Re: [PATCH for-5.0] vl.c: fix migration failure for 3.1 and older machine types

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

* Igor Mammedov (imammedo@redhat.com) wrote:
> On Wed,  4 Mar 2020 12:27:48 -0500
> Igor Mammedov <imammedo@redhat.com> wrote:
> 
> > Migration from QEMU(v4.0) fails when using 3.1 or older machine
> > type. For example if one attempts to migrate
> > QEMU-2.12 started as
> >   qemu-system-ppc64 -nodefaults -M pseries-2.12 -m 4096 -mem-path /tmp/
> > to current master, it will fail with
> >   qemu-system-ppc64: Unknown ramblock "ppc_spapr.ram", cannot accept migration
> >   qemu-system-ppc64: error while loading state for instance 0x0 of device 'ram'
> >   qemu-system-ppc64: load of migration failed: Invalid argument
> > 
> > Caused by 900c0ba373 commit which switches main RAM allocation to
> > memory backends and the fact in 3.1 and older QEMU, backends used
> > full[***] QOM path as memory region name instead of backend's name.
> > That was changed after 3.1 to use prefix-less names by default
> > (fa0cb34d22) for new machine types.
> > *** effectively makes main RAM memory region names defined by
> > MachineClass::default_ram_id being altered with '/objects/' prefix
> > and therefore migration fails as old QEMU sends prefix-less
> > name while new QEMU expects name with prefix when using 3.1 and
> > older machine types.
> > 
> > Fix it by forcing implicit[1] memory backend to always use
> > prefix-less names for its memory region by setting
> >   'x-use-canonical-path-for-ramblock-id'
> > property to false.
> > 
> > 1) i.e. memory backend created by compat glue which maps
> > -m/-mem-path/-mem-prealloc/default RAM size into
> > appropriate backend type/options to match old CLI format.
> > 
> > Fixes: 900c0ba373
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > Reported-by: Lukáš Doktor <ldoktor@redhat.com>
> 
> 
> ping,
> 
> so we don't forget to merge it

I'm queueing this.

> > ---
> > CC: ldoktor@redhat.com
> > CC: marcandre.lureau@redhat.com
> > CC: dgilbert@redhat.com
> > ---
> >  softmmu/vl.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/softmmu/vl.c b/softmmu/vl.c
> > index 5549f4b619..1101b1cb41 100644
> > --- a/softmmu/vl.c
> > +++ b/softmmu/vl.c
> > @@ -2800,6 +2800,9 @@ static void create_default_memdev(MachineState *ms, const char *path)
> >      object_property_set_int(obj, ms->ram_size, "size", &error_fatal);
> >      object_property_add_child(object_get_objects_root(), mc->default_ram_id,
> >                                obj, &error_fatal);
> > +    /* Ensure backend's memory region name is equal to mc->default_ram_id */
> > +    object_property_set_bool(obj, false, "x-use-canonical-path-for-ramblock-id",
> > +                             &error_fatal);
> >      user_creatable_complete(USER_CREATABLE(obj), &error_fatal);
> >      object_unref(obj);
> >      object_property_set_str(OBJECT(ms), mc->default_ram_id, "memory-backend",
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK