GPT aware boot loader &mdash how to install and use

Bootloader structure overview

First of all please note that despite of GPT in it's name the loader is not related to EFI/UEFI. The main goal of the loader is to provide ability to bootstrap the NetBSD from GPT partitioned disk on a PC BIOS computer.

By nature the GPT loader is very similar to usual NetBSD MBR loader. It comprised of the following three parts:

mbr_gpt or mbr_gpt_com0: LBA0 loader intended to be loaded by a PC BIOS. It's main purpose — find bootable GUID partition and pass control to its PBR. GPT have no bootable flag like on MBR, but by convention this role is assigned to the EFI System Partition type. Also, you can make mbr_gpt read PBR from any GPT partition.
bootxx_fat16: PBR loader which can be executed by either MBR loader or BIOS. Primary target for PBR is locate and execute the NetBSD boot(8) program. It was decided to follow UEFI logic and put the loader on the EFI System Partition which should be FAT formatted (per UEFI specification).
boot: NetBSD kernel loader. The secondary loader job is loading and execution of the NetBSD kernel from either a GUID or disklabel partition.

How to build the loader

In order to build and install the loader you should have the following:

syssrc, the NetBSD-current kernel sources.
sbin/gpt and usr.sbin/installboot, the loader installation tools, also -current versions
latest GPT bootloader patch. Attention: please note that despite the latest sources obviously used, the patch itself might be somewhat obsolete. Please check datestamp in the patch file.

The following steps are describing what and how you'd need to build:

Download all the sources above.
Prepare src tree to patching by making directories missing. See list of the directories inside the patchfile:
```
$ awk '/^WARNING:/ {print $(NF)}' gptboot.patch
```
Apply the patch.
Build GPT loaders and tools at the following directories:
- sys/arch/i386/stand/mbr/mbr_gpt
- sys/arch/i386/stand/fatboot/fat16
- sys/arch/i386/stand/boot/biosboot
- sbin/gpt
- usr.sbin/installboot
All of above builds just fine on NetBSD 5.0 (including amd64) without cross compilation. The installboot may require passing -DSMALLPROG to make(1) to exclude extra stuff and simplify the build process. Also, new sys/sys/bootblock.h has to be used in place of /usr/include/sys/bootblock.h.

Loader installation

First of all prepare your disk. The disk should be GPT partitioned and have at least two partitions: one for NetBSD, and one for boot loader:

# gpt create sd0
# gpt add -s 65536 -t efi sd0
# gpt add -t ffs sd0
# gpt show
then issue a set of dkctl addwedge commands or reattach the disk to configure dk wedges automatically

NB: wedges are not supported on vnd(4) devices.

Next, format partitions accordingly:

# newfs_msdos -F 16 /dev/dk0
# newfs /dev/dk1

Please note that newfs_msdos seems have a bug and can incorrectly determine filesystem size (check number of file sectors reported by newfs_msdos — it must be less or equal to partition size). It it appears, please reformat filesystem explicitly specifying correct fs size via newfs_msdos -s option.

Install loaders:

# .../gpt biosboot -c $NETBSDSRC_DIR/sys/arch/i386/stand/mbr/mbr_gpt/mbr_gpt sd0
# .../installboot /dev/rdk0 $NETBSDSRC_DIR/sys/arch/i386/stand/fatboot/fat16/bootxx_fat16
# mount -t msdos /dev/dk0 /mnt
# cp $NETBSDSRC_DIR/sys/arch/i386/stand/boot/biosboot/boot /mnt
# echo "menu=Boot NetBSD:boot hd0b:netbsd" > /mnt/boot.cfg
# umount /mnt

The gpt(8) will automatically find EFI System Partition on sd1 and instruct mbr_gpt where to load PBR from. But rest loaders should be installed on dk wedges. If you're confused about the names, you may use gpt(8) on a wedge, but in this case mbr_gpt will load PBR from the specified wedge:

# .../gpt biosboot -c $NETBSDSRC_DIR/sys/arch/i386/stand/mbr/mbr_gpt/mbr_gpt dk0

Install kernel and base system files:

# mount /dev/dk1 /mnt
# cp /netbsd /mnt
then create usual NetBSD hierarchy: /dev, /etc, /sbin, etc...
and specify root partition as it will be enumerated by DKWEDGE_AUTODISCOVER:
# echo "/dev/dk1 / ffs rw 1 1" > /etc/fstab
# umount /mnt

Please note disk names on previous steps. It might be somewhat confusing, so there is some explanation:

gpt biosboot ... sd0: LBA0 means installation in very first sector of physical disk, so we should specify parent device of our GPT wedges. But a dk device can also be used. In that case mbr_gpt will look for GUID partition matching the dk device at moment of installation.
installboot /dev/dk0 ...: bootxx_fat16 should be installed onto EFI System Partition. See gpt add commands earlier.
mount -t msdos /dev/dk0 ...: the boot(8) should be stored on EFI System Partition as well.
echo "menu=Boot NetBSD:boot hd0b:netbsd" ...: the hd0b means the second partition, which matches dk1 after boot. Please see todo list about that.
rest commands are referring to dk1 which is NetBSD FFS partition.

Reboot. Have fun.

Problems

The loader still work in progress so please check todo, bugs, and caveats page for possible misbehavior. Anyway, please feel free to contact me in case of problems, and also with ideas or comments.

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Mike M. Volokhov (mike.volokhov (at) gmail.com)
$Id: howto.html 38 2009-10-16 08:38:31Z mishka $