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 #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: a229989d975e ("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 (cherry picked from commit 2ccd42b959aaf490333dbd3b9b102eaf295c036a) Signed-off-by: David Hildenbrand david@redhat.com --- drivers/s390/virtio/virtio_ccw.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c index a10dbe632ef9b..462d67c069ac7 100644 --- a/drivers/s390/virtio/virtio_ccw.c +++ b/drivers/s390/virtio/virtio_ccw.c @@ -264,11 +264,17 @@ 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 = 0; 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]) @@ -278,7 +284,7 @@ static unsigned long get_airq_indicator(struct virtqueue *vqs[], int nvqs, if (!info) return 0; 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); @@ -287,8 +293,10 @@ static unsigned long get_airq_indicator(struct virtqueue *vqs[], int nvqs, *first = bit; *airq_info = info; indicator_addr = (unsigned long)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);