update

build.md

@@ -2,7 +2,23 @@
 
 The module building system.
 
-Kernel modules are built using a makefile located at:
+Explained at: <https://www.kernel.org/doc/Documentation/kbuild/modules.txt>
+
+Full method: get the kernel, build it to a directory `$KDIR`, and then run:
+
+	make -C "$KDIR" M="$(PWD)" modules
+
+Quick method: no need for a full build, just:
+
+    make modules_prepare
+
+but you lose some functionality. TODO: module insert on host then fails with:
+
+    insmod: ERROR: could not insert module hello.ko: Invalid module format
+
+even though kernel tree was checked out to match the host.
+
+Using your distro's kernel version:
 
 	/lib/modules/$(uname -r)/build