alt Hacker NewsI believe that's an accident of the evolutionary path chosen with syscalls. If we'd instead gone with a ring buffer approach to make requests, then you'd never need to partition the memory address space; the kernel has its memory and userspace has its and you don't need the situation where the kernel is always mapped.
Hmm. I don't understand how that would work.
I think it would be possible for e.g. microkernels to greatly reduce the size of the reservation (though not to eliminate it entirely). However, I can't imagine how you would handle the privilege escalation issue without having at least some system code in the application's virtual address space that's not modifiable by the application.