[PATCH v3 3/8] migration/savevm: Allow immutable device state to be migrated early (i.e., before RAM)

[David Hildenbrand <david@redhat.com>]

For virtio-mem, we want to have the plugged/unplugged state of memory
blocks available before migrating any actual RAM content, and perform
sanity checks before touching anything on the destination. This
information is immutable on the migration source while migration is active,

We want to use this information for proper preallocation support with
migration: currently, we don't preallocate memory on the migration target,
and especially with hugetlb, we can easily run out of hugetlb pages during
RAM migration and will crash (SIGBUS) instead of catching this gracefully
via preallocation.

Migrating device state via a vmsd before we start iterating is currently
impossible: the only approach that would be possible is avoiding a vmsd
and migrating state manually during save_setup(), to be restored during
load_state().

Let's allow for migrating device state via a vmsd early, during the
setup phase in qemu_savevm_state_setup(). To keep it simple, we
indicate applicable vmds's using an "immutable" flag.

Note that only very selected devices (i.e., ones seriously messing with
RAM setup) are supposed to make use of such early state migration.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/migration/vmstate.h |  5 +++++
 migration/savevm.c          | 14 ++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
index ad24aa1934..dd06c3abad 100644
--- a/include/migration/vmstate.h
+++ b/include/migration/vmstate.h
@@ -179,6 +179,11 @@ struct VMStateField {
 struct VMStateDescription {
     const char *name;
     int unmigratable;
+    /*
+     * The state is immutable while migration is active and is saved
+     * during the setup phase, to be restored early on the destination.
+     */
+    int immutable;
     int version_id;
     int minimum_version_id;
     MigrationPriority priority;
diff --git a/migration/savevm.c b/migration/savevm.c
index ff2b8d0064..536d6f662b 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1200,6 +1200,15 @@ void qemu_savevm_state_setup(QEMUFile *f)
 
     trace_savevm_state_setup();
     QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
+        if (se->vmsd && se->vmsd->immutable) {
+            ret = vmstate_save(f, se, ms->vmdesc);
+            if (ret) {
+                qemu_file_set_error(f, ret);
+                break;
+            }
+            continue;
+        }
+
         if (!se->ops || !se->ops->save_setup) {
             continue;
         }
@@ -1402,6 +1411,11 @@ int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
     int ret;
 
     QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
+        if (se->vmsd && se->vmsd->immutable) {
+            /* Already saved during qemu_savevm_state_setup(). */
+            continue;
+        }
+
         ret = vmstate_save(f, se, vmdesc);
         if (ret) {
             qemu_file_set_error(f, ret);
-- 
2.39.0