migration: Add error-parameterized function variants in VMSD struct

- We need to have good error reporting in the callbacks in
  VMStateDescription struct. Specifically pre_save, pre_load
  and post_load callbacks.
- It is not possible to change these functions everywhere in one
  patch, therefore, we introduce a duplicate set of callbacks
  with Error object passed to them.
- So, in this commit, we implement 'errp' variants of these callbacks,
  introducing an explicit Error object parameter.
- This is a functional step towards transitioning the entire codebase
  to the new error-parameterized functions.
- Deliberately called in mutual exclusion from their counterparts,
  to prevent conflicts during the transition.
- New impls should preferentally use 'errp' variants of
  these methods, and existing impls incrementally converted.
  The variants without 'errp' are intended to be removed
  once all usage is converted.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Arun Menon <armenon@redhat.com>
Tested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Link: https://lore.kernel.org/r/20250918-propagate_tpm_error-v14-26-36f11a6fb9d3@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

docs/devel/migration/main.rst

@@ -444,6 +444,25 @@ The functions to do that are inside a vmstate definition, and are called:
   This function is called after we save the state of one device
   (even upon failure, unless the call to pre_save returned an error).
 
+Following are the errp variants of these functions.
+
+- ``int (*pre_load_errp)(void *opaque, Error **errp);``
+
+  This function is called before we load the state of one device.
+
+- ``int (*post_load_errp)(void *opaque, int version_id, Error **errp);``
+
+  This function is called after we load the state of one device.
+
+- ``int (*pre_save_errp)(void *opaque, Error **errp);``
+
+  This function is called before we save the state of one device.
+
+New impls should preferentally use 'errp' variants of these
+methods and existing impls incrementally converted.
+The variants without 'errp' are intended to be removed
+once all usage is converted.
+
 Example: You can look at hpet.c, that uses the first three functions
 to massage the state that is transferred.
 

include/migration/vmstate.h

@@ -200,14 +200,28 @@ struct VMStateDescription {
      * exclusive. For this reason, also early_setup VMSDs are migrated in a
      * QEMU_VM_SECTION_FULL section, while save_setup() data is migrated in
      * a QEMU_VM_SECTION_START section.
+     *
+     * There are duplicate impls of the post/pre save/load hooks.
+     * New impls should preferentally use 'errp' variants of these
+     * methods and existing impls incrementally converted.
+     * The variants without 'errp' are intended to be removed
+     * once all usage is converted.
+     *
+     * For the errp variants,
+     * Returns: 0 on success,
+     *          <0 on error where -value is an error number from errno.h
      */
+
     bool early_setup;
     int version_id;
     int minimum_version_id;
     MigrationPriority priority;
     int (*pre_load)(void *opaque);
+    int (*pre_load_errp)(void *opaque, Error **errp);
     int (*post_load)(void *opaque, int version_id);
+    int (*post_load_errp)(void *opaque, int version_id, Error **errp);
     int (*pre_save)(void *opaque);
+    int (*pre_save_errp)(void *opaque, Error **errp);
     int (*post_save)(void *opaque);
     bool (*needed)(void *opaque);
     bool (*dev_unplug_pending)(void *opaque);

migration/vmstate.c

@@ -134,6 +134,7 @@ static void vmstate_handle_alloc(void *ptr, const VMStateField *field,
 int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
                        void *opaque, int version_id, Error **errp)
 {
+    ERRP_GUARD();
     const VMStateField *field = vmsd->fields;
     int ret = 0;
 
@@ -152,7 +153,16 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
         trace_vmstate_load_state_end(vmsd->name, "too old", -EINVAL);
         return -EINVAL;
     }
-    if (vmsd->pre_load) {
+    if (vmsd->pre_load_errp) {
+        ret = vmsd->pre_load_errp(opaque, errp);
+        if (ret < 0) {
+            error_prepend(errp, "pre load hook failed for: '%s', "
+                          "version_id: %d, minimum version_id: %d, "
+                          "ret: %d: ", vmsd->name, vmsd->version_id,
+                          vmsd->minimum_version_id, ret);
+            return ret;
+        }
+    } else if (vmsd->pre_load) {
         ret = vmsd->pre_load(opaque);
         if (ret) {
             error_setg(errp, "pre load hook failed for: '%s', "
@@ -245,7 +255,14 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
         qemu_file_set_error(f, ret);
         return ret;
     }
-    if (vmsd->post_load) {
+    if (vmsd->post_load_errp) {
+        ret = vmsd->post_load_errp(opaque, version_id, errp);
+        if (ret < 0) {
+            error_prepend(errp, "post load hook failed for: %s, version_id: "
+                          "%d, minimum_version: %d, ret: %d: ", vmsd->name,
+                          vmsd->version_id, vmsd->minimum_version_id, ret);
+        }
+    } else if (vmsd->post_load) {
         ret = vmsd->post_load(opaque, version_id);
         if (ret < 0) {
             error_setg(errp,
@@ -414,12 +431,20 @@ int vmstate_save_state(QEMUFile *f, const VMStateDescription *vmsd,
 int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
                          void *opaque, JSONWriter *vmdesc, int version_id, Error **errp)
 {
+    ERRP_GUARD();
     int ret = 0;
     const VMStateField *field = vmsd->fields;
 
     trace_vmstate_save_state_top(vmsd->name);
 
-    if (vmsd->pre_save) {
+    if (vmsd->pre_save_errp) {
+        ret = vmsd->pre_save_errp(opaque, errp);
+        trace_vmstate_save_state_pre_save_res(vmsd->name, ret);
+        if (ret < 0) {
+            error_prepend(errp, "pre-save for %s failed, ret: %d: ",
+                          vmsd->name, ret);
+        }
+    } else if (vmsd->pre_save) {
         ret = vmsd->pre_save(opaque);
         trace_vmstate_save_state_pre_save_res(vmsd->name, ret);
         if (ret) {