From 92a638cb3b0601746d47b4a443762fe71bb21431 Mon Sep 17 00:00:00 2001
From: Ladi Prosek <lprosek@redhat.com>
Date: Thu, 10 Nov 2016 23:00:51 +0100
Subject: [PATCH 8/8] virtio-balloon: fix stats vq migration

RH-Author: Ladi Prosek <lprosek@redhat.com>
Message-id: <1478797251-10302-2-git-send-email-lprosek@redhat.com>
Patchwork-id: 72819
O-Subject: [PATCH v2 8/6] virtio-balloon: fix stats vq migration
Bugzilla: 1377968
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
RH-Acked-by: Michael S. Tsirkin <mst@redhat.com>
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>

The statistics virtqueue is not migrated properly because virtio-balloon
does not include s->stats_vq_elem in the migration stream.

After migration the statistics virtqueue hangs because the host never
completes the last element (s->stats_vq_elem is NULL on the destination
QEMU).  Therefore the guest never submits new elements and the virtqueue
is hung.

Instead of changing the migration stream format in an incompatible way,
detect the migration case and rewind the virtqueue so the last element
can be completed.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Roman Kagan <rkagan@virtuozzo.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Suggested-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 4a1e48becab81020adfb74b22c76a595f2d02a01)
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 hw/virtio/virtio-balloon.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index faf93f7..1a60d3c 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -403,6 +403,18 @@ static void virtio_balloon_device_reset(VirtIODevice *vdev)
     }
 }
 
+static void virtio_balloon_set_status(VirtIODevice *vdev, uint8_t status)
+{
+    VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
+
+    if (!s->stats_vq_elem_pending && vdev->vm_running &&
+        (status & VIRTIO_CONFIG_S_DRIVER_OK) && virtqueue_rewind(s->svq, 1)) {
+        /* poll stats queue for the element we have discarded when the VM
+         * was stopped */
+        virtio_balloon_receive_stats(vdev, s->svq);
+    }
+}
+
 static Property virtio_balloon_properties[] = {
     DEFINE_PROP_END_OF_LIST(),
 };
@@ -419,6 +431,7 @@ static void virtio_balloon_class_init(ObjectClass *klass, void *data)
     vdc->get_config = virtio_balloon_get_config;
     vdc->set_config = virtio_balloon_set_config;
     vdc->get_features = virtio_balloon_get_features;
+    vdc->set_status = virtio_balloon_set_status;
 }
 
 static const TypeInfo virtio_balloon_info = {
-- 
1.8.3.1