[QEMU v2 6/9] balloon: migrate vq elem to destination

[Liang Li <liang.z.li@intel.com>]

After live migration, 'guest-stats' can't get the expected memory
status in the guest. This issue is caused by commit 4eae2a657d.
The value of 's->stats_vq_elem' will be NULL after live migration,
and the check in the function 'balloon_stats_poll_cb()' will
prevent the 'virtio_notify()' from executing. So guest will not
update the memory status.

Commit 4eae2a657d is doing the right thing, but 's->stats_vq_elem'
should be treated as part of balloon device state and migrated to
destination if it's not NULL to make everything works well.

For the same reason, 's->misc_vq_elem' should be migrated to
destination too.
Michael has other idea to solve this issue, but he is busy at the
moment, this patch can be used for test before his patch is ready.

Signed-off-by: Liang Li <liang.z.li@intel.com>
---
 hw/virtio/virtio-balloon.c | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index b0c09a7..f9bf26d 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -31,6 +31,7 @@
 #include "hw/virtio/virtio-access.h"
 
 #define BALLOON_PAGE_SIZE  (1 << VIRTIO_BALLOON_PFN_SHIFT)
+#define BALLOON_VERSION 2
 
 static void balloon_page(void *addr, int deflate)
 {
@@ -610,15 +611,33 @@ static void virtio_balloon_save(QEMUFile *f, void *opaque)
 static void virtio_balloon_save_device(VirtIODevice *vdev, QEMUFile *f)
 {
     VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
+    uint16_t elem_num = 0;
 
     qemu_put_be32(f, s->num_pages);
     qemu_put_be32(f, s->actual);
+    if (s->stats_vq_elem != NULL) {
+        elem_num = 1;
+    }
+    qemu_put_be16(f, elem_num);
+    if (elem_num) {
+        qemu_put_virtqueue_element(f, s->stats_vq_elem);
+    }
+
+    elem_num = 0;
+    if (s->misc_vq_elem != NULL) {
+        elem_num = 1;
+    }
+    qemu_put_be16(f, elem_num);
+    if (elem_num) {
+        qemu_put_virtqueue_element(f, s->misc_vq_elem);
+    }
 }
 
 static int virtio_balloon_load(QEMUFile *f, void *opaque, int version_id)
 {
-    if (version_id != 1)
+    if (version_id < 1 || version_id > BALLOON_VERSION) {
         return -EINVAL;
+    }
 
     return virtio_load(VIRTIO_DEVICE(opaque), f, version_id);
 }
@@ -627,9 +646,22 @@ static int virtio_balloon_load_device(VirtIODevice *vdev, QEMUFile *f,
                                       int version_id)
 {
     VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
+    uint16_t elem_num = 0;
 
     s->num_pages = qemu_get_be32(f);
     s->actual = qemu_get_be32(f);
+    if (version_id == BALLOON_VERSION) {
+        elem_num = qemu_get_be16(f);
+        if (elem_num == 1) {
+            s->stats_vq_elem =
+                    qemu_get_virtqueue_element(f, sizeof(VirtQueueElement));
+        }
+        elem_num = qemu_get_be16(f);
+        if (elem_num == 1) {
+            s->misc_vq_elem =
+                    qemu_get_virtqueue_element(f, sizeof(VirtQueueElement));
+        }
+    }
 
     if (balloon_stats_enabled(s)) {
         balloon_stats_change_timer(s, s->stats_poll_interval);
@@ -665,7 +697,7 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
     reset_stats(s);
     s->req_status = REQ_INIT;
 
-    register_savevm(dev, "virtio-balloon", -1, 1,
+    register_savevm(dev, "virtio-balloon", -1, BALLOON_VERSION,
                     virtio_balloon_save, virtio_balloon_load, s);
 }
 
-- 
1.9.1