qmp: handle stop/cont in INMIGRATE state

Right now, stop followed by an incoming migration will let the
virtual machine start.  cont before an incoming migration instead
will fail.

This is bad because the actual behavior is not predictable; it is
racy with respect to the start of the incoming migration.  That's
because incoming migration is blocking, and thus will delay the
processing of stop/cont until the end of the migration.

In addition, there's nothing that really prevents the user from
typing the block device's passwords before incoming migration is
done, so returning the DeviceEncrypted error is also helpful in
the QMP case.

Both things can be fixed by just toggling the autostart variable when
stop/cont are called in INMIGRATE state.

Note that libvirt is currently working around the race by looping
if the MigrationExpected answer is returned.  After this patch, the
command will return right away without ever raising an error.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

3 files changed, 25 insertions, 18 deletions

diff --git a/qapi-schema.json b/qapi-schema.json
index c615ee212d..6b14edcc98 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -22,15 +22,11 @@
 # @KVMMissingCap: the requested operation can't be fulfilled because a
 #                 required KVM capability is missing
 #
-# @MigrationExpected: the requested operation can't be fulfilled because a
-#                     migration process is expected
-#
 # Since: 1.2
 ##
 { 'enum': 'ErrorClass',
   'data': [ 'GenericError', 'CommandNotFound', 'DeviceEncrypted',
-            'DeviceNotActive', 'DeviceNotFound', 'KVMMissingCap',
-            'MigrationExpected' ] }
+            'DeviceNotActive', 'DeviceNotFound', 'KVMMissingCap' ] }
 
 ##
 # @add_client
@@ -149,7 +145,11 @@
 #
 # @finish-migrate: guest is paused to finish the migration process
 #
-# @inmigrate: guest is paused waiting for an incoming migration
+# @inmigrate: guest is paused waiting for an incoming migration.  Note
+# that this state does not tell whether the machine will start at the
+# end of the migration.  This depends on the command-line -S option and
+# any invocation of 'stop' or 'cont' that has happened since QEMU was
+# started.
 #
 # @internal-error: An internal error that prevents further guest execution
 # has occurred
@@ -1210,7 +1210,9 @@
 # Since:  0.14.0
 #
 # Notes:  This function will succeed even if the guest is already in the stopped
-#         state
+#         state.  In "inmigrate" state, it will ensure that the guest
+#         remains paused once migration finishes, as if the -S option was
+#         passed on the command line.
 ##
 { 'command': 'stop' }
 
@@ -1299,11 +1301,14 @@
 # Since:  0.14.0
 #
 # Returns:  If successful, nothing
-#           If the QEMU is waiting for an incoming migration, MigrationExpected
 #           If QEMU was started with an encrypted block device and a key has
 #              not yet been set, DeviceEncrypted.
 #
-# Notes:  This command will succeed if the guest is currently running.
+# Notes:  This command will succeed if the guest is currently running.  It
+#         will also succeed if the guest is in the "inmigrate" state; in
+#         this case, the effect of the command is to make sure the guest
+#         starts once migration finishes, removing the effect of the -S
+#         command line option if it was passed.
 ##
 { 'command': 'cont' }
 
diff --git a/qerror.h b/qerror.h
index c91708cc3c..5e98a39ae1 100644
--- a/qerror.h
+++ b/qerror.h
@@ -165,9 +165,6 @@ void assert_no_error(Error *err);
 #define QERR_MIGRATION_NOT_SUPPORTED \
     ERROR_CLASS_GENERIC_ERROR, "State blocked by non-migratable device '%s'"
 
-#define QERR_MIGRATION_EXPECTED \
-    ERROR_CLASS_MIGRATION_EXPECTED, "An incoming migration is expected before this command can be executed"
-
 #define QERR_MISSING_PARAMETER \
     ERROR_CLASS_GENERIC_ERROR, "Parameter '%s' is missing"
 
diff --git a/qmp.c b/qmp.c
index 36c54c57cf..2c8d559609 100644
--- a/qmp.c
+++ b/qmp.c
@@ -85,7 +85,11 @@ void qmp_quit(Error **err)
 
 void qmp_stop(Error **errp)
 {
-    vm_stop(RUN_STATE_PAUSED);
+    if (runstate_check(RUN_STATE_INMIGRATE)) {
+        autostart = 0;
+    } else {
+        vm_stop(RUN_STATE_PAUSED);
+    }
 }
 
 void qmp_system_reset(Error **errp)
@@ -144,10 +148,7 @@ void qmp_cont(Error **errp)
 {
     Error *local_err = NULL;
 
-    if (runstate_check(RUN_STATE_INMIGRATE)) {
-        error_set(errp, QERR_MIGRATION_EXPECTED);
-        return;
-    } else if (runstate_check(RUN_STATE_INTERNAL_ERROR) ||
+    if (runstate_check(RUN_STATE_INTERNAL_ERROR) ||
                runstate_check(RUN_STATE_SHUTDOWN)) {
         error_set(errp, QERR_RESET_REQUIRED);
         return;
@@ -162,7 +163,11 @@ void qmp_cont(Error **errp)
         return;
     }
 
-    vm_start();
+    if (runstate_check(RUN_STATE_INMIGRATE)) {
+        autostart = 1;
+    } else {
+        vm_start();
+    }
 }
 
 void qmp_system_wakeup(Error **errp)