EFI is this other Intel brain-damage (the first one being ACPI). It's
totally different from a normal BIOS, and was brought on by ia64, which
never had a BIOS, of course.
Sadly, Apple bought into the whole "BIOS bad, EFI good" hype, so we now
have x86 machines with EFI as the native boot protocol.
The original EFI code in the kernel basically duplicates all the BIOS
interfaces (ie everything that looks at a memory map comes in two
varieties: the normal and tested BIOS e820 variety, and the usually broken
and hacked-up EFI memory map variety).
Translating the EFI memory map to e820 is very much the sane thing to do,
and should have been done by ia64 in the first place. Sadly, EFI people
(a) think that their stinking mess is better than a BIOS and (b) are
historically ia64-only, so they didn't do that, but went the "we'll just
duplicate everything using our inferior EFI interfaces" way.
- LKML, July 2006