Created Simple remote-building (markdown)

Simple-remote-building.md

@@ -0,0 +1,52 @@
+Building remotely with qemu-user-aarch64,
+using a prebuilt qemu user emulation binary.
+
+# Prerequisites:
+1. a powerful NixOS 20.09 x86_64 machine
+2. that you can ssh to using `ssh HOST` without passwords (e.g., with a passwordless key).
+
+# Builder
+
+Add the following to the system configuration:
+
+``` nix
+let
+  qemu-aarch64-static = pkgs.stdenv.mkDerivation {
+    name = "qemu-aarch64-static";
+
+    src = builtins.fetchurl {
+      url = "https://github.com/multiarch/qemu-user-static/releases/download/v5.
+1.0-7/qemu-aarch64-static";
+      sha256 = "0yzlrlknslvas58msrbbq3hazphyydrbaqrd840bd1c7vc9lcrh6";
+    };
+
+    dontUnpack = true;
+    installPhase = "install -D -m 0755 $src $out/bin/qemu-aarch64-static";
+  };
+in
+{
+  # ...
+
+  boot.binfmt.registrations.aarch64 = {
+    interpreter = "${qemu-aarch64-static}/bin/qemu-aarch64-static";
+    magicOrExtension = ''\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\xb7\x00'';
+    mask = ''\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\x00\xff\xfe\xff\xff\xff'';
+  };
+  nix.extraOptions = ''
+    extra-platforms = aarch64-linux
+    trusted-users = USER
+  '';
+  nix.sandboxPaths = [ "/run/binfmt/aarch64=${qemu-aarch64-static}/bin/qemu-aarch64-static" ];
+}
+```
+
+CAUTION: if you're using current unstable/flake-powered Nix without https://github.com/NixOS/nixpkgs/pull/103137,
+don't set `nix.sandboxPaths`, but set
+`sandbox-paths = /bin/sh=${pkgs.busybox-sandbox-shell}/bin/busybox /run/binfmt/aarch64=${qemu-aarch64-static}/bin/qemu-aarch64-static` instead.
+
+# Android side
+
+```
+mkdir -p ~/.config/nix
+echo -e "builders-use-substitutes = true\nbuilders = ssh-ng://USER@HOST" >> ~/.config/nix/nix.conf
+```