From ccf9b1cf4df79309d44a20f002a055441c4729eb Mon Sep 17 00:00:00 2001
From: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>
Date: Fri, 7 Jun 2024 10:40:37 +0200
Subject: [PATCH] Add documentation section about GRUB chain loading

---
 doc/install.md | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/doc/install.md b/doc/install.md
index d5e279dd..cee67c9c 100644
--- a/doc/install.md
+++ b/doc/install.md
@@ -150,3 +150,27 @@ And update grub.
 ```
 # nix build .#nixosConfigurations.xeon02.config.system.build.kexecTree -v
 ```
+
+## Chain NixOS in same disk with other systems
+
+To install NixOS on a partition along another system which controls the GRUB,
+first disable the grub device, so the GRUB is not installed in the disk by
+NixOS (only the /boot files will be generated):
+
+```
+boot.loader.grub.device = "nodev";
+```
+
+Then add the following entry to the old GRUB configuration:
+
+```
+menuentry 'NixOS' {
+        insmod chain
+        search --no-floppy --label nixos --set root
+        configfile /boot/grub/grub.cfg
+}
+```
+
+The partition with NixOS must have the label "nixos" for it to be found. New
+system configuration entries will be stored in the GRUB configuration managed
+by NixOS, so there is no need to change the old GRUB settings.