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




On Sun 13 May 2018 at 19:08:48 (+0200), Pascal Hambourg wrote:
> Le 13/05/2018 à 17:18, David Wright a écrit :
> >On Fri 11 May 2018 at 15:13:04 (-0500), Kent West wrote:
> >>
> >>That's good to know. I guess my source material (
> >>https://www.happyassassin.net/2014/01/25/uefi-boot-how-does-that-actually-work-then/)
> >>is wrong. Or I misunderstood it.
> >
> >While a lot of the detail on that long page might be correct, there
> >are also statements there that don't seem to agree with reality.
> 
> Most of the statements you quoted agree with my (admittedly limited)
> experience with UEFI. There is a difference between the theory
> (specifications) and the reality (implementations), and some pieces
> of software may have extra requirements beyond the sole UEFI
> specification.
> 
> >     "I really can’t recommend strongly enough that you do not attempt
> >     to mix UEFI-native and BIOS-compatible booting of
> >     permanently-installed operating systems on the same computer, and
> >     especially not on the same disk. It is a terrible terrible idea
> >     and will cause you heartache and pain. If you decide to do it,
> >     don’t come crying to me." (under "UEFI booting: background").
> 
> I would not be as much adamant as the author, but my experience says :
> it can work, but expect trouble.
> 
> Most of my early experience with UEFI boot comes from a rather old
> Intel motherboard. Beside crippled UEFI support (no UEFI boot from
> USB or SATA in AHCI mode), it had a couple of annoying requirements :
> - boot in legacy mode only if the MBR contains a partition entry
> with the boot flag set, regardless of whether the disk has a MSDOS
> or GPT partition table. This behaviour is beyond any common BIOS
> standard, but I have observed it on many other systems, mostly Dell
> and HP ;

In the case of GPT, I assume the partition entry with the boot flag set
is the protective MBR.

> - boot in EFI mode from GPT only if the protective partition entry
> in the MBR has the boot flag unset. I admit this requirement is part
> of the GPT specification, but really do not see the point in
> enforcing such a minor detail.
> 
> Anyway, these two requirements put together make it impossible to
> boot in both legacy and EFI mode from the same GPT disk with this
> motherboard. However they allow to boot in both modes from the same
> MSDOS disk. But who still wants to use MSDOS format nowadays ?

Is it impossible, then, to change the boot flag in a protective MBR?

> >     "Disk formats (MBR vs. GPT)
> >
> >       Here’s another very important consideration:
> >
> >       If you want to do a ‘BIOS compatibility’ type installation, you
> >     probably want to install to an MBR formatted disk. If you want to
> >     do a UEFI native installation, you probably want to install to a
> >     GPT formatted disk."
> 
> I do not agree so much with this one when it comes to install
> GNU/Linux. But it is an absolute requirement when installing
> Windows.

Yes, though I assume few people install Windows. It's more likely to
be pre-installed.

> >     "A specific example
> >
> >     To boil down the above: if you bought a Windows 8 or later system,
> >     you almost certainly have a UEFI native install of Windows to a
> >     GPT-formatted disk. This means that if you want to install another
> >     OS alongside that Windows install, you almost certainly want to do
> >     a UEFI-native installation of your other OS. If you don’t like all
> >     this UEFI nonsense and want to go back to the good old world
> >     you’re familiar with, you will, I’m afraid, have to blow away the
> >     UEFI-native Windows installation, and it would be a good idea to
> >     reformat the disk to MBR."
> 
> I agree with the author. If you want to keep the existing EFI
> Windows installation and have a convenient dual boot with GRUB,
> you'll have to set up your favourite distribution to boot in EFI
> mode. If you want to go back to legacy boot, including for Windows,
> you'll have to repartition the disk to MSDOS format and reinstall
> Windows.

"convenient dual boot with GRUB" moves the goalposts.

> >I can't reconcile that with the system here, a Windows 8→10 UEFI laptop
> >and GPT disk running linux in BIOS compatibility mode (here called
> >Legacy mode by Lenovo) booting from an MBR on an ATA disk:
> 
> That is not very convenient, is it ? You cannot boot Windows boot
> manager from GRUB nor you can boot GRUB from Windows boot manager
> and must select the boot mode in the UEFI firmware setup whenever
> you want to switch the operating system.

It's very convenient for me. It means I haven't had to interfere with
the way windows chooses to boot, or its configuration of the disk, at all.

> >Switching over involves going through the "BIOS Setup", reached
> >by a separate button (almost recessed).
> 
> As expected.

Not by the author, who would have me reformat the disk as MBR and then
install windows…from where exactly?

Cheers,
David.