On Thu, Dec 11, 2025 at 03:05:49AM -0600, Aaron Lo wrote:
> The VirtIO specification (section 5.5.2) states that the stats queue
> is only present if the VIRTIO_BALLOON_F_STATS_VQ feature is
> negotiated. QEMU currently creates the statsq unconditionally.
>
> This patch guards statsq creation so it occurs only when the
> feature bit is enabled.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3188
>
> Signed-off-by: Aaron Lo <[email protected]>
did you test this with linux guests and with
free page hinting?
The issue is that sadly everyone is out of spec here.
We will have to fix the spec.
> ---
> hw/virtio/virtio-balloon.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 02cdd807d7..f5d4d5f60c 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -892,7 +892,10 @@ static void virtio_balloon_device_realize(DeviceState
> *dev, Error **errp)
>
> s->ivq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
> s->dvq = virtio_add_queue(vdev, 128, virtio_balloon_handle_output);
> - s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
> +
> + if (virtio_has_feature(s->host_features, VIRTIO_BALLOON_F_STATS_VQ)) {
> + s->svq = virtio_add_queue(vdev, 128, virtio_balloon_receive_stats);
> + }
>
> if (virtio_has_feature(s->host_features,
> VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE,
> @@ -932,7 +935,9 @@ static void virtio_balloon_device_unrealize(DeviceState
> *dev)
>
> virtio_delete_queue(s->ivq);
> virtio_delete_queue(s->dvq);
> - virtio_delete_queue(s->svq);
> + if (s->svq) {
> + virtio_delete_queue(s->svq);
> + }
> if (s->free_page_vq) {
> virtio_delete_queue(s->free_page_vq);
> }
>
> ---
> base-commit: 9c23f2a7b0b45277693a14074b1aaa827eecdb92
> change-id: 20251211-balloon-check-stats-feature-7ea658e038ce
>
> Best regards,
> --
> Aaron Lo <[email protected]>
