A MsgSend is to another service process, hopefully waiting on a MsgReceive. For the case where the service process is idle, waiting on a MsgReceive, there is a fast path where the sending thread is blocked, the receiving thread is unblocked, and control is immediately transferred without a trip through the scheduler. The receiving process inherits the sender's priority and CPU quantum. When the service process does a MsgReply, control is transferred back in a similar way.
This fast path offers some big advantages. There's no scheduling delay; the control transfer happens immediately, almost like a coroutine. There's no CPU switch, so the data that's being sent is in the cache the service process will need. This minimizes the penalty for data copying; the message being copied is usually in the highest level cache.
Inheriting the sender's priority avoids priority inversions, where a high-priority process calls a lower-priority one and stalls. QNX is a real-time system, and priorities are taken very seriously. MsgSend/Receive is priority based; higher priorities preempt lower ones. This gives QNX the unusual property that file system and network access are also priority based. I've run hard real time programs while doing compiles and web browsing on the same machine. The real-time code wasn't slowed by that. (Sadly, with the latest release, QNX is discontinuing support for self-hosted development. QNX is mostly being used for auto dashboards and mobile devices now, so everybody is cross-developing. The IDE is Eclipse, by the way.)
Inheriting the sender's CPU quantum (time left before another task at the same priority gets to run) means that calling a server neither puts you at the end of the line for CPU nor puts you at the head of the line. It's just like a subroutine call for scheduling purposes.
MsgReceive returns an ID for replying to the message; that's used in the MsgReply. So one server can serve many clients. You can have multiple threads in MsgReceive/process/MsgReply loops, so you can have multiple servers running in parallel for concurrency.
This isn't that hard to implement. It's not a secret; it's in the QNX documentation. But few OSs work that way. Most OSs (Linux-domain messaging, System V messaging) have unidirectional message passing, so when the caller sends, the receiver is unblocked, and the sender continues to run. The sender then typically reads from a channel for a reply, which blocks it. This approach means several trips through the CPU scheduler and behaves badly under heavy CPU load. Most of those systems don't support the many-one or many-many case.
Somebody really should write a microkernel like this in Rust. The actual QNX kernel occupies only about 60K bytes on an IA-32 machine, plus a process called "proc" which does various privileged functions but runs as a user process. So it's not a huge job.
All drivers are user processes. There is no such thing as a kernel driver in QNX. Boot images can contain user processes to be started at boot time, which is how initial drivers get loaded. Almost everything is an optional component, including the file system. Code is ROMable, and for small embedded devices, all the code may be in ROM. On the other hand, QNX can be configured as a web server or a desktop system, although this is rarely done.
There's no paging or swapping. This is real-time, and there may not even be a disk. (Paging can be supported within a process, and that's done for gcc, but not much else.) This makes for a nicely responsive desktop system.
The design is great. I get most of it. Security is a big, use case for microkernels these days. One part that jumps out at me is that receiver inherits sendor's priority and quantum during a transfer of control. I'm guessing the kernel manages the inheritance in storage only it controls, right? Otherwise, I could see issues coming up.
So, it made a robust desktop, eh? I considered getting a copy of it but didn't have time. What do you think about integrating a knock-off of this kernel with MINIX 3 or GenodeOS? They seem to be the furthest ahead of the open, microkernel-based systems at getting to something usable.