The UserMode Guide says to do this. # emerge sys-kernel/usermode-sources # cd /usr/src/uml/linux # make menuconfig ARCH=um # make linux ARCH=um # cp linux /usr/local/bin/linux However, make linux ARCH=i386 gives a Rule not found for target. Using make vmlinux ARCH=um allows the make to run.
Never mind. I went back through some bug reports on uml - again! - and found that arch must be um. I thought it was supposed to be mine arch!