On Wed, Dec 02, 2009 at 07:42:35PM +0100, Juan Quintela wrote: > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > On Wed, Dec 02, 2009 at 07:19:17PM +0100, Juan Quintela wrote: > >> "Michael S. Tsirkin" <m...@redhat.com> wrote: > >> > On Wed, Dec 02, 2009 at 01:04:04PM +0100, Juan Quintela wrote: > >> >> virtio_common_init() creates a struct with the right size, DO_UPCAST > >> >> is the appropiate thing here > >> >> > >> >> Signed-off-by: Juan Quintela <quint...@redhat.com> > >> > > >> > BTW why not container_of? That one does not require > >> > field to be at the beginning of structure. > >> > >> VirtIO devices (and PCIDevices) are declared in this way: > >> > >> typedef struct VirtIOBalloon > >> { > >> VirtIODevice vdev; > >> VirtQueue *ivq, *dvq; > >> uint32_t num_pages; > >> uint32_t actual; > >> } VirtIOBalloon; > >> > >> > >> I.e. the virtioDevice is always the 1st element, otherwise things don't > >> work. There are code that requires it to be the 1st element. > > > > I know. But I think we should slowly fix these assumptions, and not > > introduce more of them. IOW: don't use DO_UPCAST. > > It is inherent in how to implement OOP in C. You want to use things as > pci devices and as pci specific devices. Then you need to put the pci > common fields at the beggining. Yes, for virtio devices they want to be > pci and virtio devices, but neither pci or virtio nor qemu in general > allow to be derived of two types (a.k.a. as multiple inheritance). > > I think that we should continue using DO_UPCAST() until there are some > design that allows you to change that. This particular case "requires" > that VirtioDevice is the 1st field of the struct. If you change the > code enough to make container_of() work, doing the > s/DO_UPCAST/container_of/ is going to be the less of your problems. > > Later, Juan.
I don't understand. container_of is just more generic than DO_UPCAST. So why *ever* use DO_UPCAST? Let's get rid of it. -- MST