[Qemu-devel] [PATCH 6/9] migration: create new section to store global state

[Juan Quintela <quintela@redhat.com>]

This includes a new section that for now just stores the current qemu state.

Right now, there are only one way to control what is the state of the
target after migration.

- If you run the target qemu with -S, it would start stopped.
- If you run the target qemu without -S, it would run just after migration finishes.

The problem here is what happens if we start the target without -S and
there happens one error during migration that puts current state as
-EIO.  Migration would ends (notice that the error happend doing block
IO, network IO, i.e. nothing related with migration), and when
migration finish, we would just "continue" running on destination,
probably hanging the guest/corruption data, whatever.

Signed-off-by: Juan Quintela <quintela@redhat.com>
---
 include/migration/migration.h |  4 ++
 migration/migration.c         | 85 +++++++++++++++++++++++++++++++++++++++++--
 vl.c                          |  1 +
 3 files changed, 87 insertions(+), 3 deletions(-)

diff --git a/include/migration/migration.h b/include/migration/migration.h
index a6e025a..785c2dc 100644
--- a/include/migration/migration.h
+++ b/include/migration/migration.h
@@ -180,4 +180,8 @@ size_t ram_control_save_page(QEMUFile *f, ram_addr_t block_offset,
                              ram_addr_t offset, size_t size,
                              uint64_t *bytes_sent);

+void register_global_state(void);
+void global_state_store(void);
+char *global_state_get_runstate(void);
+
 #endif
diff --git a/migration/migration.c b/migration/migration.c
index 732d229..ab69f81 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -133,10 +133,20 @@ static void process_incoming_migration_co(void *opaque)
         exit(EXIT_FAILURE);
     }

-    if (autostart) {
+    /* runstate == "" means that we haven't received it through the
+     * wire, so we obey autostart.  runstate == runing means that we
+     * need to run it, we need to make sure that we do it after
+     * everything else has finished.  Every other state change is done
+     * at the post_load function */
+
+    if (strcmp(global_state_get_runstate(), "running") == 0) {
         vm_start();
-    } else {
-        runstate_set(RUN_STATE_PAUSED);
+    } else if (strcmp(global_state_get_runstate(), "") == 0) {
+        if (autostart) {
+            vm_start();
+        } else {
+            runstate_set(RUN_STATE_PAUSED);
+        }
     }
     migrate_decompress_threads_join();
 }
@@ -760,6 +770,7 @@ static void *migration_thread(void *opaque)
                 qemu_system_wakeup_request(QEMU_WAKEUP_REASON_OTHER);
                 old_vm_running = runstate_is_running();

+                global_state_store();
                 ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
                 if (ret >= 0) {
                     qemu_file_set_rate_limit(s->file, INT64_MAX);
@@ -855,3 +866,71 @@ void migrate_fd_connect(MigrationState *s)
     qemu_thread_create(&s->thread, "migration", migration_thread, s,
                        QEMU_THREAD_JOINABLE);
 }
+
+typedef struct {
+    int32_t size;
+    uint8_t runstate[100];
+} GlobalState;
+
+static GlobalState global_state;
+
+void global_state_store(void)
+{
+    if (runstate_store((char *)global_state.runstate,
+                       sizeof(global_state.runstate)) == -1) {
+        error_report("Runstate is too big: %s\n", global_state.runstate);
+        exit(-1);
+    }
+}
+
+char *global_state_get_runstate(void)
+{
+    return (char *)global_state.runstate;
+}
+
+static int global_state_post_load(void *opaque, int version_id)
+{
+    GlobalState *s = opaque;
+    int ret = 0;
+    char *runstate = (char *)s->runstate;
+
+    if (strcmp(runstate, "running") != 0) {
+
+        RunState r = runstate_index(runstate);
+
+        if (r == -1) {
+            error_report("Unknown received state %s\n", runstate);
+            return -1;
+        }
+        ret = vm_stop_force_state(r);
+    }
+
+   return ret;
+}
+
+static void global_state_pre_save(void *opaque)
+{
+    GlobalState *s = opaque;
+
+    s->size = strlen((char *)s->runstate) + 1;
+}
+
+static const VMStateDescription vmstate_globalstate = {
+    .name = "globalstate",
+    .version_id = 1,
+    .minimum_version_id = 1,
+    .post_load = global_state_post_load,
+    .pre_save = global_state_pre_save,
+    .fields = (VMStateField[]) {
+        VMSTATE_INT32(size, GlobalState),
+        VMSTATE_BUFFER(runstate, GlobalState),
+        VMSTATE_END_OF_LIST()
+    },
+};
+
+void register_global_state(void)
+{
+    /* We would use it independently that we receive it */
+    strcpy((char *)&global_state.runstate, "");
+    vmstate_register(NULL, 0, &vmstate_globalstate, &global_state);
+}
diff --git a/vl.c b/vl.c
index b8844f6..8af6c79 100644
--- a/vl.c
+++ b/vl.c
@@ -4387,6 +4387,7 @@ int main(int argc, char **argv, char **envp)
         return 0;
     }

+    register_global_state();
     if (incoming) {
         Error *local_err = NULL;
         qemu_start_incoming_migration(incoming, &local_err);
-- 
2.4.0