Re: [Qemu-devel] [PATCH v2 1/2] spapr: introduce SPAPR_MACHINE_NOASSERT()

[Greg Kurz <groug@kaod.org>]

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On Mon, 9 Oct 2017 10:26:38 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Sun, Oct 08, 2017 at 09:05:37PM +0100, Peter Maydell wrote:
> > On 8 October 2017 at 18:17, Greg Kurz <groug@kaod.org> wrote:  
> > > A spapr-cpu-core device can only be plugged into a pseries machine. This
> > > is checked in spapr_cpu_core_realize() with object_dynamic_cast() instead
> > > of the usual SPAPR_MACHINE() macro because we don't want QEMU to abort.
> > >
> > > This patch moves the gory details to a SPAPR_MACHINE_NOASSERT() macro
> > > that acts like the SPAPR_MACHINE() one, except it returns NULL instead
> > > of aborting if its argument doesn't point to a pseries machine type.
> > >
> > > This is done for two reasons:
> > > - it makes the code nicer
> > > - it may be used by other pseries-specific devices like PHBs for example
> > >
> > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > ---
> > >  hw/ppc/spapr_cpu_core.c |    7 +++----
> > >  include/hw/ppc/spapr.h  |    3 +++
> > >  2 files changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> > > index 37beb56e8b18..5fde07614218 100644
> > > --- a/hw/ppc/spapr_cpu_core.c
> > > +++ b/hw/ppc/spapr_cpu_core.c
> > > @@ -199,7 +199,7 @@ error:
> > >
> > >  static void spapr_cpu_core_realize(DeviceState *dev, Error **errp)
> > >  {
> > > -    sPAPRMachineState *spapr;
> > > +    sPAPRMachineState *spapr = SPAPR_MACHINE_NOASSERT(qdev_get_machine());
> > >      sPAPRCPUCore *sc = SPAPR_CPU_CORE(OBJECT(dev));
> > >      sPAPRCPUCoreClass *scc = SPAPR_CPU_CORE_GET_CLASS(OBJECT(dev));
> > >      CPUCore *cc = CPU_CORE(OBJECT(dev));
> > > @@ -209,9 +209,8 @@ static void spapr_cpu_core_realize(DeviceState *dev, Error **errp)
> > >      void *obj;
> > >      int i, j;
> > >
> > > -    spapr = (sPAPRMachineState *) qdev_get_machine();
> > > -    if (!object_dynamic_cast((Object *) spapr, TYPE_SPAPR_MACHINE)) {
> > > -        error_setg(errp, "spapr-cpu-core needs a pseries machine");
> > > +    if (!spapr) {
> > > +        error_setg(errp, TYPE_SPAPR_CPU_CORE " needs a pseries machine");
> > >          return;
> > >      }
> > >
> > > diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> > > index c1b365f56431..4933da8083df 100644
> > > --- a/include/hw/ppc/spapr.h
> > > +++ b/include/hw/ppc/spapr.h
> > > @@ -43,6 +43,9 @@ typedef struct sPAPRMachineClass sPAPRMachineClass;
> > >  #define SPAPR_MACHINE_CLASS(klass) \
> > >      OBJECT_CLASS_CHECK(sPAPRMachineClass, klass, TYPE_SPAPR_MACHINE)
> > >
> > > +#define SPAPR_MACHINE_NOASSERT(obj) \
> > > +    ((sPAPRMachineState *) object_dynamic_cast(obj, TYPE_SPAPR_MACHINE))
> > > +  
> > 
> > I don't think this is a great idea. Doing things with pointers
> > that might not be of the right type should be obvious, not hidden
> > under macros. An opencoded call to object_dynamic_cast is how the
> > rest of the codebase does this; it's a bit of a weird way
> > to write "isinstance()" but there you go. If we want to
> > improve the way we write this sort of thing we should do
> > so as a general improvement to the QOM APIs and conventions,
> > not just a single thing in SPAPR code.  
> 

Ok, I agree.

> Yeah, I tend to agree.  Sorry, the original advice I gave on not using
> object_dynamic_cast() directly was bogus - I didn't think it through
> clearly.
> 

Then maybe you can apply the previous version or do you have another
suggestion ?

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