KVM: Terminate memslot walks via used_slots

Refactor memslot handling to treat the number of used slots as the de
facto size of the memslot array, e.g. return NULL from id_to_memslot()
when an invalid index is provided instead of relying on npages==0 to
detect an invalid memslot.  Rework the sorting and walking of memslots
in advance of dynamically sizing memslots to aid bisection and debug,
e.g. with luck, a bug in the refactoring will bisect here and/or hit a
WARN instead of randomly corrupting memory.

Alternatively, a global null/invalid memslot could be returned, i.e. so
callers of id_to_memslot() don't have to explicitly check for a NULL
memslot, but that approach runs the risk of introducing difficult-to-
debug issues, e.g. if the global null slot is modified.  Constifying
the return from id_to_memslot() to combat such issues is possible, but
would require a massive refactoring of arch specific code and would
still be susceptible to casting shenanigans.

Add function comments to update_memslots() and search_memslots() to
explicitly (and loudly) state how memslots are sorted.

Opportunistically stuff @hva with a non-canonical value when deleting a
private memslot on x86 to detect bogus usage of the freed slot.

No functional change intended.

Tested-by: Christoffer Dall <christoffer.dall@arm.com>
Tested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 158 insertions, 52 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c115b78e85c3..62e45c64e443 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -566,7 +566,7 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
 		return NULL;
 
 	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
-		slots->id_to_index[i] = slots->memslots[i].id = i;
+		slots->id_to_index[i] = slots->memslots[i].id = -1;
 
 	return slots;
 }
@@ -870,63 +870,162 @@ static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
 }
 
 /*
- * Insert memslot and re-sort memslots based on their GFN,
- * so binary search could be used to lookup GFN.
- * Sorting algorithm takes advantage of having initially
- * sorted array and known changed memslot position.
+ * Delete a memslot by decrementing the number of used slots and shifting all
+ * other entries in the array forward one spot.
  */
-static void update_memslots(struct kvm_memslots *slots,
-			    struct kvm_memory_slot *new,
-			    enum kvm_mr_change change)
+static inline void kvm_memslot_delete(struct kvm_memslots *slots,
+				      struct kvm_memory_slot *memslot)
 {
-	int id = new->id;
-	int i = slots->id_to_index[id];
 	struct kvm_memory_slot *mslots = slots->memslots;
+	int i;
 
-	WARN_ON(mslots[i].id != id);
-	switch (change) {
-	case KVM_MR_CREATE:
-		slots->used_slots++;
-		WARN_ON(mslots[i].npages || !new->npages);
-		break;
-	case KVM_MR_DELETE:
-		slots->used_slots--;
-		WARN_ON(new->npages || !mslots[i].npages);
-		break;
-	default:
-		break;
-	}
+	if (WARN_ON(slots->id_to_index[memslot->id] == -1))
+		return;
 
-	while (i < KVM_MEM_SLOTS_NUM - 1 &&
-	       new->base_gfn <= mslots[i + 1].base_gfn) {
-		if (!mslots[i + 1].npages)
-			break;
+	slots->used_slots--;
+
+	for (i = slots->id_to_index[memslot->id]; i < slots->used_slots; i++) {
 		mslots[i] = mslots[i + 1];
 		slots->id_to_index[mslots[i].id] = i;
-		i++;
 	}
+	mslots[i] = *memslot;
+	slots->id_to_index[memslot->id] = -1;
+}
+
+/*
+ * "Insert" a new memslot by incrementing the number of used slots.  Returns
+ * the new slot's initial index into the memslots array.
+ */
+static inline int kvm_memslot_insert_back(struct kvm_memslots *slots)
+{
+	return slots->used_slots++;
+}
+
+/*
+ * Move a changed memslot backwards in the array by shifting existing slots
+ * with a higher GFN toward the front of the array.  Note, the changed memslot
+ * itself is not preserved in the array, i.e. not swapped at this time, only
+ * its new index into the array is tracked.  Returns the changed memslot's
+ * current index into the memslots array.
+ */
+static inline int kvm_memslot_move_backward(struct kvm_memslots *slots,
+					    struct kvm_memory_slot *memslot)
+{
+	struct kvm_memory_slot *mslots = slots->memslots;
+	int i;
+
+	if (WARN_ON_ONCE(slots->id_to_index[memslot->id] == -1) ||
+	    WARN_ON_ONCE(!slots->used_slots))
+		return -1;
 
 	/*
-	 * The ">=" is needed when creating a slot with base_gfn == 0,
-	 * so that it moves before all those with base_gfn == npages == 0.
-	 *
-	 * On the other hand, if new->npages is zero, the above loop has
-	 * already left i pointing to the beginning of the empty part of
-	 * mslots, and the ">=" would move the hole backwards in this
-	 * case---which is wrong.  So skip the loop when deleting a slot.
+	 * Move the target memslot backward in the array by shifting existing
+	 * memslots with a higher GFN (than the target memslot) towards the
+	 * front of the array.
 	 */
-	if (new->npages) {
-		while (i > 0 &&
-		       new->base_gfn >= mslots[i - 1].base_gfn) {
-			mslots[i] = mslots[i - 1];
-			slots->id_to_index[mslots[i].id] = i;
-			i--;
-		}
-	} else
-		WARN_ON_ONCE(i != slots->used_slots);
+	for (i = slots->id_to_index[memslot->id]; i < slots->used_slots - 1; i++) {
+		if (memslot->base_gfn > mslots[i + 1].base_gfn)
+			break;
+
+		WARN_ON_ONCE(memslot->base_gfn == mslots[i + 1].base_gfn);
+
+		/* Shift the next memslot forward one and update its index. */
+		mslots[i] = mslots[i + 1];
+		slots->id_to_index[mslots[i].id] = i;
+	}
+	return i;
+}
+
+/*
+ * Move a changed memslot forwards in the array by shifting existing slots with
+ * a lower GFN toward the back of the array.  Note, the changed memslot itself
+ * is not preserved in the array, i.e. not swapped at this time, only its new
+ * index into the array is tracked.  Returns the changed memslot's final index
+ * into the memslots array.
+ */
+static inline int kvm_memslot_move_forward(struct kvm_memslots *slots,
+					   struct kvm_memory_slot *memslot,
+					   int start)
+{
+	struct kvm_memory_slot *mslots = slots->memslots;
+	int i;
+
+	for (i = start; i > 0; i--) {
+		if (memslot->base_gfn < mslots[i - 1].base_gfn)
+			break;
+
+		WARN_ON_ONCE(memslot->base_gfn == mslots[i - 1].base_gfn);
 
-	mslots[i] = *new;
-	slots->id_to_index[mslots[i].id] = i;
+		/* Shift the next memslot back one and update its index. */
+		mslots[i] = mslots[i - 1];
+		slots->id_to_index[mslots[i].id] = i;
+	}
+	return i;
+}
+
+/*
+ * Re-sort memslots based on their GFN to account for an added, deleted, or
+ * moved memslot.  Sorting memslots by GFN allows using a binary search during
+ * memslot lookup.
+ *
+ * IMPORTANT: Slots are sorted from highest GFN to lowest GFN!  I.e. the entry
+ * at memslots[0] has the highest GFN.
+ *
+ * The sorting algorithm takes advantage of having initially sorted memslots
+ * and knowing the position of the changed memslot.  Sorting is also optimized
+ * by not swapping the updated memslot and instead only shifting other memslots
+ * and tracking the new index for the update memslot.  Only once its final
+ * index is known is the updated memslot copied into its position in the array.
+ *
+ *  - When deleting a memslot, the deleted memslot simply needs to be moved to
+ *    the end of the array.
+ *
+ *  - When creating a memslot, the algorithm "inserts" the new memslot at the
+ *    end of the array and then it forward to its correct location.
+ *
+ *  - When moving a memslot, the algorithm first moves the updated memslot
+ *    backward to handle the scenario where the memslot's GFN was changed to a
+ *    lower value.  update_memslots() then falls through and runs the same flow
+ *    as creating a memslot to move the memslot forward to handle the scenario
+ *    where its GFN was changed to a higher value.
+ *
+ * Note, slots are sorted from highest->lowest instead of lowest->highest for
+ * historical reasons.  Originally, invalid memslots where denoted by having
+ * GFN=0, thus sorting from highest->lowest naturally sorted invalid memslots
+ * to the end of the array.  The current algorithm uses dedicated logic to
+ * delete a memslot and thus does not rely on invalid memslots having GFN=0.
+ *
+ * The other historical motiviation for highest->lowest was to improve the
+ * performance of memslot lookup.  KVM originally used a linear search starting
+ * at memslots[0].  On x86, the largest memslot usually has one of the highest,
+ * if not *the* highest, GFN, as the bulk of the guest's RAM is located in a
+ * single memslot above the 4gb boundary.  As the largest memslot is also the
+ * most likely to be referenced, sorting it to the front of the array was
+ * advantageous.  The current binary search starts from the middle of the array
+ * and uses an LRU pointer to improve performance for all memslots and GFNs.
+ */
+static void update_memslots(struct kvm_memslots *slots,
+			    struct kvm_memory_slot *memslot,
+			    enum kvm_mr_change change)
+{
+	int i;
+
+	if (change == KVM_MR_DELETE) {
+		kvm_memslot_delete(slots, memslot);
+	} else {
+		if (change == KVM_MR_CREATE)
+			i = kvm_memslot_insert_back(slots);
+		else
+			i = kvm_memslot_move_backward(slots, memslot);
+		i = kvm_memslot_move_forward(slots, memslot, i);
+
+		/*
+		 * Copy the memslot to its new position in memslots and update
+		 * its index accordingly.
+		 */
+		slots->memslots[i] = *memslot;
+		slots->id_to_index[memslot->id] = i;
+	}
 }
 
 static int check_memory_region_flags(const struct kvm_userspace_memory_region *mem)
@@ -1106,7 +1205,14 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	 * memslot needs to be referenced after calling update_memslots(), e.g.
 	 * to free its resources and for arch specific behavior.
 	 */
-	old = *id_to_memslot(__kvm_memslots(kvm, as_id), id);
+	tmp = id_to_memslot(__kvm_memslots(kvm, as_id), id);
+	if (tmp) {
+		old = *tmp;
+		tmp = NULL;
+	} else {
+		memset(&old, 0, sizeof(old));
+		old.id = id;
+	}
 
 	if (!mem->memory_size)
 		return kvm_delete_memslot(kvm, mem, &old, as_id);
@@ -1224,7 +1330,7 @@ int kvm_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log,
 
 	slots = __kvm_memslots(kvm, as_id);
 	*memslot = id_to_memslot(slots, id);
-	if (!(*memslot)->dirty_bitmap)
+	if (!(*memslot) || !(*memslot)->dirty_bitmap)
 		return -ENOENT;
 
 	kvm_arch_sync_dirty_log(kvm, *memslot);
@@ -1282,10 +1388,10 @@ static int kvm_get_dirty_log_protect(struct kvm *kvm, struct kvm_dirty_log *log)
 
 	slots = __kvm_memslots(kvm, as_id);
 	memslot = id_to_memslot(slots, id);
+	if (!memslot || !memslot->dirty_bitmap)
+		return -ENOENT;
 
 	dirty_bitmap = memslot->dirty_bitmap;
-	if (!dirty_bitmap)
-		return -ENOENT;
 
 	kvm_arch_sync_dirty_log(kvm, memslot);
 
@@ -1393,10 +1499,10 @@ static int kvm_clear_dirty_log_protect(struct kvm *kvm,
 
 	slots = __kvm_memslots(kvm, as_id);
 	memslot = id_to_memslot(slots, id);
+	if (!memslot || !memslot->dirty_bitmap)
+		return -ENOENT;
 
 	dirty_bitmap = memslot->dirty_bitmap;
-	if (!dirty_bitmap)
-		return -ENOENT;
 
 	n = ALIGN(log->num_pages, BITS_PER_LONG) / 8;