Re: [Qemu-devel] [PATCH] report serial devices created with -device in the PIIX4 config space

[Paolo Bonzini]

> > > +static void piix4_pm_machine_ready(struct Notifier* n)
> > > +{
> > > + PIIX4PMState *s = container_of(n, PIIX4PMState, machine_ready);
> >
> > DO_UPCAST()? I assume we have it for a reason.
>
> NIH is the reason we have it.

DO_UPCAST checks that the offset of the field is zero:

#ifdef __GNUC__
#define DO_UPCAST(type, field, dev) ( __extension__ ( { \
    char __attribute__((unused)) offset_must_be_zero[ \
        -offsetof(type, field)]; \
    container_of(dev, type, field);}))
#else
#define DO_UPCAST(type, field, dev) container_of(dev, type, field)
#endif

This isn't the case here, we really want container_of.

BTW, DO_UPCAST actually is used to do a _down_cast (base to derived).  A 
compile-time checked upcast (derived to base) could be done like this:

#ifdef __GNUC__
#define DO_UPCAST(type, field, dev) ( __extension__ ( { \
    char __attribute__((unused)) offset_must_be_zero[ \
        -offsetof(type, field)]; \
    char __attribute__((unused)) type_matches = \
        type_check(type, __typeof__(dev));
    &(dev)->field);}))
#else
#define DO_UPCAST(type, field, dev) &(dev)->field
#endif

Paolo