# EFI notes

## Dual Boot

We can use the EFI `NextBoot` feature to signal that the system should boot the 2nd OS.

For initial setup, find the 2nd OS boot entry:

```sh
PS C:\Users\pleb\Documents\windows-scripts> PowerShell.exe -ExecutionPolicy Bypass -File .\List-BootEntries.ps1
Found Boot0000
Found Boot0002
```

Hard-code the boot entry in the `Set-BootNext.ps1` script:

```powershell
# NixOS boot entry ID
$NixOSBootEntryID = "0002"
```

Then whenever you want to run NixOS, run the script:

```sh
PowerShell.exe -ExecutionPolicy Bypass -File \Users\pleb\Documents\windows-scripts\Set-BootNext.ps1
```

## EFI Recovery

Sometimes Windows Update will trample the EFI boot entry for NixOS.

## Linux

* Boot into a NixOS live environment (USB or CD).

* Mount your NixOS root partition and the ESP (EFI System Partition):

   ```sh
   mount /dev/nvme0n1p4 /mnt
   mount /dev/nvme0n1p1 /mnt/boot
   ```

* Chroot into your NixOS installation:

   ```sh
   nixos-enter --root /mnt
   ```

* Manually reinstall the bootloader:

   ```sh
   bootctl install --path /boot
   ```

* Exit the chroot environment:

   ```sh
   exit
   ```

* Unmount the partitions:

   ```sh
   umount /mnt/boot
   umount /mnt
   ```

8. Ensure the boot order is to your satisfaction

```sh
efibootmgr -v
efibootmgr -o XXXX,YYYY
```

9. Reboot your system.

By following these steps, you should be able to recover your NixOS boot option if it disappears after a Windows update.

## Windows

In the unlikely scenario that the Windows boot entry is removed, here's how to recover from that:

* Mount the EFI System Partition if it's not already mounted:

   ```sh
   mount /dev/nvme0n1p1 /mnt/boot
   ```

   (Replace nvme0n1p1 with the correct partition if different)

* Verify that the Windows bootloader files exist:

   ```sh
   ls /mnt/boot/EFI/Microsoft/Boot/
   ```

   You should see a file named `bootmgfw.efi`.

* If the files are present, you can create a new boot entry for Windows using `efibootmgr`:

   ```sh
   efibootmgr -c -d /dev/nvme0n1 -p 1 -L "Windows" -l '\EFI\Microsoft\Boot\bootmgfw.efi'
   ```

   This command creates a new boot entry labeled "Windows" that points to the Windows bootloader file.

* Verify that the new entry was created:

   ```sh
   efibootmgr -v
   ```