Add migration-capability boolean for postcopy-ram.

The 'postcopy ram' capability allows postcopy migration of RAM;
note that the migration starts off in precopy mode until
postcopy mode is triggered (see the migrate_start_postcopy
patch later in the series).

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/migration/migration.h

@@ -186,6 +186,7 @@ void migrate_add_blocker(Error *reason);
  */
 void migrate_del_blocker(Error *reason);
 
+bool migrate_postcopy_ram(void);
 bool migrate_zero_blocks(void);
 
 bool migrate_auto_converge(void);

migration/migration.c

@@ -578,6 +578,20 @@ void qmp_migrate_set_capabilities(MigrationCapabilityStatusList *params,
     for (cap = params; cap; cap = cap->next) {
         s->enabled_capabilities[cap->value->capability] = cap->value->state;
     }
+
+    if (migrate_postcopy_ram()) {
+        if (migrate_use_compression()) {
+            /* The decompression threads asynchronously write into RAM
+             * rather than use the atomic copies needed to avoid
+             * userfaulting.  It should be possible to fix the decompression
+             * threads for compatibility in future.
+             */
+            error_report("Postcopy is not currently compatible with "
+                         "compression");
+            s->enabled_capabilities[MIGRATION_CAPABILITY_X_POSTCOPY_RAM] =
+                false;
+        }
+    }
 }
 
 void qmp_migrate_set_parameters(bool has_compress_level,
@@ -956,6 +970,15 @@ void qmp_migrate_set_downtime(double value, Error **errp)
     max_downtime = (uint64_t)value;
 }
 
+bool migrate_postcopy_ram(void)
+{
+    MigrationState *s;
+
+    s = migrate_get_current();
+
+    return s->enabled_capabilities[MIGRATION_CAPABILITY_X_POSTCOPY_RAM];
+}
+
 bool migrate_auto_converge(void)
 {
     MigrationState *s;

qapi-schema.json

@@ -540,11 +540,15 @@
 # @auto-converge: If enabled, QEMU will automatically throttle down the guest
 #          to speed up convergence of RAM migration. (since 1.6)
 #
+# @x-postcopy-ram: Start executing on the migration target before all of RAM has
+#          been migrated, pulling the remaining pages along as needed. NOTE: If
+#          the migration fails during postcopy the VM will fail.  (since 2.5)
+#
 # Since: 1.2
 ##
 { 'enum': 'MigrationCapability',
   'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
-           'compress', 'events'] }
+           'compress', 'events', 'x-postcopy-ram'] }
 
 ##
 # @MigrationCapabilityStatus