Re: UEFI/"BIOS" booting, was Re: USB Install Fails, Complains about CD-ROM

[David Wright <deblis@lionunicorn.co.uk>]

On Wed 16 May 2018 at 00:35:33 (+0200), Pascal Hambourg wrote:
> Le 15/05/2018 à 03:23, David Wright a écrit :
> >
> >In this particular instance (the Lenovo), its PDF says:
> (...)
> >     The default boot mode for your computer is UEFI mode. If you need to
> >     install a legacy operating system, such as Windows (that is, any operating
> >     system before Windows 8)
> 
> This is so wrong. Windows has supported EFI since Vista.

I offer no defence of what Lenovo choose to write. What I do know is
that in order to make this laptop available to myself, I had to be
able to install linux without disturbing the Windows/GPT/UEFI setup
already present.

Reading the page at
https://www.happyassassin.net/2014/01/25/uefi-boot-how-does-that-actually-work-then/
would have been most discouraging, so I'm glad I didn't chance upon
it and, in the light of my experience, I consider some of *it* to be
garbage, a word it's happy to use to condemn 95% of what's written
about UEFI elsewhere on the internet.

But then, I also chose to ignore what's written in the Debian
Installation Guide §3.6.3:
    "Booting from a disk with GPT is only possible in native UEFI mode".

> >AIUI what's unspecified is what you choose to put in the code section
> >of the MBR. In my case, and probably for most people here, it will be
> >Stage 1 of Grub.
> 
> Note : "stage 1" is a GRUB legacy thing. GRUB 2 calls it "boot image".

I wasn't aware that the "Stage 1" name was eliminated. I don't find
"boot image" adding any clarity to the process. Anyway, someone needs
to tell https://en.wikipedia.org/wiki/GNU_GRUB which says:

  Version 2 (GRUB)
    Stage 1: boot.img is stored in the master boot record (MBR) or
    optionally in any of the volume boot records (VBRs), and addresses
    the next stage by an LBA48 address (thus, the 1024-cylinder
    limitation of GRUB legacy is avoided); at installation time it is
    configured to load the first sector of core.img.

> >And if one has any sense, that will be pointing to a
> >BIOS boot partition, which is one of the first things I set up on a
> >GPT disk so that I can use it in my old BIOS machines. It makes things
> >far more bullet-proof if Grub knows there's a safe place to put its
> >later Stages. I suspect that systems without ones are likely to be the
> >ones causes much of the reported trouble.
> 
> Moreover, GRUB BIOS cannot be installed without embedding (in a BIOS
> boot partition or any other suitable location) when the /boot
> filesystem is anywhere else than a plain partition (LVM, RAID,
> crypto...) or a filesystem type which may move block around such as
> btrfs, defeating the hardcoded blocklists.

Agreed: that's why I'm making sure I add a BIOS boot partition to
any disk I convert from MBR to GPT, allowing it to be a candidate
for booting from.

Cheers,
David.