colo: Setup ram cache in normal migration path

Since
121ccedc2b migration: block incoming colo when capability is disabled

x-colo capability needs to be always enabled on the incoming side.
So migration_incoming_colo_enabled() and migrate_colo() are equivalent
with migrate_colo() being easier to reason about since it is always true
during the whole migration.

Use migrate_colo() to initialize the ram cache in the normal migration path.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Link: https://lore.kernel.org/qemu-devel/20260302-colo_unit_test_multifd-v11-3-d653fb3b1d80@web.de
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/migration.c

@@ -630,10 +630,6 @@ int migration_incoming_enable_colo(Error **errp)
         return -EINVAL;
     }
 
-    if (ram_block_discard_disable(true)) {
-        error_setg(errp, "COLO: cannot disable RAM discard");
-        return -EBUSY;
-    }
     migration_colo_enabled = true;
     return 0;
 }
@@ -770,6 +766,20 @@ process_incoming_migration_co(void *opaque)
 
     assert(mis->from_src_file);
 
+    if (migrate_colo()) {
+        if (ram_block_discard_disable(true)) {
+            error_setg(&local_err, "COLO: cannot disable RAM discard");
+            goto fail;
+        }
+
+        ret = colo_init_ram_cache(&local_err);
+        if (ret) {
+            error_prepend(&local_err, "failed to init colo RAM cache: %d: ",
+                          ret);
+            goto fail;
+        }
+    }
+
     mis->largest_page_size = qemu_ram_pagesize_largest();
     postcopy_state_set(POSTCOPY_INCOMING_NONE);
     migrate_set_state(&mis->state, MIGRATION_STATUS_SETUP,

migration/savevm.c

@@ -2427,19 +2427,7 @@ static int loadvm_process_enable_colo(MigrationIncomingState *mis,
                                       Error **errp)
 {
     ERRP_GUARD();
-    int ret;
-
-    ret = migration_incoming_enable_colo(errp);
-    if (ret < 0) {
-        return ret;
-    }
-
-    ret = colo_init_ram_cache(errp);
-    if (ret) {
-        error_prepend(errp, "failed to init colo RAM cache: %d: ", ret);
-        migration_incoming_disable_colo();
-    }
-    return ret;
+    return migration_incoming_enable_colo(errp);
 }
 
 static int loadvm_postcopy_handle_switchover_start(Error **errp)