Re: [PATCH v9 07/27] migration: push Error **errp into qemu_loadvm_state()

[Arun Menon <armenon@redhat.com>]

Hi Akihiko,

Thanks for the review.

On Wed, Aug 06, 2025 at 02:17:02PM +0900, Akihiko Odaki wrote:
> On 2025/08/06 3:25, Arun Menon wrote:
> > This is an incremental step in converting vmstate loading
> > code to report error via Error objects instead of directly
> > printing it to console/monitor.
> > It is ensured that qemu_loadvm_state() must report an error
> > in errp, in case of failure.
> > 
> > When postcopy live migration runs, the device states are loaded by
> > both the qemu coroutine process_incoming_migration_co() and the
> > postcopy_ram_listen_thread(). Therefore, it is important that the
> > coroutine also reports the error in case of failure, with
> > error_report_err(). Otherwise, the source qemu will not display
> > any errors before going into the postcopy pause state.
> > 
> > Signed-off-by: Arun Menon <armenon@redhat.com>
> > ---
> >   migration/migration.c |  7 ++++---
> >   migration/savevm.c    | 31 +++++++++++++++++++------------
> >   migration/savevm.h    |  2 +-
> >   3 files changed, 24 insertions(+), 16 deletions(-)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index 10c216d25dec01f206eacad2edd24d21f00e614c..bb7d5e1dee52692cbea1d2c8fdca541e6a75bedb 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -881,7 +881,7 @@ process_incoming_migration_co(void *opaque)
> >                         MIGRATION_STATUS_ACTIVE);
> >       mis->loadvm_co = qemu_coroutine_self();
> > -    ret = qemu_loadvm_state(mis->from_src_file);
> > +    ret = qemu_loadvm_state(mis->from_src_file, &local_err);
> >       mis->loadvm_co = NULL;
> >       trace_vmstate_downtime_checkpoint("dst-precopy-loadvm-completed");
> > @@ -908,7 +908,8 @@ process_incoming_migration_co(void *opaque)
> >       }
> >       if (ret < 0) {
> > -        error_setg(&local_err, "load of migration failed: %s", strerror(-ret));
> > +        error_prepend(&local_err, "load of migration failed: %s: ",
> > +                      strerror(-ret));
> >           goto fail;
> >       }
> > @@ -924,7 +925,7 @@ fail:
> >       migrate_set_state(&mis->state, MIGRATION_STATUS_ACTIVE,
> >                         MIGRATION_STATUS_FAILED);
> >       migrate_set_error(s, local_err);
> > -    error_free(local_err);
> > +    error_report_err(local_err);
> 
> There is "error_report_err(s->error)" after this so this can result in
> duplicate error printing.

Yes, we can replace this with error_free(s->error).

> 
> >       migration_incoming_state_destroy();
> > diff --git a/migration/savevm.c b/migration/savevm.c
> > index 1bd27efe437d4d911728d776e995490d0a45dcf5..ca166ebd397ad80836ed2f9cb20a92f704fd4ed5 100644
> > --- a/migration/savevm.c
> > +++ b/migration/savevm.c
> > @@ -3149,28 +3149,24 @@ out:
> >       return ret;
> >   }
> > -int qemu_loadvm_state(QEMUFile *f)
> > +int qemu_loadvm_state(QEMUFile *f, Error **errp)
> >   {
> >       MigrationState *s = migrate_get_current();
> >       MigrationIncomingState *mis = migration_incoming_get_current();
> > -    Error *local_err = NULL;
> >       int ret;
> > -    if (qemu_savevm_state_blocked(&local_err)) {
> > -        error_report_err(local_err);
> > +    if (qemu_savevm_state_blocked(errp)) {
> >           return -EINVAL;
> >       }
> >       qemu_loadvm_thread_pool_create(mis);
> > -    ret = qemu_loadvm_state_header(f, &local_err);
> > +    ret = qemu_loadvm_state_header(f, errp);
> >       if (ret) {
> > -        error_report_err(local_err);
> >           return ret;
> >       }
> > -    if (qemu_loadvm_state_setup(f, &local_err) != 0) {
> > -        error_report_err(local_err);
> > +    if (qemu_loadvm_state_setup(f, errp) != 0) {
> >           return -EINVAL;
> >       }
> > @@ -3181,6 +3177,9 @@ int qemu_loadvm_state(QEMUFile *f)
> >       cpu_synchronize_all_pre_loadvm();
> >       ret = qemu_loadvm_state_main(f, mis);
> > +    if (ret < 0) {
> > +        error_setg(errp, "Load VM state failed: %d", ret);
> > +    }
> >       qemu_event_set(&mis->main_thread_load_event);
> >       trace_qemu_loadvm_state_post_main(ret);
> > @@ -3198,8 +3197,14 @@ int qemu_loadvm_state(QEMUFile *f)
> >           if (migrate_has_error(migrate_get_current()) ||
> >               !qemu_loadvm_thread_pool_wait(s, mis)) {
> >               ret = -EINVAL;
> > +            error_setg(errp,
> > +                       "Error while loading VM state: "
> > +                       "Migration stream has error");
> >           } else {
> >               ret = qemu_file_get_error(f);
> > +            if (ret < 0) {
> > +                error_setg(errp, "Error while loading vmstate : %d", ret);
> > +            }
> >           }
> >       }
> >       /*
> > @@ -3464,6 +3469,7 @@ void qmp_xen_save_devices_state(const char *filename, bool has_live, bool live,
> >   void qmp_xen_load_devices_state(const char *filename, Error **errp)
> >   {
> > +    ERRP_GUARD();
> >       QEMUFile *f;
> >       QIOChannelFile *ioc;
> >       int ret;
> > @@ -3485,10 +3491,10 @@ void qmp_xen_load_devices_state(const char *filename, Error **errp)
> >       f = qemu_file_new_input(QIO_CHANNEL(ioc));
> >       object_unref(OBJECT(ioc));
> > -    ret = qemu_loadvm_state(f);
> > +    ret = qemu_loadvm_state(f, errp);
> >       qemu_fclose(f);
> >       if (ret < 0) {
> > -        error_setg(errp, "loading Xen device state failed");
> > +        error_prepend(errp, "loading Xen device state failed: ");
> >       }
> >       migration_incoming_state_destroy();
> >   }
> > @@ -3496,6 +3502,7 @@ void qmp_xen_load_devices_state(const char *filename, Error **errp)
> >   bool load_snapshot(const char *name, const char *vmstate,
> >                      bool has_devices, strList *devices, Error **errp)
> >   {
> > +    ERRP_GUARD();
> >       BlockDriverState *bs_vm_state;
> >       QEMUSnapshotInfo sn;
> >       QEMUFile *f;
> > @@ -3559,13 +3566,13 @@ bool load_snapshot(const char *name, const char *vmstate,
> >           ret = -EINVAL;
> >           goto err_drain;
> >       }
> > -    ret = qemu_loadvm_state(f);
> > +    ret = qemu_loadvm_state(f, errp);
> >       migration_incoming_state_destroy();
> >       bdrv_drain_all_end();
> >       if (ret < 0) {
> > -        error_setg(errp, "Error %d while loading VM state", ret);
> > +        error_prepend(errp, "Error %d while loading VM state: ", ret);
> >           return false;
> >       }
> > diff --git a/migration/savevm.h b/migration/savevm.h
> > index 2d5e9c716686f06720325e82fe90c75335ced1de..b80770b7461a60e2ad6ba5e24a7baeae73d90955 100644
> > --- a/migration/savevm.h
> > +++ b/migration/savevm.h
> > @@ -64,7 +64,7 @@ void qemu_savevm_send_colo_enable(QEMUFile *f);
> >   void qemu_savevm_live_state(QEMUFile *f);
> >   int qemu_save_device_state(QEMUFile *f);
> > -int qemu_loadvm_state(QEMUFile *f);
> > +int qemu_loadvm_state(QEMUFile *f, Error **errp);
> >   void qemu_loadvm_state_cleanup(MigrationIncomingState *mis);
> >   int qemu_loadvm_state_main(QEMUFile *f, MigrationIncomingState *mis);
> >   int qemu_load_device_state(QEMUFile *f);
> > 
> 
Regards,
Arun Menon