[Qemu-devel] [RFC PATCH v1 2/3] provenance: serialize MigrationInfo across the wire

[mrhines]

From: "Michael R. Hines" <address@hidden>

At the end of the migration, we use the existing QMP json manipulation
functions and the query-migrate QMP function to serialize the MigrationInfo
structure that was populated in the 'COMPLETED' state of the migration.

This code does not need to know anything about the actual contents of the
MigrationInfo structure, as the QMP code has taken care of all of the hard
work of serializing and visiting all of the individual fields of the structure.
Once we have a QObject, we convert that to JSON and send it across the
connection at the end of the migration.

The only exception to the serialization is the 'downtime' field. Previously
this was calculated inside the migration_thread, but because want to ensure
that this field is serialized as well, we need to calculate it just a little
bit earlier.

Once the json is received on the other side, we perform the reverse of
all the previous steps and convert the json string back into a MigrationInfo
structure just as it was before.

The information in this structure will only last as long as the next migration,
so if another migration occurs, it is the responsibility of the management
software to extract the information before the next migration begins or else
the information will be lost.

Signed-off-by: Michael R. Hines <address@hidden>
---
 migration.c | 126 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 121 insertions(+), 5 deletions(-)

diff --git a/migration.c b/migration.c
index 25add6f..8acabf0 100644
--- a/migration.c
+++ b/migration.c
@@ -24,6 +24,10 @@
 #include "migration/block.h"
 #include "qemu/thread.h"
 #include "qmp-commands.h"
+#include "qapi/qmp/qobject.h"
+#include "qapi/qmp/qstring.h"
+#include "qapi/qmp/qint.h"
+#include "qapi/qmp/qjson.h"
 #include "trace.h"
 
 //#define DEBUG_MIGRATION
@@ -183,6 +187,115 @@ static void get_xbzrle_cache_stats(MigrationInfo *info)
     }
 }
 
+/*
+ * 'last_info' is not a device, per-se, but the existing device state
+ * state transfer mechanism is very easy to use for this purpose, as
+ * we're only (currently) interested in communicating this information
+ * in-band after the last migration round has completed (as opposed to
+ * out-of-band, which would require more intelligence in the management
+ * software, for example).
+ */
+void migrate_info_save(QEMUFile *f, void *opaque)
+{
+    MigrationState *s = migrate_get_current();
+    QObject *info;
+    QString * qjson;
+    QInt *downtime;
+    uint32_t len;
+    const char * json;
+    int64_t end_time;
+
+    /*
+     * Use the existing QMP command to get the statistics.
+     */
+    qmp_marshal_input_query_migrate(NULL, NULL, &info);
+
+    /*
+     * Update the structure with the final downtime of the migration.
+     */
+    end_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
+    s->total_time = end_time - s->total_time;
+    s->downtime = end_time - s->start_time;
+    downtime = qint_from_int(s->downtime);
+
+    qdict_put_obj(qobject_to_qdict(info), "downtime", QOBJECT(downtime));
+
+    /*
+     * Serialize into raw JSON and send to other side only
+     * after the last migration round has completed.
+     */
+    qjson = qobject_to_json(info);
+    json = qstring_get_str(qjson);
+
+    len = strlen(json);
+    qemu_put_be32(f, len);
+    qemu_put_buffer(f, (uint8_t *)json, len);
+
+    qemu_fflush(f);
+
+    qobject_decref(QOBJECT(qjson));
+    qobject_decref(info);
+    qobject_decref(QOBJECT(downtime));
+}
+
+/*
+ * The source has nearly completed the migration and has sent
+ * out a JSON description of the migration performance statistics.
+ * Load it and use the QMP 'migrate-set-last-info' command to automatically
+ * convert it into a usable structure.
+ */
+int migrate_info_load(QEMUFile *f, void *opaque, int version_id)
+{
+    uint32_t len = qemu_get_be32(f);
+    char * json = g_malloc0(len + 1);
+    QDict *info = qdict_new();
+
+    qemu_get_buffer(f, (uint8_t *)json, len);
+
+    /*
+     * Construct a { 'info' : MigrationInfo } structure
+     * out of the resulting JSON for QMP to parse the input.
+     */
+    qdict_put_obj(info, "info", qobject_from_json(json));
+    DPRINTF("migrate_info_load: %s\n", json);
+    g_free(json);
+    qmp_marshal_input_migrate_set_last_info(NULL, info, NULL);
+    qobject_decref(QOBJECT(info));
+
+    return 0;
+}
+
+/*
+ * JSON from the migration source was received - now save it.
+ */
+void qmp_migrate_set_last_info(MigrationInfo *info, Error **errp)
+{
+    MigrationState *s = migrate_get_current();
+
+    if (!info) {
+        fprintf(stderr, "error: migrate-set-last-info dictionary is empty!\n");
+        return;
+    }
+
+    if (!s->last_info) {
+        s->last_info = g_malloc0(sizeof(*s->last_info));
+    } else {
+        g_free(s->last_info->ram);
+    }
+
+    memcpy(s->last_info, info, sizeof(*s->last_info));
+    s->last_info->ram = NULL;
+
+    if (info->has_ram) {
+        s->last_info->ram = g_malloc0(sizeof(*s->last_info->ram));
+        memcpy(s->last_info->ram, info->ram, sizeof(*s->last_info->ram));
+    }
+
+    s->last_info->has_status = false;
+    s->migrated_before = true;
+}
+
+
 MigrationInfo *qmp_query_migrate(Error **errp)
 {
     MigrationInfo *info = g_malloc0(sizeof(*info));
@@ -191,6 +304,11 @@ MigrationInfo *qmp_query_migrate(Error **errp)
     switch (s->state) {
     case MIG_STATE_NONE:
         /* no migration has happened ever */
+        if (s->migrated_before) {
+            memcpy(info, s->last_info, sizeof(*info));
+            info->has_status = true;
+            info->status = g_strdup("last");
+        }
         break;
     case MIG_STATE_SETUP:
         info->has_status = true;
@@ -580,9 +698,10 @@ static void *migration_thread(void *opaque)
     int64_t setup_start = qemu_clock_get_ms(QEMU_CLOCK_HOST);
     int64_t initial_bytes = 0;
     int64_t max_size = 0;
-    int64_t start_time = initial_time;
     bool old_vm_running = false;
 
+    s->start_time = initial_time;
+
     DPRINTF("beginning savevm\n");
     qemu_savevm_state_begin(s->file, &s->params);
 
@@ -607,7 +726,7 @@ static void *migration_thread(void *opaque)
 
                 DPRINTF("done iterating\n");
                 qemu_mutex_lock_iothread();
-                start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
+                s->start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
                 qemu_system_wakeup_request(QEMU_WAKEUP_REASON_OTHER);
                 old_vm_running = runstate_is_running();
 
@@ -665,9 +784,6 @@ static void *migration_thread(void *opaque)
 
     qemu_mutex_lock_iothread();
     if (s->state == MIG_STATE_COMPLETED) {
-        int64_t end_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
-        s->total_time = end_time - s->total_time;
-        s->downtime = end_time - start_time;
         runstate_set(RUN_STATE_POSTMIGRATE);
     } else {
         if (old_vm_running) {
-- 
1.8.1.2