disk: handle adding PReP partition on PPC64/s390x (HMS-8884)

[mvo5]

This commit adds support for PPC64/s390x in the
addPartitionsForBootMode. Here we need to handle
things slightly differently from x86_64/aarch64:

We need to add the PReP partition type automatically
on ppc64le when disk customizations are used.

When the user provides an "incomplete" partition table in
the disk customizations (e.g. missing the bios boot partition
on x86_64) we automatically add it. This support was missing
for the ppc64le and the PReP partition (type "41").

Note that this change also means we no longer add an EFI
partition on s390x/ppc64le. Adding them was a side-effect
of the code in addPartitionsForBootMode() but according
to wikipedia [0] UEFI is not specificied for powerpc CPUs.

Closes: https://issues.redhat.com/browse/HMS-8884

[0] https://en.wikipedia.org/wiki/UEFI#Compatibility

[mvo5]

Fwiw, I tested this manually via bp-ppc.toml.txt

$ sudo ./image-builder build -v --blueprint=bp-ppc.toml --arch ppc64le server-qcow2
$ qemu-system-ppc64   -machine pseries   -m 4096   -smp 2   -hda fedora-42-server-qcow2-ppc64le.qcow2   -netdev user,id=net0,hostfwd=tcp::2222-:22   -device virtio-net-pci,netdev=net0   -vga std
[boots into multiuser]
$ guestfish -a fedora-42-server-qcow2-ppc64le.qcow2
><fs> man
><fs> list-filesystems 
/dev/sda1: unknown
/dev/sda2: xfs
/dev/rootvg/rootlv: ext4
/dev/rootvg/swaplv: swap
/dev/rootvg/tmplv: ext4
/dev/rootvg/varlogauditlv: ext4
/dev/rootvg/varloglv: ext4
/dev/rootvg/varlogtmplv: ext4
/dev/rootvg/varlv: ext4

[lzap]

Is my understanding correct that this partition is not user facing, thus no UBP change is needed? I do not see it in the BP as well so that makes me to believe that.

[supakeen]

I don't think a PReP partition is necessary on s390x, it's purely a ppc64le thing. The s390x partition table needs a type 83 partition of size 1 GiB I think?

This is what I see the partition tables as on Fedora Server:

€ fdisk -l ppc64le.raw 
Disk ppc64le.raw: 10 GiB, 10737418240 bytes, 20971520 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 817C70CD-2D3A-4A86-AF15-FB671EAC686A

Device         Start      End  Sectors  Size Type
ppc64le.raw1    2048    18431    16384    8M PowerPC PReP boot
ppc64le.raw2   18432  2066431  2048000 1000M Linux extended boot
ppc64le.raw3 2066432 20971486 18905055    9G Linux root (PPC64LE)

€ fdisk -l s390x.raw  
Disk s390x.raw: 10 GiB, 10737418240 bytes, 20971520 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xaefb5223

Device     Boot   Start      End  Sectors  Size Id Type
s390x.raw1         2048  2050047  2048000 1000M 83 Linux
s390x.raw2      2050048 20971519 18921472    9G 8e Linux LVM

[mvo5]

I don't think a PReP partition is necessary on s390x, it's purely a ppc64le thing. The s390x partition table needs a type 83 partition of size 1 GiB I think?
[..]

Indeed, nice catch! I misread https://github.com/osbuild/images/blob/v0.164.0/pkg/distro/defs/fedora/distro.yaml#L421 but that is just the boot not the bios, my bad.I will remove this part. I guess we still don't need to add UEFI there, but I guess that should be a separate PR/commit.

[mvo5]

Is my understanding correct that this partition is not user facing, thus no UBP change is needed? I do not see it in the BP as well so that makes me to believe that.

Yes, that is correct, this is a partition that is automatically and transparently added if missing.