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.
