migration: Append JSON description of migration stream

One of the annoyances of the current migration format is the fact that
it's not self-describing. In fact, it's not properly describing at all.
Some code randomly scattered throughout QEMU elaborates roughly how to
read and write a stream of bytes.

We discussed an idea during KVM Forum 2013 to add a JSON description of
the migration protocol itself to the migration stream. This patch
adds a section after the VM_END migration end marker that contains
description data on what the device sections of the stream are composed of.

This approach is backwards compatible with any QEMU version reading the
stream, because QEMU just stops reading after the VM_END marker and ignores
any data following it.

With an additional external program this allows us to decipher the
contents of any migration stream and hopefully make migration bugs easier
to track down.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

1 files changed, 48 insertions, 6 deletions

diff --git a/savevm.c b/savevm.c
index 98895fee81..80407662ad 100644
--- a/savevm.c
+++ b/savevm.c
@@ -572,14 +572,34 @@ static int vmstate_load(QEMUFile *f, SaveStateEntry *se, int version_id)
     return vmstate_load_state(f, se->vmsd, se->opaque, version_id);
 }
 
-static void vmstate_save(QEMUFile *f, SaveStateEntry *se)
+static void vmstate_save_old_style(QEMUFile *f, SaveStateEntry *se, QJSON *vmdesc)
+{
+    int64_t old_offset, size;
+
+    old_offset = qemu_ftell_fast(f);
+    se->ops->save_state(f, se->opaque);
+    size = qemu_ftell_fast(f) - old_offset;
+
+    if (vmdesc) {
+        json_prop_int(vmdesc, "size", size);
+        json_start_array(vmdesc, "fields");
+        json_start_object(vmdesc, NULL);
+        json_prop_str(vmdesc, "name", "data");
+        json_prop_int(vmdesc, "size", size);
+        json_prop_str(vmdesc, "type", "buffer");
+        json_end_object(vmdesc);
+        json_end_array(vmdesc);
+    }
+}
+
+static void vmstate_save(QEMUFile *f, SaveStateEntry *se, QJSON *vmdesc)
 {
     trace_vmstate_save(se->idstr, se->vmsd ? se->vmsd->name : "(old)");
-    if (!se->vmsd) {         /* Old style */
-        se->ops->save_state(f, se->opaque);
+    if (!se->vmsd) {
+        vmstate_save_old_style(f, se, vmdesc);
         return;
     }
-    vmstate_save_state(f, se->vmsd, se->opaque);
+    vmstate_save_state(f, se->vmsd, se->opaque, vmdesc);
 }
 
 bool qemu_savevm_state_blocked(Error **errp)
@@ -692,6 +712,8 @@ int qemu_savevm_state_iterate(QEMUFile *f)
 
 void qemu_savevm_state_complete(QEMUFile *f)
 {
+    QJSON *vmdesc;
+    int vmdesc_len;
     SaveStateEntry *se;
     int ret;
 
@@ -721,6 +743,9 @@ void qemu_savevm_state_complete(QEMUFile *f)
         }
     }
 
+    vmdesc = qjson_new();
+    json_prop_int(vmdesc, "page_size", TARGET_PAGE_SIZE);
+    json_start_array(vmdesc, "devices");
     QTAILQ_FOREACH(se, &savevm_handlers, entry) {
         int len;
 
@@ -728,6 +753,11 @@ void qemu_savevm_state_complete(QEMUFile *f)
             continue;
         }
         trace_savevm_section_start(se->idstr, se->section_id);
+
+        json_start_object(vmdesc, NULL);
+        json_prop_str(vmdesc, "name", se->idstr);
+        json_prop_int(vmdesc, "instance_id", se->instance_id);
+
         /* Section type */
         qemu_put_byte(f, QEMU_VM_SECTION_FULL);
         qemu_put_be32(f, se->section_id);
@@ -740,11 +770,23 @@ void qemu_savevm_state_complete(QEMUFile *f)
         qemu_put_be32(f, se->instance_id);
         qemu_put_be32(f, se->version_id);
 
-        vmstate_save(f, se);
+        vmstate_save(f, se, vmdesc);
+
+        json_end_object(vmdesc);
         trace_savevm_section_end(se->idstr, se->section_id, 0);
     }
 
     qemu_put_byte(f, QEMU_VM_EOF);
+
+    json_end_array(vmdesc);
+    qjson_finish(vmdesc);
+    vmdesc_len = strlen(qjson_get_str(vmdesc));
+
+    qemu_put_byte(f, QEMU_VM_VMDESCRIPTION);
+    qemu_put_be32(f, vmdesc_len);
+    qemu_put_buffer(f, (uint8_t *)qjson_get_str(vmdesc), vmdesc_len);
+    object_unref(OBJECT(vmdesc));
+
     qemu_fflush(f);
 }
 
@@ -843,7 +885,7 @@ static int qemu_save_device_state(QEMUFile *f)
         qemu_put_be32(f, se->instance_id);
         qemu_put_be32(f, se->version_id);
 
-        vmstate_save(f, se);
+        vmstate_save(f, se, NULL);
     }
 
     qemu_put_byte(f, QEMU_VM_EOF);