virtio-balloon: fix free page hinting without an iothread

In case we don't have an iothread, we mark the feature as abscent but
still add the queue. 'free_page_bh' remains set to NULL.

qemu-system-i386 \
        -M microvm \
        -nographic \
        -device virtio-balloon-device,free-page-hint=true \
        -nographic \
        -display none \
        -monitor none \
        -serial none \
        -qtest stdio

Doing a "write 0xc0000e30 0x24
0x030000000300000003000000030000000300000003000000030000000300000003000000"

We will trigger a SEGFAULT. Let's move the check and bail out.

While at it, move the static initializations to instance_init().
free_page_report_status and block_iothread are implicitly set to the
right values (0/false) already, so drop the initialization.

Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Fixes: c13c415 ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
Reported-by: Alexander Bulekov <alxndr@bu.edu>
Cc: qemu-stable@nongnu.org
Cc: Wei Wang <wei.w.wang@intel.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200520100439.19872-2-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-balloon.c

@@ -789,6 +789,13 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
         return;
     }
 
+    if (virtio_has_feature(s->host_features, VIRTIO_BALLOON_F_FREE_PAGE_HINT) &&
+        !s->iothread) {
+        error_setg(errp, "'free-page-hint' requires 'iothread' to be set");
+        virtio_cleanup(vdev);
+        return;
+    }
+
     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);
@@ -797,24 +804,11 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
                            VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
         s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE,
                                            virtio_balloon_handle_free_page_vq);
-        s->free_page_report_status = FREE_PAGE_REPORT_S_STOP;
-        s->free_page_report_cmd_id =
-                           VIRTIO_BALLOON_FREE_PAGE_REPORT_CMD_ID_MIN;
-        s->free_page_report_notify.notify =
-                                       virtio_balloon_free_page_report_notify;
         precopy_add_notifier(&s->free_page_report_notify);
-        if (s->iothread) {
-            object_ref(OBJECT(s->iothread));
-            s->free_page_bh = aio_bh_new(iothread_get_aio_context(s->iothread),
-                                       virtio_ballloon_get_free_page_hints, s);
-            qemu_mutex_init(&s->free_page_lock);
-            qemu_cond_init(&s->free_page_cond);
-            s->block_iothread = false;
-        } else {
-            /* Simply disable this feature if the iothread wasn't created. */
-            s->host_features &= ~(1 << VIRTIO_BALLOON_F_FREE_PAGE_HINT);
-            virtio_error(vdev, "iothread is missing");
-        }
+
+        object_ref(OBJECT(s->iothread));
+        s->free_page_bh = aio_bh_new(iothread_get_aio_context(s->iothread),
+                                     virtio_ballloon_get_free_page_hints, s);
     }
     reset_stats(s);
 }
@@ -892,6 +886,11 @@ static void virtio_balloon_instance_init(Object *obj)
 {
     VirtIOBalloon *s = VIRTIO_BALLOON(obj);
 
+    qemu_mutex_init(&s->free_page_lock);
+    qemu_cond_init(&s->free_page_cond);
+    s->free_page_report_cmd_id = VIRTIO_BALLOON_FREE_PAGE_REPORT_CMD_ID_MIN;
+    s->free_page_report_notify.notify = virtio_balloon_free_page_report_notify;
+
     object_property_add(obj, "guest-stats", "guest statistics",
                         balloon_stats_get_all, NULL, NULL, s);