[PATCH 5/5] qemu: migration: Reactivate block nodes after migration if VM is left paused

[Peter Krempa <pkrempa@xxxxxxxxxx>]

On incoming migration qemu doesn't activate the block graph nodes right
away. This is to properly facilitate locking of the images.

The block nodes are normally re-activated when starting the CPUs after
migration, but in cases (e.g. when a paused VM was migrated) when the VM
is left paused the block nodes are not re-activated by qemu.

This means that blockjobs which would want to write to an existing
backing chain member would fail. Generally read-only jobs would succeed
with older qemu's but this was not intended.

Instead with new qemu you'll always get an error if attempting to access
a inactive node:

 error: internal error: unable to execute QEMU command 'blockdev-mirror': Inactive 'libvirt-1-storage' can't be a backing child of active '#block052'

This is the case for explicit blockjobs (virsh blockcopy) but also for
non shared-storage migration (virsh migrate --copy-storage-all).

Since qemu now provides 'blockdev-set-active' QMP command which can
on-demand re-activate the nodes we can re-activate them in similar cases
as when we'd be starting vCPUs if the VM weren't left paused.

The only exception is on the source in case of a failed post-copy
migration as the VM already ran on destination so it won't ever run on
the source even when recovered.

Resolves: https://issues.redhat.com/browse/RHEL-78398
Signed-off-by: Peter Krempa <pkrempa@xxxxxxxxxx>
---
 src/qemu/qemu_migration.c | 46 +++++++++++++++++++++++++++++++++++----
 1 file changed, 42 insertions(+), 4 deletions(-)

diff --git a/src/qemu/qemu_migration.c b/src/qemu/qemu_migration.c
index 53bbbee629..159521fbf5 100644
--- a/src/qemu/qemu_migration.c
+++ b/src/qemu/qemu_migration.c
@@ -220,6 +220,39 @@ qemuMigrationSrcStoreDomainState(virDomainObj *vm)
 }


+/**
+ * qemuMigrationBlockNodesReactivate:
+ *
+ * In case when we're keeping the VM paused qemu will not re-activate the block
+ * device backend tree so blockjobs would fail. In case when qemu supports the
+ * 'blockdev-set-active' command this function will re-activate the block nodes.
+ */
+static void
+qemuMigrationBlockNodesReactivate(virDomainObj *vm,
+                                  virDomainAsyncJob asyncJob)
+{
+    qemuDomainObjPrivate *priv = vm->privateData;
+    int rc;
+
+    if (!virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKDEV_SET_ACTIVE))
+        return;
+
+    VIR_DEBUG("re-activating block nodes");
+
+    if (qemuDomainObjEnterMonitorAsync(vm, asyncJob) < 0)
+        return;
+
+    rc = qemuMonitorBlockdevSetActive(priv->mon, NULL, true);
+
+    qemuDomainObjExitMonitor(vm);
+
+    if (rc < 0) {
+        VIR_WARN("failed to re-activate block nodes after migration of VM '%s'", vm->def->name);
+        virResetLastError();
+    }
+}
+
+
 static void
 qemuMigrationSrcRestoreDomainState(virQEMUDriver *driver, virDomainObj *vm)
 {
@@ -238,11 +271,14 @@ qemuMigrationSrcRestoreDomainState(virQEMUDriver *driver, virDomainObj *vm)

     if (preMigrationState != VIR_DOMAIN_RUNNING ||
         state != VIR_DOMAIN_PAUSED ||
-        reason == VIR_DOMAIN_PAUSED_POSTCOPY_FAILED)
-        return;
+        reason == VIR_DOMAIN_PAUSED_POSTCOPY_FAILED) {
+        if (reason == VIR_DOMAIN_PAUSED_IOERROR)
+            VIR_DEBUG("Domain is paused due to I/O error, skipping resume");
+
+        /* Don't reactivate disks on post-copy failure */
+        if (reason != VIR_DOMAIN_PAUSED_POSTCOPY_FAILED)
+            qemuMigrationBlockNodesReactivate(vm, VIR_ASYNC_JOB_MIGRATION_OUT);

-    if (reason == VIR_DOMAIN_PAUSED_IOERROR) {
-        VIR_DEBUG("Domain is paused due to I/O error, skipping resume");
         return;
     }

@@ -6795,6 +6831,8 @@ qemuMigrationDstFinishFresh(virQEMUDriver *driver,

         if (*inPostCopy)
             *doKill = false;
+    } else {
+        qemuMigrationBlockNodesReactivate(vm, VIR_ASYNC_JOB_MIGRATION_IN);
     }

     if (mig->jobData) {
-- 
2.48.1