Re: [PATCH 02/18] migration/vmstate: Introduce vmstate_no_state_to_migrate

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On Fri, 3 Jul 2020 at 21:19, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
> >
> > Introduce a special state to indicate when an object doesn't
> > have anything in its state to migrate.
> >
> > Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> > Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> > ---
> >  include/migration/vmstate.h | 1 +
> >  migration/vmstate.c         | 7 +++++++
> >  2 files changed, 8 insertions(+)
> >
> > diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
> > index af7d80cd4e..0922bc8efa 100644
> > --- a/include/migration/vmstate.h
> > +++ b/include/migration/vmstate.h
> > @@ -195,6 +195,7 @@ struct VMStateDescription {
> >  };
> >
> >  extern const VMStateDescription vmstate_dummy; /* Exclusively for linux-user */
> > +extern const VMStateDescription vmstate_no_state_to_migrate;
> >
> >  extern const VMStateInfo vmstate_info_bool;
> >
> > diff --git a/migration/vmstate.c b/migration/vmstate.c
> > index bafa890384..d640cafad3 100644
> > --- a/migration/vmstate.c
> > +++ b/migration/vmstate.c
> > @@ -20,6 +20,13 @@
> >  #include "trace.h"
> >  #include "qjson.h"
> >
> > +const VMStateDescription vmstate_no_state_to_migrate = {
> > +    .name = "empty-state",
> > +    .fields = (VMStateField[]) {
> > +        VMSTATE_END_OF_LIST()
> > +    }
> > +};
> 
> Does this definitely not put any data into the migration stream?
> We don't want to change what's on the wire for machines that
> use devices that start using this. (If it does by default, it
> would be easy to make the migration code special case the
> magic symbol to act like "no vmsd specified").

I'd need to test it to be sure, but I think if we added a .needed
to vmstate_no_state_to_migrate with a function that always returned
false, then I think the stream would stay unchanged.

Dave

> thanks
> -- PMM
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK