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[libvirt] [PATCHv4 06/51] snapshot: improve reverting to qemu paused snapshots



If you take a checkpoint snapshot of a running domain, then pause
qemu, then restore the snapshot, the result should be a running
domain, but the code was leaving things paused.  Furthermore, if
you take a checkpoint of a paused domain, then run, then restore,
there was a brief but non-deterministic window of time where the
domain was running rather than paused.  Fix both of these
discrepancies by always pausing before restoring.

Also, check that the VM is active every time lock is dropped
between two monitor calls.

Finally, straighten out the events that get emitted on each
transition.

* src/qemu/qemu_driver.c (qemuDomainRevertToSnapshot): Always
pause before reversion, and improve events.
---
 src/qemu/qemu_driver.c |  114 +++++++++++++++++++++++++++++++++++++++--------
 1 files changed, 94 insertions(+), 20 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index 914d2be..87888b6 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -8871,11 +8871,25 @@ static int qemuDomainRevertToSnapshot(virDomainSnapshotPtr snapshot,
     char uuidstr[VIR_UUID_STRING_BUFLEN];
     virDomainEventPtr event = NULL;
     virDomainEventPtr event2 = NULL;
+    int detail;
     qemuDomainObjPrivatePtr priv;
     int rc;

     virCheckFlags(0, -1);

+    /* We have the following transitions, which create the following events:
+     * 1. inactive -> inactive: none
+     * 2. inactive -> running:  EVENT_STARTED
+     * 3. inactive -> paused:   EVENT_STARTED, EVENT_PAUSED
+     * 4. running  -> inactive: EVENT_STOPPED
+     * 5. running  -> running:  none
+     * 6. running  -> paused:   EVENT_PAUSED
+     * 7. paused   -> inactive: EVENT_STOPPED
+     * 8. paused   -> running:  EVENT_RESUMED
+     * 9. paused   -> paused:   none
+     * Also, several transitions occur even if we fail partway through.
+     */
+
     qemuDriverLock(driver);
     virUUIDFormat(snapshot->domain->uuid, uuidstr);
     vm = virDomainFindByUUID(&driver->domains, snapshot->domain->uuid);
@@ -8910,44 +8924,102 @@ static int qemuDomainRevertToSnapshot(virDomainSnapshotPtr snapshot,

     if (snap->def->state == VIR_DOMAIN_RUNNING
         || snap->def->state == VIR_DOMAIN_PAUSED) {
-
+        /* Transitions 2, 3, 5, 6, 8, 9 */
+        bool was_running = false;
+        bool was_stopped = false;
+
+        /* When using the loadvm monitor command, qemu does not know
+         * whether to pause or run the reverted domain, and just stays
+         * in the same state as before the monitor command, whether
+         * that is paused or running.  We always pause before loadvm,
+         * to have finer control.  */
         if (virDomainObjIsActive(vm)) {
+            /* Transitions 5, 6, 8, 9 */
             priv = vm->privateData;
+            if (virDomainObjGetState(vm, NULL) == VIR_DOMAIN_RUNNING) {
+                /* Transitions 5, 6 */
+                was_running = true;
+                if (qemuProcessStopCPUs(driver, vm,
+                                        VIR_DOMAIN_PAUSED_FROM_SNAPSHOT,
+                                        QEMU_ASYNC_JOB_NONE) < 0)
+                    goto endjob;
+                /* Create an event now in case the restore fails, so
+                 * that user will be alerted that they are now paused.
+                 * If restore later succeeds, we might replace this. */
+                detail = VIR_DOMAIN_EVENT_SUSPENDED_FROM_SNAPSHOT;
+                event = virDomainEventNewFromObj(vm,
+                                                 VIR_DOMAIN_EVENT_SUSPENDED,
+                                                 detail);
+                if (!virDomainObjIsActive(vm)) {
+                    qemuReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                                    _("guest unexpectedly quit"));
+                    goto endjob;
+                }
+            }
             qemuDomainObjEnterMonitorWithDriver(driver, vm);
             rc = qemuMonitorLoadSnapshot(priv->mon, snap->def->name);
             qemuDomainObjExitMonitorWithDriver(driver, vm);
-            if (rc < 0)
+            if (rc < 0) {
+                /* XXX resume domain if it was running before the
+                 * failed loadvm attempt? */
                 goto endjob;
+            }
         } else {
+            /* Transitions 2, 3 */
+            was_stopped = true;
             rc = qemuProcessStart(snapshot->domain->conn, driver, vm, NULL,
-                                  false, false, -1, NULL, snap,
+                                  true, false, -1, NULL, snap,
                                   VIR_VM_OP_CREATE);
             virDomainAuditStart(vm, "from-snapshot", rc >= 0);
+            detail = VIR_DOMAIN_EVENT_STARTED_FROM_SNAPSHOT;
+            event = virDomainEventNewFromObj(vm,
+                                             VIR_DOMAIN_EVENT_STARTED,
+                                             detail);
             if (rc < 0)
                 goto endjob;
         }

-        event = virDomainEventNewFromObj(vm,
-                                         VIR_DOMAIN_EVENT_STARTED,
-                                         VIR_DOMAIN_EVENT_STARTED_FROM_SNAPSHOT);
+        /* Touch up domain state.  */
         if (snap->def->state == VIR_DOMAIN_PAUSED) {
-            /* qemu unconditionally starts the domain running again after
-             * loadvm, so let's pause it to keep consistency
-             * XXX we should have used qemuProcessStart's start_paused instead
-             */
-            rc = qemuProcessStopCPUs(driver, vm,
-                                     VIR_DOMAIN_PAUSED_FROM_SNAPSHOT,
-                                     QEMU_ASYNC_JOB_NONE);
+            /* Transitions 3, 6, 9 */
+            virDomainObjSetState(vm, VIR_DOMAIN_PAUSED,
+                                 VIR_DOMAIN_PAUSED_FROM_SNAPSHOT);
+            if (was_stopped) {
+                /* Transition 3, use event as-is and add event2 */
+                detail = VIR_DOMAIN_EVENT_SUSPENDED_FROM_SNAPSHOT;
+                event2 = virDomainEventNewFromObj(vm,
+                                                  VIR_DOMAIN_EVENT_SUSPENDED,
+                                                  detail);
+            } /* else transition 6 and 9 use event as-is */
+        } else {
+            /* Transitions 2, 5, 8 */
+            if (!virDomainObjIsActive(vm)) {
+                qemuReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                                _("guest unexpectedly quit"));
+                goto endjob;
+            }
+            rc = qemuProcessStartCPUs(driver, vm, snapshot->domain->conn,
+                                      VIR_DOMAIN_RUNNING_FROM_SNAPSHOT,
+                                      QEMU_ASYNC_JOB_NONE);
             if (rc < 0)
                 goto endjob;
-            event2 = virDomainEventNewFromObj(vm,
-                                              VIR_DOMAIN_EVENT_SUSPENDED,
-                                              VIR_DOMAIN_EVENT_SUSPENDED_FROM_SNAPSHOT);
-        } else {
-            virDomainObjSetState(vm, VIR_DOMAIN_RUNNING,
-                                 VIR_DOMAIN_RUNNING_FROM_SNAPSHOT);
+            virDomainEventFree(event);
+            if (was_stopped) {
+                /* Transition 2 */
+                detail = VIR_DOMAIN_EVENT_STARTED_FROM_SNAPSHOT;
+                event = virDomainEventNewFromObj(vm,
+                                                 VIR_DOMAIN_EVENT_STARTED,
+                                                 detail);
+            } else if (was_running) {
+                /* Transition 8 */
+                detail = VIR_DOMAIN_EVENT_RESUMED;
+                event = virDomainEventNewFromObj(vm,
+                                                 VIR_DOMAIN_EVENT_RESUMED,
+                                                 detail);
+            }
         }
     } else {
+        /* Transitions 1, 4, 7 */
         /* qemu is a little funny with running guests and the restoration
          * of snapshots.  If the snapshot was taken online,
          * then after a "loadvm" monitor command, the VM is set running
@@ -8959,11 +9031,13 @@ static int qemuDomainRevertToSnapshot(virDomainSnapshotPtr snapshot,
          */

         if (virDomainObjIsActive(vm)) {
+            /* Transitions 4, 7 */
             qemuProcessStop(driver, vm, 0, VIR_DOMAIN_SHUTOFF_FROM_SNAPSHOT);
             virDomainAuditStop(vm, "from-snapshot");
+            detail = VIR_DOMAIN_EVENT_STOPPED_FROM_SNAPSHOT;
             event = virDomainEventNewFromObj(vm,
                                              VIR_DOMAIN_EVENT_STOPPED,
-                                             VIR_DOMAIN_EVENT_STOPPED_FROM_SNAPSHOT);
+                                             detail);
             if (!vm->persistent) {
                 if (qemuDomainObjEndJob(driver, vm) > 0)
                     virDomainRemoveInactive(&driver->domains, vm);
-- 
1.7.4.4


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