Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for non-existing queues

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 02, 2025 at 10:36:21PM +0200, David Hildenbrand wrote:
> 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>
> Cc: Cornelia Huck <cohuck@redhat.com>
> Cc: Thomas Huth <thuth@redhat.com>
> Cc: Halil Pasic <pasic@linux.ibm.com>
> Cc: Eric Farman <farman@linux.ibm.com>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> Cc: Sven Schnelle <svens@linux.ibm.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Wei Wang <wei.w.wang@intel.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>


feel free to merge.

> ---
>  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 21fa7ac849e5c..4904b831c0a75 100644
> --- a/drivers/s390/virtio/virtio_ccw.c
> +++ b/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);
> -- 
> 2.48.1