Re: [Qemu-devel] [PATCH v2 01/10] qapi: add Visitor interfaces for uint*_t and int*_t

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/21/2011 03:45 PM, Anthony Liguori wrote:
> On 12/21/2011 06:35 AM, Paolo Bonzini wrote:
>> On 12/20/2011 09:56 PM, Anthony Liguori wrote:
>>>> As always, you can implement that in many ways. However, I think the
>>>> point of using Visitors is not to remove QEMUFile.
>>>
>>> Yes, it is.
>>
>> The point of using Visitors is to provide a standard representation of
>> device
>> state. QEMUFile is but one consumer of that representation, along with
>> any other
>> migration filter.
>
> Can you do a quick code mock up of how you'd expect this to work?

void
vmstate_get(Device *obj, Visitor *v, void *opaque, const char *name)
{
     /* Use the VMStateDescription in opaque to add fields to Visitor
        from the struct.  */
}

void
vmstate_set(Device *obj, Visitor *v, void *opaque, const char *name)
{
     /* Use the VMStateDescription in opaque to retrieve fields from
        Visitor and store them in the struct.  */
}

void
vmstate_load_state(Device *obj, Visitor *v, QEMUFile )
{
     VMStateDescription *vmsd = ???; /* something from the DeviceClass */

     /* Use the VMStateDescription in opaque to retrieve fields from
        Visitor and store them in the struct.  This is basically the
        VMState interpreter currently in savevm.c, only fetching fields
        from v rather than the file.  */
}

void
vmstate_save_state(Device *obj, Visitor *v, QEMUFile *out)
{
     VMStateDescription *vmsd = ???; /* something from the DeviceClass */

     /* Use the VMStateDescription in opaque to fetch fields from
        Visitor and store them in the file.  This is basically the
        other VMState interpreter currently in savevm.c, but fetching
        fields from v rather than the struct.  */
}

void
qdev_add_vmstate(Device *obj, VMStateDescription *vmsd)
{
     qdev_add_property(obj, "vmstate", vmstate_get, vmstate_set, vmsd);
     qdev_add_interface(obj, "QEMUFileSerializable", vmstate_load_state,
                        vmstate_save_state);
}


savevm.c:

Visitor *v = qmp_output_visitor_new();
qdev_property_get(obj, "vmstate", v);
QObject *qobj = qmp_visitor_get_obj(v);
qmp_visitor_free(v);

Visitor *v = qmp_input_visitor_new(qobj);
QEMUFileSerializable *s = QEMU_FILE_SERIALIZABLE(obj);
s->save_state(obj, v, outfile);
qmp_visitor_free(v);

...

Visitor *v = qmp_output_visitor_new();
QEMUFileSerializable *s = QEMU_FILE_SERIALIZABLE(obj);
s->load_state(obj, v, outfile);
QObject *qobj = qmp_visitor_get_obj(v);
qmp_visitor_free(v);

Visitor *v = qmp_input_visitor_new(qobj);
QEMUFileSerializable *s = QEMU_FILE_SERIALIZABLE(obj);
qdev_property_set(obj, "vmstate", v);
qmp_visitor_free(v);

---------------------

Yes, this makes QEMUFile serialization special.  But that's because it's 
legacy, and it needs to do strange things and store things beyond the 
contents of the vmstate.

Take for example "unused" fields.  In Mike's implementation, if you try 
to pass a QMPOutputVisitor to save_state you might get a "unused" entry 
that is an array of zeros.  I have no idea what happens if a single 
VMStateDescription has more than one VMSTATE_UNUSED field. 
QEMUFileOutputVisitor completely ignores the field names, so it probably 
works but would break with QMPOutputVisitor.

Other serialization backends should not need any hooks in the devices 
beyond save_state and load_state.

Paolo