Re: [Qemu-devel] [RFC] New Migration Protocol using Visitor Interface

[Anthony Liguori]

On 10/03/2011 09:11 AM, Michael S. Tsirkin wrote:
> On Mon, Oct 03, 2011 at 08:43:54AM -0500, Anthony Liguori wrote:
> > > > visit_start_array(v, "entries", errp);
> > > > for (int i = 0; i<   s->size; i++) {
> > > >      visit_type_int(v, NULL,&s->entry[i], errp);
> > > > }
> > > > visit_end_array(v, errp);
> > >
> > > Sequences can encode structures not just arrays.
> > > How would you encode this for example:
> > >
> > > SEQUENCE OF { VQN: INTEGER, SEQUENCE { OPTIONAL VECTOR: INTEGER}  }
> >
> > visit_start_array(v, "vqs", errp);
> > for (i = 0; i<  s->n_vqs; i++) {
> >      // Array elements never have a name, hence NULL name
> >      visit_start_struct(v, "VirtQueue", NULL, errp);
> >      visit_type_int(v,&s->vq[i].num, "vqn", errp);
> >
> >      // Given this sub-struct an arbitrary name.  It could also be anonymous.
> >      visit_start_struct(v, "MsixInfo", "msix_info", errp);
> >      if (s->vq[i].msix_enabled) {
> >          visit_type_int(v,&s->vq[i].vector, "vector", errp);
>
> Why is this a pointer to vector, btw?

So you can write a single visit function that works for input or output.

Think of the simple case like:

void visit_simple_type(Visitor *v, SimpleType *t, const char *name, Error 
**errp)
{
   visit_start_struct(v, "SimpleType", name, errp);
   visit_type_int(v, &t->a, "a", errp);
   visit_type_int(v, &t->b, "b", errp);
   visit_end_struct(v, errp);
}

For complex types like Virtio, you need to do a bit more.  You wouldn't do a 
simple for () {} loop but instead use the Visitor list mechanism.  That would 
eliminate the need to have to marshal n_vqs.

> >      }
> >      visit_end_struct(v, errp);
> >
> >      visit_end_struct(v, errp);
> > }
> > visit_end_array(v, errp);
> >
> > This would also generate JSON of:
> >
> > 'vqs': [ { 'vqn': 2, 'msix_info': { 'vector': 3 } } ]
>
> How would optional fields be handled?

As far as the Visitor goes, if something is optional you just don't encode it. 
If you need to key off the presence of a field, presumably you could just check 
to see whether it succeeded or failed to visit that field.  I'm not 100% sure if 
you can do a single input/output visitor when you have optional fields.

My rough thinking is that each device would have a input/output visitor callback 
that took the same signature.  That gives the flexibility of having two separate 
interfaces but in the common case, you just pass the same function for both.

> Specifically
> the case where first field in a sequence tells
> you the meaning of the following ones?

Can you give me the example in ASN.1?

Regards,

Anthony Liguori