docs: Add note on RuntimeBuildSources= to hacking guide

diff --git a/docs/HACKING.md b/docs/HACKING.md
index 8abb5e040f21a800b59a66adeaf6b4137b26567e..23ea7b10b16cc162fdf0f9228e60dbe19877605e 100644 (file)
--- a/docs/HACKING.md
+++ b/docs/HACKING.md
@@ -82,6 +82,21 @@ to `mkosi.local.conf`:
 QemuFirmware=uefi
 ```
 
+To avoid having to build a new image all the time when iterating on a patch, add
+the following to `mkosi.local.conf`:
+
+```conf
+[Host]
+RuntimeBuildSources=yes
+```
+
+After enabling this setting, the source and build directories will be mounted to
+`/work/src` and `/work/build` respectively when booting the image as a container
+or virtual machine. To build the latest changes and re-install, run
+`meson install -C /work/build --only-changed` in the container or virtual machine
+and optionally restart the daemon(s) you're working on using
+`systemctl restart <units>` or `systemctl daemon-reexec` if you're working on pid1.
+
 Putting this all together, here's a series of commands for preparing a patch
 for systemd: