Re: [PATCH v3 1/1] vhost-user-fs: add migration type property

[Vladimir Sementsov-Ogievskiy]

On 22.02.23 17:25, Anton Kuchin wrote:
> > > > +static int vhost_user_fs_pre_save(void *opaque)
> > > > +{
> > > > +    VHostUserFS *fs = opaque;
> > > > +    g_autofree char *path = object_get_canonical_path(OBJECT(fs));
> > > > +
> > > > +    switch (fs->migration_type) {
> > > > +    case VHOST_USER_MIGRATION_TYPE_NONE:
> > > > +        error_report("Migration is blocked by device %s", path);
> > > > +        break;
> > > > +    case VHOST_USER_MIGRATION_TYPE_EXTERNAL:
> > > > +        return 0;
> > > > +    default:
> > > > +        error_report("Migration type '%s' is not supported by device %s",
> > > > +                     VhostUserMigrationType_str(fs->migration_type), path);
> > > > +        break;
> > > > +    }
> > > > +
> > > > +    return -1;
> > > > +}
> > > Should we also add this as .pre_load, to force user select correct 
> > > migration_type on target too?
> > In fact, I would claim we only want pre_load.
> > When qemu is started on destination we know where it's migrated
> > from so this flag can be set.
> > When qemu is started on source we generally do not yet know so
> > we don't know whether it's safe to set this flag.

But destination is a "source" for next migration, so there shouldn't be real 
difference.
The new property has ".realized_set_allowed = true", so, as I understand it may 
be changed at any time, so that's not a problem.

> This property selects if VM can migrate and if it can what should qemu put
> to the migration stream. So we select on source what type of migration is
> allowed for this VM, destination can't check anything at load time.

OK, so the new field "migration" regulates only outgoing migration and do 
nothing for incoming. On incoming migration the migration stream itself defines the type 
of device migration.
Worth mentioning in doc?

--
Best regards,
Vladimir