sneaky dual-boot [LWN.net]

sneaky dual-boot

Posted Apr 25, 2022 0:46 UTC (Mon) by pabs (subscriber, #43278)
In reply to: sneaky dual-boot by ballombe
Parent article: Fedora considers deprecating legacy BIOS

What about the opposite; is it possible to set up a system so that it can boot in either UEFI or BIOS mode? IIRC current install systems don't do that, but can themselves be booted in either UEFI or BIOS mode.

sneaky dual-boot

Posted Apr 27, 2022 11:57 UTC (Wed) by plugwash (subscriber, #29694) [Link] (1 responses)

Many distro install images nowadays support four different boot methods.

Legacy HD-media
Legacy optical media
UEFI HD-media
UEFI CD

In principle it would be possible for an installed system to support both UEFI and Legacy. I do see a few issues though.

1. Operating systems using UEFI installed on fixed drives are supposed to register themselves with the firmware(which requires the installer to berunning in UEFImode) rather than relying on a fixed entry point on the drive. In practice it's possible to boot using the "removable media path" even on a fixed drive but it's not the official way to do things.
2. While the combination of BIOS boot and GPT is technically feasible it's notexactly standard, I'm not sure if grub supports it or if a hybrid partion table (which is it's own can of worms) is needed.
3. There is no gaurantee that what works in one mode will work in the other.
4. Multiboot becomes even more of a can of worms than usual.

sneaky dual-boot

Posted Apr 27, 2022 14:55 UTC (Wed) by jem (subscriber, #24231) [Link]

While the combination of BIOS boot and GPT is technically feasible it's notexactly standard, I'm not sure if grub supports it or if a hybrid partion table (which is it's own can of worms) is needed.

GRUB supports booting from a GPT disk in BIOS mode, but you will need an extra BIOS boot partition. A disk with an MBR partition table usually contains a gap between the boot record and the first partition, which GRUB takes advantage of. GPT-partitioned disks do not (typically) have this "no man's land", so a separate partition is used instead. Using a partition is cleaner anyway, and GPT also does not have a practical limit on the number of partitions a disk can contain, so one extra partition doesn't make a big difference.