HowTo: Making a bootable ISO w/ miniroot.kmod etc.

I've asked how building a bootable ISO works now that we have kernel
modules and a bootloader that can load kernels before the kernel,
e.g. for a ramdisk. After some digging, I've managed to fit the pieces
together, get things going[1], and here is a write-up so others that
are intestested don't have to wade through that swamp again.

Old (pre-5.0/current):

  * build INSTALL kernel, which reserves fixed space for a ramdisk
    image (pseudo-device md, options MEMORY_DISK_HOOKS,
    MEMORY_DISK_IS_ROOT, MEMORY_DISK_SERVER=0, MEMORY_DISK_ROOT_SIZE)
  * build ramdisk image (done in src/distrib/i386/ramdisks). This uses
    crunchgen(1) and makefs(8) to assemble things, configuration is in one
    or several "list" files.
  * insert ramdisk image into kernel to get an instkernel (done in
    src/distrib/i386/instkernel). This is done via mdsetimage(8).
  * build ISO with instkernel and bootloderer (done in
    src/distrib/i386/cdroms/bootcd). This uses makefs(8) again.

New (5.0/current):

  * build GENERIC kernel, which tells there will be a ramdisk, but size
    will be detected at load-time (pseudo-device md, options
    MEMORY_DISK_HOOKS, MEMORY_DISK_DYNAMIC)
  * build ramdisk image as before (in src/distrib/i386/ramdisks)
  * build the miniroot.kmod (in src/sys/modules/miniroot). This is an
    empty module for a start, which will get the ramdisk appended in
    the next step.
  * add the ramdisk image to the miniroot.kmod module (done in
    src/distrib/i386/kmod). This uses the GNU bintools objcopy(1) to
    append the ramdisk into a special ELF section.
  * prepare an instkernel that is basically just a compressed version
    of the GENERIC kernel (done in src/distrib/i386/instkernel). This
    is the same code as before, but for each kernel, variables can be
    set to prevent it from getting stripped and a ramdisk image
    inserted (MDSET_NOSTRIP.netbsd-GENERIC,
    MDSET_NOIMAGE.netbsd-GENERIC).
  * build ISO with the instkernel kernel, the miniroot.kmod file, the
    bootloader and a config file for the bootloader (done in
    distrib/i386/cdroms/bootcd).

Enjoy!


  - Hubert

P.S.: I wonder if/how module loading works in a PXE boot environment.
       anyone got experiences with that?


[1] g4u 2.4alpha3 is available for testing
     http://www.feyrer.de/NetBSD/blog.html/nb_20081122_1744.html