Reference darwin.builder and historical context

README.md

@@ -1,3 +1,35 @@
+You should look at the
+[darwin.builder](https://nixos.org/manual/nixpkgs/unstable/#sec-darwin-builder),
+which is now part of nixpkgs.
+
+linuxkit-nix was started in 2017 when QEMU did not support macOS'
+Hypervisor.framework API. This meant that QEMU had to use full
+emulation with no hardware acceleration. Not ideal for building large
+software.
+
+QEMU also had some issues on macOS with userspace networking.
+
+At the time, LinuxKit was the easiest way to spin up a VM for builds,
+because it spun up HyperKit for hardware accelerated virtualisation
+and VPNKit for userspace networking - both used in Docker for Mac.
+
+Theoretically the underlying technology was stable but it was bit
+tricky to get everything working well together. There were
+bootstrapping issues. For example, we had to be careful when
+referencing `linux-x86_64` packages because we were on `darwin-x86_64`
+and it could only fetch from Hydra - it couldn't even build a custom
+shell script for the Linux VM until we got that initial VM running.
+
+This project also had issues with daemons, permissions and race
+conditions.
+
+In 2018, QEMU got experimental support for Hypervisor.framework and
+that got promoted to stable in 2019. QEMU is now fast and since
+nixpkgs has great support for building and running QEMU virtual
+machines, there's little need for this project.
+
+---
+
 # LinuxKit Nix - Linux on Mac Nix builder
 
 [hydra jobset](https://hydra.nixos.org/jobset/linux-on-mac-builder/master)