Bootstrap System for i386

Boot modules: The new Mach boot mechanism introduces the concept of "boot modules". Basically, any number of "boot modules" can be thrown together to create a single boot image. That boot image is what LILO or whatever loads, and when the system starts up, all of the boot modules will be already in memory and immediately available. At least one of those modules is the microkernel itself; others may be device drivers, other kernel modules, user-level servers (e.g. the bootstrap code), etc. Currently only the kernel itself and the single server bootstrap program go into the boot image, but that'll probably change soon. I'd like to be able to get rid of the server bootstrap code entirely, and simply embed the appropriate server(s) directly into the boot image: for example, the LITES, or the basic Hurd administration servers (exec, proc, etc.) and a file server for the root filesystem. Eventually it should be possible (though optional) to throw an entire root filesystem snapshot into the boot image, so upon startup you can get a Unix shell and a minimal set of utilities without depending on the successful initialization of any devices other than the console.

The exact structure of a boot module and a boot image is by nature somewhat system-dependent. On the i386, a boot module is a relocatable object file created with the `mkbmod' script. A boot module must contain no external references (all externs must be resolved), and usually must have all symbols stripped, leaving only relocations to be fixed up. To facilitate debugging of boot modules, _one_ boot module in a boot image may be left unstripped; you can tell `mkbmod' to leave a boot module unstripped by giving it the `-no-strip' option.

Creating boot modules: The `mkbmod' program assumes when it creates boot modules that certain entry points exist in the object. It links a bootmodule specific runtime that calls the routine `mach_bmod_init' at boot time. (Actually, the kernel calls this routine by following pointers stored in the boot module header.) This init routine returns information about the object file that `kernel/kern/bootstrap.c' uses to execute the module in a thread.

Encapsulating user-level programs in boot modules: The `mkubmod' command takes a more conventionally linked program (a linked executable) and adds this `mach_bmod_init' routine to it. This allows the kernel to load some arbitrary linked executable into a user-level Mach task and start it running before any personality servers are available. The executable you give to `mkubmod' can be in any of the standard i386 a.out variants that Mach understands, including:

Once your program is compiled and linked, use mkubmod to embed it in a bmod object and you can add it to your larger boot image.

Boot images: A boot image is simply a set of boot modules glued together with the `mkbootimage' script, with some extra header/trailer code linked in with it (see mach4-i386/boot/bootimage_*), with all relocations fixed, and with a LILO-compatible boot sector and setup program tacked onto the front.

Boot module installation: All boot modules created, along with the auxiliary stuff needed to create boot images, gets thrown into the directory /lib/mach-boot. The idea is that some or all of the boot modules in .../lib/mach-boot will be glued together into a single boot image to be placed on the root filesystem in `/' or `/boot' or whatever. The boot modules and other files used in constructing boot images don't themselves need to be available immediately on bootup, so they can be located in the "usual" places, even shared among multiple machines. Eventually, the mach-boot directory will probably contain a whole slew of boot modules (e.g. both uniprocessor and SMP kernels, various device drivers, file system servers for various file system formats, etc.). You will then be able to type something like `mkbootimage -o /boot/machboot /usr/lib/mach-boot/*.bmod' and get a fully functional, generic boot image with "all" the possible features and device drivers; or you might include only specific modules to get a stripped-down image.