Created Use a remote builder with qemu (markdown)

Use-a-remote-builder-with-qemu.md

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+# Use a remote builder with qemu
+
+It seems possible to use a remote builder to avoid building packages on Android.
+
+qemu.nix: https://github.com/bqv/nixos/blob/live/profiles/misc/qemu.nix
+overlay: https://github.com/bqv/nixos/tree/live/overlays/qemu
+
+## Builder side
+
+```nix
+let
+  qemuOverlay = (import ./overlays/qemu);
+in
+{
+  imports = [ ./qemu.nix ];
+
+  boot.kernelModules = [ "kvm-intel" ];
+  qemu-user.arm = true;
+  boot.binfmt.emulatedSystems = [ "aarch64-linux" ];
+
+  nix = {
+    trustedUsers = [ "builder" ];
+  };
+
+  users.users.builder = {
+    createHome = true;
+    isNormalUser = true;
+  };
+}
+```
+
+## Android side
+
+~/.config/nix/nix.conf
+```conf
+builders-use-substitutes = true
+builders = ssh://builder
+```
+
+~/.ssh/config
+```conf
+Host builder
+    HostName <the ip>
+    User builder
+    IdentitiesOnly yes
+    IdentityFile ~/.ssh/nix_remote
+```
+
+~/.ssh/nix_remote is a ssh key without a password. Add the pub key on the builder machine.
+
+You need to ssh first to the host to make ssh accept the host key.
+
+You can test with `nix build '(with import <nixpkgs> { }; runCommand "foo" {} "uname -a > $out")' --builders 'ssh://builder' -j0`
+
+or `nix-on-droid switch --max-job 0`