I am not aware of any document, but here is how UEFI boot works: Harddrive is GPT-labeled and it has a partition with a special EFI flag. The partition is formatted as Fat32.
Firmware (people tend to call it BIOS) contains configuration in non-volatile memory. This configuration consists of paths to .efi files, something like (harddisk_[disk_id[partition_id])/efi/debian/bootx64.efi .efi is a special file that firmware can run. One file is for grub, another is for windows loader. So, you need to have two efi files on this partition (one file for each OS) and add records about them in firmware configuration. Linux does it with efibootmgr tool (grub2-install calls it under the hood) If you have Windows installed, then you can simply install Debian, and I believe the installation process will add Debian .efi file preserving Windows file. If not, then simply run `update-grub` after the installation. I just played with uefi dualboot on hyper-v (second generation VM supports EFI), so here is how I did it: I installed windows, and got several partitions, one of them is called EFI and contains "\EFI\Microsoft\Boot\boomgfw.efi". This partition doesn't have letter associated, but you can use disk manager to assign letter and check its contents. My EFI configuration contains following record: Firmware device path: HD(1,GPT,47D66F17-77D9-4C7D-8993-733E9FDF5459,0x800,0x32000)/\EFI\Microsoft\Boot\bootmgfw.efi So far, so good. I then used disk manager to shrink disk "C" (to get some free space for Debian) and installed Debian there. I disabled Secure boot for now. Debian's grub installed its own efi file there: /boot/efi/EFI/debian/shimx64.efi Here is my new firmware config record: Firmware device path: HD(1,GPT,47D66F17-77D9-4C7D-8993-733E9FDF5459,0x800,0x32000)/\EFI\debian\shimx64.efi So I can now choose between Windows and Debian by choosing .efi file to boot in firmware settings (aka boot menu) But `update-grub` found Windows .efi file and added it to its configuration to implement chain loading: PC boots grub efi file, and grub then loads Windows efi file https://www.gnu.org/software/grub/manual/grub/grub.html#chainloader I suggest you to install hyper-v on Windows, create Gen2 VM, install Windows there, disable secure boot, then do everything I wrote about, and then do it on your real laptop On Sat, Apr 30, 2022 at 5:54 PM Richard Owlett <rcowl...@cloud85.net> wrote: > On 04/30/2022 09:01 AM, IL Ka wrote: > > this is possible: you just need to have two .efi files for your OSes: one > > for Windows and one for Linux. > > Use ``efibootmgr`` to manage it. > > If you have secure boot enabled, you need shim: > > https://wiki.debian.org/SecureBoot > > Following links from there suggests I know even less than I thought I > did. Confirms I need to read newbie oriented material about dual booting > Debian on a UEFI equipped Windows machine (with or without Secure Boot). > > Suggestions? > > TIA > >
