EFI's GPT disklabels present a number of benefits to the traditional MBR scheme. For instance, not having to deal with CHS addressing, better data integrity (including a backup header as data redundancy) and 64bit LBA addressing, allowing partitions to go beyond the 2Tb limit all the way up to 9.4 Zb.
These nice features don't come free, however, having to deal with older legacy systems (normally BIOS-based) that only use MBR, and do not know about GPT. For example, users who have an EFI system (say a Mac), dual booting with an older, non-EFI version of Windows. While OSX knows GPT and uses the GPT partition(s), Windows doesn't, so you cannot dual boot without creating a hybrid MBR - the standard protective MBR (pMBR) won't allow Windows to boot. This hybrid MBR will extend the regular pMBR (containing a 0xEE GPT partition) so that it contains up to three primary partitions that point to the same disk locations that the GPT partitions point to. Hybrid MBRs are unofficial workarounds to the GPT specs, but necessary for backward compatibility. Furthermore, most bootloaders are now acknowledging this kind of scheme.
In order for Linux to properly discover protective MBRs, it must be made
aware of devices that have hybrid MBRs. To this end, Linux v3.12 will now be able to detect these partitioning schemes.
Furthermore, the kernel will no longer require the GPT partition to begin at sector 1, enabling Linux to be more flexible when
probing for GPT disklabels. Linux was the only OS that enforced this, and apart from it not being enforced by UEFI, it caused Linux to potentially
fail to detect valid partitions on the disk.
For compatibility reasons, if the first partition is hybridized, the 0xEE
partition must be small enough to ensure that it only protects the GPT
data structures - as opposed to the the whole disk in a protective MBR. Note that these changes do not affect already existing partitions.
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