan of the whole memslot set.
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
[sean: tweaked params to avoid churn in future cleanup]
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <a4795e5c2f624754e9c0aab023ebda1966feb3e1.1638817641.git.maciej.szmigiero@oracle.com>

virt/kvm/kvm_main.c

@@ -1815,6 +1815,19 @@ static int kvm_set_memslot(struct kvm *kvm,
 	return 0;
 }
 
+static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
+				      gfn_t start, gfn_t end)
+{
+	struct kvm_memslot_iter iter;
+
+	kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
+		if (iter.slot->id != id)
+			return true;
+	}
+
+	return false;
+}
+
 /*
  * Allocate some memory and give it an address in the guest physical address
  * space.
@@ -1826,8 +1839,9 @@ static int kvm_set_memslot(struct kvm *kvm,
 int __kvm_set_memory_region(struct kvm *kvm,
 			    const struct kvm_userspace_memory_region *mem)
 {
-	struct kvm_memory_slot *old, *tmp;
+	struct kvm_memory_slot *old;
 	struct kvm_memory_slot new;
+	struct kvm_memslots *slots;
 	enum kvm_mr_change change;
 	int as_id, id;
 	int r;
@@ -1856,11 +1870,13 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
 		return -EINVAL;
 
+	slots = __kvm_memslots(kvm, as_id);
+
 	/*
 	 * Note, the old memslot (and the pointer itself!) may be invalidated
 	 * and/or destroyed by kvm_set_memslot().
 	 */
-	old = id_to_memslot(__kvm_memslots(kvm, as_id), id);
+	old = id_to_memslot(slots, id);
 
 	if (!mem->memory_size) {
 		if (!old || !old->npages)
@@ -1909,18 +1925,10 @@ int __kvm_set_memory_region(struct kvm *kvm,
 			return 0;
 	}
 
-	if ((change == KVM_MR_CREATE) || (change == KVM_MR_MOVE)) {
-		int bkt;
-
-		/* Check for overlaps */
-		kvm_for_each_memslot(tmp, bkt, __kvm_memslots(kvm, as_id)) {
-			if (tmp->id == id)
-				continue;
-			if (!((new.base_gfn + new.npages <= tmp->base_gfn) ||
-			      (new.base_gfn >= tmp->base_gfn + tmp->npages)))
+	if ((change == KVM_MR_CREATE || change == KVM_MR_MOVE) &&
+	    kvm_check_memslot_overlap(slots, id, new.base_gfn,
+				      new.base_gfn + new.npages))
 		return -EEXIST;
-		}
-	}
 
 	return kvm_set_memslot(kvm, old, &new, change);
 }