Re: [Xen-devel] [PATCH v6 08/12] libxl: ocaml: drop the ocaml heap lock before calling into libxl

[Rob Hoes <Rob.Hoes@xxxxxxxxxx>]

> > > Can't you just use caml_register_global_root on the address of your
> > > value for_callback? I presume you can call this register/unregister
> > > pair many times (for different events etc)?
> >
> > I think you need to register a value, not a pointer to a value.
> 
> CAMLextern void caml_register_global_root (value *);
> 
> It takes a pointer to the value to register. Is there any reason why you
> can't pass &for_callback given "value for_callback" in the local context?

I think we'd then have to rely on libxl to never copy to pointer internally, 
and throw away the original one. I am worried that this may happen, because the 
ao_how value is temporary.

> >
> > > Otherwise you need to build some sort of C-side data structure to
> > > hold all the various values, which is doable but a bit tedious I
> imagine.
> >
> > I think we can malloc a new "value" (pointer to ocaml heap), make it
> > equal to the for_callback value, register this with the GC (so it can
> > update this heap pointer when the actual value moves), and give a
> > pointer to it to libxl. And then of course deregister and free before
> > we give it back to ocaml.
> 
> I think this (moving the heap pointer from the GC) would require the
> prototype to be "value **" not "value *".
> 
> Ian.

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