In https://bugzilla.redhat.com/show_bug.cgi?id=2328590 we got into a rabbit hole around disk label (partition table) stuff, where we wound up enhancing Kiwi to allow use of straight GPT and also 'hybrid' disk labels on live images. For now we set Fedora lives to use a straight GPT disk label, as this is the same thing livemedia-creator does. However, we might consider using a 'hybrid' disk label. This is what Ubuntu does. In this config, xorriso writes a GPT with a 'protective' MBR as in a straight GPT setup, but the MBR is a bit out of spec. As well as the single, ee-type partition required by the UEFI spec, it contains an additional partition of type 00 and with the 'boot' flag set.

This can help the image to boot on CSMs and real BIOS firmwares which might refuse to boot the disk if they see no partition with a 'boot' flag. We're aware of some real-life examples of this - or rather, xorriso's maintainer, Thomas Schmitt, cited some - but they're rare and old.

Theoretically it might prevent the image booting in UEFI mode on a firmware which refuses to boot a disk that does not exactly comply with the UEFI spec's 'protective MBR' requirements, but we're not aware of any such case in real life.

To do this, we'd just have to set the gpt_hybrid_mbr setting, and use a Kiwi version with https://github.com/OSInside/kiwi/pull/2695 merged or backported.

Tagging @bcl for thoughts as he's involved in the maintenance of the other tools for this (lmc and osbuild).