Simultaneous EFI and Legacy bootloader installation

[Lennart Sorensen]

On Wed, Mar 30, 2016 at 09:16:35AM -0500, Mario Limonciello wrote:
> Can you comment what version of Windows you had noticed this behavior? 
> We actually factory install as old as Windows 7 with GPT disks in legacy
> mode at Dell.  We don't factory install Windows 8 or Windows 10 in
> legacy mode.

Well certainly with Windows 7 and Windows 10, if you boot legacy mode,
you don't get an option to use GPT.  And everything I have ever read
says you can't do that with Windows because Microsoft says so.
Certainly the hybrid partition table option is a problem since Linux
and BSD prioritize GPT while windows prioritizes MBR.

Every answer I can find to how to boot windows on GPT with legacy involves
using an MBR USB key or a floppy as the boot media with the main install
on GPT (but in that case it is in fact NOT booting from GPT).

I can't find anything anywhere that says you can boot on a legacy system
from GPT with Windows 7, 8, 8.1 or 10.  I can only find lots of people
asking how to do it and being told you can't without a separate boot
device that isn't GPT.

> Shouldn't it be possible to install GRUB into the partition boot record
> (PBR) of the ESP?

Certainly when I have used grub to run legacy systems with GPT due to
large disks, I have used the BIOS Boot partition and grub has been happy,
while without it you need to use the block map mode that grub highly
discourages because it is as fragile as lilo always was.  The BIOS Boot
partition contains the stage2 of grub raw, it does not have a filesystem
unlike the ESP.  To put stage2 on the ESP you would have to then use
the block map mode which defeats the purpose of using it and you might
as well just block map the file in /boot.  Since you don't want to use
the block map mode, the BIOS Boot Partition is the better option when
you want grub on a legacy system with GPT.

The < 500 bytes in the PBR is way too small for stage2 of grub and no
better than using the MBR area (which works for a GPT disk too).
It could only hold stage1.

> Yes, this scenario is why I was recommending in legacy mode to install
> the removable path bootloader (\efi\boot\boot$ARCH.efi) by default. 

-- 
Len Sorensen