Compilation

Now that the UML has been configured, it is time to build it. On 2.6 hosts, we need to take care of one more detail. If the UML instance is to be built to use AIO support on the host, a header file, include/ linux/aio_abi.h in the UML tree, must be copied to /usr/ include/linux/aio_abi.h on the host.

With this taken care of, building UML is as simple as this:

host% make ARCH=um

If you have built Linux kernels before, you will see that the UML build is very similar to what you have seen before. When it finishes, you will get two identical files, called vmlinux and linux. In fact, they are hard links to the same file. Traditionally, the UML build produced a file called linux rather than the vmlinux or vmlinuz that the kernel build normally produces. I did this on purpose, believing that having the binary be named linux was more intuitive than vmlinux or vmlinuz.

This was true, and most people like the name, but some kernel hackers are very used to an output file named vmlinux. Also, the kernel build became stricter over time, and it became very hard to avoid having a final binary named vmlinux. So, I made the UML build produce the vmlinux file, and as a final step, link the name linux to that file. This way, everyone is happy.