Well, Mill is vaporware but its portals would be a (micro-kernel ready but Linux-usable) syscall boundary, defining at least the core of the ABI -- and they're

Well, Mill is vaporware but its portals would be a (micro-kernel ready but Linux-usable) syscall boundary, defining at least the core of the ABI -- and they're pretty much just a flavor of the regular function call on the platform.