Update README

README.md

@@ -39,34 +39,33 @@ The worksites at the moment are:
 
  - ARM, i686
 
- - Write a proper test framework, or better, reuse the NixOS one.
-
- - Patching standard Nixpkgs packages by overlay. Explore Workflow and tooling 
+ - Patching standard Nixpkgs packages by overlay. Explore Workflow and tooling
    for building emulated UNIX environments.
 
  - LLVM testing and upstreaming patches.
 
  - Formalizing Dhall configuration types.
 
- - NixOS modules for constructing a reasonably usable Genode system. Reusing the 
-   module system from NixOS is advantagous because a dual boot system can 
-   share a `/nix/store` and be updated simultaneously. Work-in-progress on a 
-   topic branch.
+ - NixOS modules for constructing a reasonably usable Genode system. Reusing the
+   module system from NixOS is advantageous because a dual boot system can
+   share a `/nix/store` and be updated simultaneously.
+
+ - NixOS module for NixOS guests with the ARM VMM.
 
  - Configuration validation via a service routing prover.
 
 # Nix Flakes
 
 This repository is structured as a Nix flake, which is an experimental extension
-of Nix tooling. See https://www.tweag.io/posts/2020-05-25-flakes.html for an 
+of Nix tooling. See https://www.tweag.io/posts/2020-05-25-flakes.html for an
 introduction and tutorial.
 
 ## Build Caching
 
-The builds produced by Hydra are available as a binary cache, using the cache is 
+The builds produced by Hydra are available as a binary cache, using the cache is
 strongly recommended.
 
-The global (IPv6 only) address for the cache can be tested by fetching the 
+The global (IPv6 only) address for the cache can be tested by fetching the
 following: http://cache.server7.hq.c3d2.de/nix-cache-info.
 
 If that fails the cache is also available within the
@@ -113,6 +112,17 @@ $ nix log .#checks.x86_64-linux.nova-x86
 Packaging is done using standard Nixpkgs methods, a `stdenv` is available for
 cross-compilation. See [Solo5](./packages/solo5/default.nix) as an example.
 
+## Cross-compiling Nixpkgs
+
+Some existing packages from Nixpkgs can be built with little or no modification.
+Nixpkgs is available at the path `.#legacyPackages.x86_64-linux-x86_64-genode`
+(or `…-aarch64-genode`) and modifications to packages are made at an
+[overlay](./overlay/default.nix). Some packages need patched, others require
+additional features added to the Genode libc.
+
+Packages that run on Linux as well as Genode should not be packaged here, those
+should be added to Nixpkgs.
+
 # Toolchain
 
 The Genode Labs toolchain is available here only for Linux and in binary form.
@@ -122,7 +132,7 @@ has not been practical due to its recursive autotools build system.
 The enter a shell with the toolchain:
 
 ```shell
-nix run .#legacyPackages.x86_64-linux-x86_64-genode.stdenv.cc
+nix shell .#legacyPackages.x86_64-linux-x86_64-genode.stdenv.cc
 
 x86_64-unknown-genode-clang++ -v
 ```
@@ -279,9 +289,9 @@ Others are simply introduced as an aid to the reader:
 
 # Legal
 
-This repository is for trivial packaging and metadata that exists only for the 
-sake of richer and more meaningful code in external repositories. All works 
-committed to this repository, unless otherwise noted, shall be implicitly 
+This repository is for trivial packaging and metadata that exists only for the
+sake of richer and more meaningful code in external repositories. All works
+committed to this repository, unless otherwise noted, shall be implicitly
 licensed under a [CC0 1.0](https://creativecommons.org/publicdomain/zero/1.0/deed)
 public domain dedication.