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Date: Mon, 25 Jul 2011 11:11:08 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] report serial devices created with -device
 in the PIIX4 config space
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: =?windows-1252?Q?Andreas_F=E4rber?= <andreas.faerber@web.de>, QEMU Developers <qemu-devel@nongnu.org>, Gleb Natapov <gleb@redhat.com>, Markus Armbruster <armbru@redhat.com>


>>> +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