s390/virtio_ccw: Don't allocate/assign airqs for non-existing queues

If we finds a vq without a name in our input array in
virtio_ccw_find_vqs(), we treat it as "non-existing" and set the vq pointer
to NULL; we will not call virtio_ccw_setup_vq() to allocate/setup a vq.

Consequently, we create only a queue if it actually exists (name != NULL)
and assign an incremental queue index to each such existing queue.

However, in virtio_ccw_register_adapter_ind()->get_airq_indicator() we
will not ignore these "non-existing queues", but instead assign an airq
indicator to them.

Besides never releasing them in virtio_ccw_drop_indicators() (because
there is no virtqueue), the bigger issue seems to be that there will be a
disagreement between the device and the Linux guest about the airq
indicator to be used for notifying a queue, because the indicator bit
for adapter I/O interrupt is derived from the queue index.

The virtio spec states under "Setting Up Two-Stage Queue Indicators":

	... indicator contains the guest address of an area wherein the
	indicators for the devices are contained, starting at bit_nr, one
	bit per virtqueue of the device.

And further in "Notification via Adapter I/O Interrupts":

	For notifying the driver of virtqueue buffers, the device sets the
	bit in the guest-provided indicator area at the corresponding
	offset.

For example, QEMU uses in virtio_ccw_notify() the queue index (passed as
"vector") to select the relevant indicator bit. If a queue does not exist,
it does not have a corresponding indicator bit assigned, because it
effectively doesn't have a queue index.

Using a virtio-balloon-ccw device under QEMU with free-page-hinting
disabled ("free-page-hint=off") but free-page-reporting enabled
("free-page-reporting=on") will result in free page reporting
not working as expected: in the virtio_balloon driver, we'll be stuck
forever in virtballoon_free_page_report()->wait_event(), because the
waitqueue will not be woken up as the notification from the device is
lost: it would use the wrong indicator bit.

Free page reporting stops working and we get splats (when configured to
detect hung wqs) like:

 INFO: task kworker/1:3:463 blocked for more than 61 seconds.
       Not tainted 6.14.0 CachyOS#4
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 task:kworker/1:3 [...]
 Workqueue: events page_reporting_process
 Call Trace:
  [<000002f404e6dfb2>] __schedule+0x402/0x1640
  [<000002f404e6f22e>] schedule+0x3e/0xe0
  [<000002f3846a88fa>] virtballoon_free_page_report+0xaa/0x110 [virtio_balloon]
  [<000002f40435c8a4>] page_reporting_process+0x2e4/0x740
  [<000002f403fd3ee2>] process_one_work+0x1c2/0x400
  [<000002f403fd4b96>] worker_thread+0x296/0x420
  [<000002f403fe10b4>] kthread+0x124/0x290
  [<000002f403f4e0dc>] __ret_from_fork+0x3c/0x60
  [<000002f404e77272>] ret_from_fork+0xa/0x38

There was recently a discussion [1] whether the "holes" should be
treated differently again, effectively assigning also non-existing
queues a queue index: that should also fix the issue, but requires other
workarounds to not break existing setups.

Let's fix it without affecting existing setups for now by properly ignoring
the non-existing queues, so the indicator bits will match the queue
indexes.

[1] https://lore.kernel.org/all/cover.1720611677.git.mst@redhat.com/

Fixes: a229989 ("virtio: don't allocate vqs when names[i] = NULL")
Reported-by: Chandra Merla <cmerla@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: David Hildenbrand <david@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Link: https://lore.kernel.org/r/20250402203621.940090-1-david@redhat.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Author: davidhildenbrand

drivers/s390/virtio/virtio_ccw.c

@@ -302,11 +302,17 @@ static struct airq_info *new_airq_info(int index)
 static unsigned long *get_airq_indicator(struct virtqueue *vqs[], int nvqs,
 					 u64 *first, void **airq_info)
 {
-	int i, j;
+	int i, j, queue_idx, highest_queue_idx = -1;
 	struct airq_info *info;
 	unsigned long *indicator_addr = NULL;
 	unsigned long bit, flags;
 
+	/* Array entries without an actual queue pointer must be ignored. */
+	for (i = 0; i < nvqs; i++) {
+		if (vqs[i])
+			highest_queue_idx++;
+	}
+
 	for (i = 0; i < MAX_AIRQ_AREAS && !indicator_addr; i++) {
 		mutex_lock(&airq_areas_lock);
 		if (!airq_areas[i])
@@ -316,7 +322,7 @@ static unsigned long *get_airq_indicator(struct virtqueue *vqs[], int nvqs,
 		if (!info)
 			return NULL;
 		write_lock_irqsave(&info->lock, flags);
-		bit = airq_iv_alloc(info->aiv, nvqs);
+		bit = airq_iv_alloc(info->aiv, highest_queue_idx + 1);
 		if (bit == -1UL) {
 			/* Not enough vacancies. */
 			write_unlock_irqrestore(&info->lock, flags);
@@ -325,8 +331,10 @@ static unsigned long *get_airq_indicator(struct virtqueue *vqs[], int nvqs,
 		*first = bit;
 		*airq_info = info;
 		indicator_addr = info->aiv->vector;
-		for (j = 0; j < nvqs; j++) {
-			airq_iv_set_ptr(info->aiv, bit + j,
+		for (j = 0, queue_idx = 0; j < nvqs; j++) {
+			if (!vqs[j])
+				continue;
+			airq_iv_set_ptr(info->aiv, bit + queue_idx++,
 					(unsigned long)vqs[j]);
 		}
 		write_unlock_irqrestore(&info->lock, flags);