L4/Darwin (aka Darbat)

The L4/Darwin project was an experimental port of Darwin to the L4 microkernel to study the characteristics of a large-scale microkernel-based system. It includes a port of IOKit to L4, a modified libc to communicate with the Darbat Server, and of course XNU with many of the machine-dependent parts heavily modified (pmap, thread/task creation, etc) but much left unchanged (most of BSD, and large parts of OSFMK work without modification).

Current Plans

The aim was for Darbat to be fully binary compatible with existing userlevel Darwin binaries. There were also plans to integrate paravirtualised L4 Linux (Wombat) to run side-by-side with Darbat on the same machine and allow them to communicate via L4 IPC.

Given the amount of existing OS X code that depends on Mach, we are currently not aiming at completely removing Mach from Darwin. Instead we are modifying Darwin to make better use of L4 and profit from its performance. This includes:

• Translating current MIG-based RPC calls to make direct L4-IPC calls, bypassing much of the Mach IPC infrastructure
• Modifying launchd's launch_msg primitive, and other services, to use L4 IPC directly
• Providing a user-level library for optimising certain types of mach_msg calls to go directly via L4 IPC (without indirecting through the L4/Darwin kernel server)
• Modifying the Mach scheduler to move the higher-frequency scheduling decisions into L4

Driver support

Driver support is provided by the IOKit. A range of drivers are supported, including PIIX ATA, USB (keyboard and mouse, USB stick), Intel 8255x ethernet, and generic serial. There is almost complete support for binary-loading of existing drivers.

Status

Supported Hardware

The following systems are supported through a FireWire based bootloader and debug console. Support for the builtin display and USB keyboard is available to use the shell.

• Intel Mac Mini (Core Solo)
• Intel iMac (17", 1.83GHz Core Duo)
• Intel iMac (20", 2GHz Core Duo)
• MacBook (White, 1.83GHz Core Duo)
• MacBook Pro (15.4", 1.83GHz Core Duo)

While other systems may work, we haven't tested them yet.

Software support

Darbat boots into single user mode (launchd -s) and is able to run a range of commands including (but in no way limited to):

• ps
• cat
• vim (although display is slightly buggy)
• emacs (requires 10.4.7 on the host machine)
• make
• gcc/as (some programs generated by gcc can also be run)
• python
• perl

Things that still need to be implemented are:

• support for more advanced commands (eg top, gdb)
• BSD signals (currently only very specific cases of signal delivery are supported)
• Kernel preemption
• Timer interrupts and clock support

There is full read/write support for the internal HD, and for testing purposes it is best to mount this drive and run commands directly from it. Currently 10.4.6 binaries found on release hardware will load successfully in Darbat.

Downloads

Darbat is available as both source and binaries. We are currently at Release 0.2. For more details and downloads, see the release page.

Conference Publications

Geoffrey Lee and Charles Gray L4/Darwin: Evolving UNIX Conference for Unix, Linux and Open Source Professionals (AUUG), Melbourne, Vic, Australia, October, 2006 Slides

Theses

		Joshua Root Virtualising Darwin on L4 BE Thesis, School of Computer Science and Engineering, Sydney, Australia, February, 2007
		Tom Birch Performance limits of Darwin on L4 BE Thesis, School of Computer Science and Engineering, Sydney, Australia, October, 2006
		Geoffrey Lee I/O kit drivers for L4 BE Thesis, School of Computer Science and Engineering, Sydney, Australia, November, 2005
		Ka-shu Wong MacOS X on L4 BE Thesis, School of Computer Science and Engineering, Sydney, Australia, December, 2003