The big question with IPC is what problem you are trying to solve. For microkernels you want blazingly fast cross-process function calls, but IPC mecanism that

The big question with IPC is what problem you are trying to solve. For microkernels you want blazingly fast cross-process function calls, but IPC mecanism that can do only that is mostly useless for Unix userspace. For example all of QNX style IPC, traditional SysV-style unix IPC and pthreads have no API for select()-on-IPC-primitive (VMS and Symbian have that and NT has mechanism for this that mostly works)