Re: [patch] improved multiboot support in kernel and boot(8)

[Grégoire Sutre <gregoire.sutre%gmail.com@localhost>]

rjs%fdy2.demon.co.uk@localhost wrote:

> The existing multiboot code was only intended to be enough to load Xen,
> the expectation was that native kernels would still be loaded using
> the old method.

But the NetBSD kernel is also multiboot-compliant.  Wouldn't it be
better if multibooting the kernel from the boot-loader also worked?

> What is the benefit of adding your patch ?

Let me quote the boot(8) man page [1]:

   NetBSD multiboot kernels
           A NetBSD kernel that was built with options MULTIBOOT
           (see multiboot(8)) may be booted with either the boot
           or multiboot command, passing the same arguments in
           either case.

   Non-NetBSD kernels
           A kernel for a non-NetBSD operating system that expects
           to be booted using the multiboot protocol (such as by
           the GNU ``GRUB'' boot loader) may be booted using the
           multiboot command.  See the foreign operating system's
           documentation for the available arguments.

IMHO, the patch makes the implementation closer to the man page.

> > To properly set the boot_device field of the MBI, the DOS partition
> > number is now stored in the biosdisk structure in biosdisk.c.
>
> What happens if there is no DOS partition table on the boot disk ?

Indeed, this is a delicate point.  I find the multiboot specification
ambiguous on the boot_device field (section 3.3 in [2]).  I asked on the
grub-devel mailing list [3], but I did not get any answer yet.  The way
GRUB does it would give a clue, but I couldn't make GRUB accept disks
that only have a NetBSD disklabel.

So the patch makes the following choice (which is consistent with the
kernel part):

- the NetBSD disklabel partition (a, b, ...) shall always be part2,
  whether the NetBSD disklabel is top-level or contained in a DOS
  partition.

- the DOS partition number is stored in part1, and this field defaults
  to 0xFF if there is no DOS partition table.

- part3 is set to 0xFF.

An alternative solution would be to store the NetBSD disklabel partition
in part1 (and set part2 = part3 = 0xFF) when the NetBSD disklabel is the
top-level one.

What do you think?

Grégoire

[1] http://netbsd.gw.com/cgi-bin/man-cgi?boot++NetBSD-current
[2] http://www.gnu.org/software/grub/manual/multiboot/multiboot.html
[3] http://lists.gnu.org/archive/html/grub-devel/2010-01/msg00351.html