[Qemu-devel] Re: [PATCH 06/41] virtio: Use DO_UPCAST instead of a cast

[Michael S. Tsirkin]

On Wed, Dec 02, 2009 at 07:42:35PM +0100, Juan Quintela wrote:
> "Michael S. Tsirkin" <address@hidden> wrote:
> > On Wed, Dec 02, 2009 at 07:19:17PM +0100, Juan Quintela wrote:
> >> "Michael S. Tsirkin" <address@hidden> 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 <address@hidden>
> >> >
> >> > 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