Missing the AF_BUS

I think I was unclear: I'm not suggesting that DBus do some sort of socketpair thing to provide high-speed dbus links; I'm suggesting that use cases that don't really care about DBus, but need to negotiate with another program for their input, could get back to their normal operation (input from a fd) by having the other side send a fd back when it's all set up. That is, user input doesn't normally go over DBus, so applications are already designed around a non-DBus input path; even if you need IPC to set up your input device appropriately, it doesn't make too much sense to get user input over DBus, particularly if you can't get corresponding user input over DBus if the system doesn't have Chinese input or multi-touch.

Of course, it's certainly possible that people will want high-speed IPC with DBus properties also, and it makes sense for DBus to be efficient regardless of whether it's running into performance constraints. But it doesn't make sense to use DBus for all communication, even if its performance could be made good enough.