[Qemu-devel] [PATCH v8 08/20] virtio: add subsections to the migration stream

[Greg Kurz]

There is a need to add some more fields to VirtIODevice that should be
migrated (broken status, endianness). The problem is that we do not
want to break compatibility while adding a new feature... This issue has
been addressed in the generic VMState code with the use of optional
subsections. As a *temporary* alternative to port the whole virtio
migration code to VMState, this patch mimics a similar subsectionning
ability for virtio, using the VMState code.

Since each virtio device is streamed in its own section, the idea is to
stream subsections between the end of the device section and the start
of the next sections. This allows an older QEMU to complain and exit
when fed with subsections:

Unknown savevm section type 5
load of migration failed

Suggested-by: Alexander Graf <address@hidden>
Signed-off-by: Greg Kurz <address@hidden>
---

 Changes since last version (RFC):
 - use the VMState framework as suggested by Andreas Färber

 hw/virtio/virtio.c |   21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 44517fc..16b73d9 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -19,6 +19,7 @@
 #include "hw/virtio/virtio.h"
 #include "qemu/atomic.h"
 #include "hw/virtio/virtio-bus.h"
+#include "migration/migration.h"
 
 /*
  * The alignment to use between consumer and producer parts of vring.
@@ -839,6 +840,16 @@ void virtio_notify_config(VirtIODevice *vdev)
     virtio_notify_vector(vdev, vdev->config_vector);
 }
 
+static const VMStateDescription vmstate_virtio = {
+    .name = "virtio",
+    .version_id = 1,
+    .minimum_version_id = 1,
+    .minimum_version_id_old = 1,
+    .fields = (VMStateField[]) {
+        VMSTATE_END_OF_LIST()
+    }
+};
+
 void virtio_save(VirtIODevice *vdev, QEMUFile *f)
 {
     BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
@@ -882,6 +893,9 @@ void virtio_save(VirtIODevice *vdev, QEMUFile *f)
     if (vdc->save != NULL) {
         vdc->save(vdev, f);
     }
+
+    /* Subsections */
+    vmstate_save_state(f, &vmstate_virtio, vdev);
 }
 
 int virtio_set_features(VirtIODevice *vdev, uint32_t val)
@@ -985,10 +999,13 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int 
version_id)
     virtio_notify_vector(vdev, VIRTIO_NO_VECTOR);
 
     if (vdc->load != NULL) {
-        return vdc->load(vdev, f, version_id);
+        ret = vdc->load(vdev, f, version_id);
+        if (ret) {
+            return ret;
+        }
     }
 
-    return 0;
+    return vmstate_load_state(f, &vmstate_virtio, vdev, 1);
 }
 
 void virtio_cleanup(VirtIODevice *vdev)