Problems arises when supporting EFI + GRUB2 + Xen with Multiboot2 boot specification

Previously I wrote a post to discuss the limitations for Multiboot boot specification, today I saw that XEN hypervisor also has the similar problems. 

Multiboot Specification and limitations (Bootloader -> OS)

Typically in a computer system, the boot sequence is like this: firmware -> bootloader -> Operating System.  The problem is that nowadays we have the largest variety of operating systems, boot loaders, and firmwares (UEFI/Legacy), and even some of them are proprietary software that are not public. So, the interaction might be a problem if the interface is not well-defined.