I'm managing a number of Arch Linux clients that PXE boot and connect to read-only root file systems over NFS. In order to avoid conflating permissions on the client file systems with permissions on the server, I have the file systems saved as partition images rather than being part of mounted file systems on the server. This means that early userspace needs to loopback mount the partition image before switching root into the read-only file system. I've added that functionality into the net hook in mkinitcpio. Which image to loopback mount and what format to expect the image to be in are specified by two new kernel boot parameters (loop and loopfmt) specified in the PXE configuration. I also made the net hook install the loop module by default. Having an appropriate file system module in the initcpio is, of course, also required.

I'm attaching a patch to mkinitcpio. The patch was originally based on 0.6.3. The initcpio images we have in production were made with a patched version of 0.6.3. The patch I've uploaded is rebased on the current git head. Rebasing didn't produce any conflicts; the net stuff doesn't look like it's undergone any changes since then. If this is a feature that you'd like to include in mkinitcpio, I'm happy to do further testing with initcpio images made after applying the patch to the current version of mkinitcpio.