Please give us control over values returned by GetSupportedInterfaceOrientations() implemented in Xamarin_Forms_Platform_iOS_PlatformRenderer class, which is internal root view in Xamarin.Forms. Not having portrait upside down mode is killing my app. Xamarin should provide access to native iOS framework as promised in docs, but it doesn't in this case!
This problem is also addressed here: http://forums.xamarin.com/discussion/37379/getsupportedinterfaceorientations-is-never-called-in-pagerenderer-subclass-in-ios