Comment by vacuity

> The kernel and memory manager: probably yes, the device drivers: not necessarily, the language runtimes for managed languages: not necessarily.

The parts that touch hardware (or similarly bare kernel interfaces) must be so. Sure, you could split device driver implementations and so on, but somewhere there's a meaningful lower level of software within the system.

> Because any other solution will quickly run into chicken-and-the-egg style problems.

No. The kernel must provide for context switching. It would be like migrating threads IPC, but one-way. No threads, no scheduler, no dedicated data transfer. In other words, the bare minimum necessary to make a sensible abstraction around switching processes.

seL4, according to its developers, is not absolutely a microkernel. I believe the rationale mainly points to the in-kernel scheduler, but seL4's IPC interacts with the scheduler and is noticeably more elaborate than a mere context switch. Even if seL4's IPC is, by most standards, minimal, I do not consider it to be so objectively. I described a meaningfully more minimal alternative.

Delegating scheduling to userspace is trivial. If necessary, designate a scheduler to run if no scheduling decision is available. It has been done before, and the only usual objection is performance.

> No. A syscall is usually defined as a call to a ring one level in from the one where you currently are. But lots of things that are syscalls right now do not necessarily have to be.

Just as hardware interrupts can be abstracted into messages, syscalls can be abstracted into messages. I'm not saying that the hardware implementation directly conforms to the abstraction.