In order for Xen to act as an Hypervisor, it must be loaded before the Linux Kernel. On a regular install using BIOS Boot, this is easy because the Xen packages provided in the AUR already take care of building and copying the Xen GZ image to /boot, so you just have to edit the Boot Loader config file to make use of it. However, if you want to do UEFI Boot with Xen, you need to use an EFI executable instead of the GZ image. Xen can already build the EFI executable, but as a prerequisite, binutils must include --enable-targets=x86_64-pep. As the default binutils does not include it, currently in order to get a Xen EFI executable, you have to install ABS (Arch Build System), download the core/binutils PKGBUILD, edit it to include --enable-targets=x86_64-pep, build it, and install it with pacman -U. Having this included in the default binutils would simplify this, and possibily could allow for improvements of the Xen AUR packages for UEFI-related stuff if they can rely on binutils being ready.
So far, the only thing which I know that uses --enable-targets=x86_64-pep is just Xen, and only for that purpose. Googled around a lot and x86_64-pep doesn't seem to have any other use at all. I also don't know if it could conflict with anything else or is safe to add, but should the second case be confirmed, I see absolutely no reason why this can't be added to the default binutils config.


This request has already been made last year, but was close cause it was for another version of the binutils package: https://bugs.archlinux.org/task/36713
Build guide for Xen EFI executable for Arch Linux: https://bbs.archlinux.org/viewtopic.php?id=174226
Xen Documentation for UEFI Boot stating the requeriment: http://xenbits.xen.org/docs/4.2-testing/misc/efi.html
Debian already decided to include this option in their binutils package: http://metadata.ftp-master.debian.org/changelogs//main/b/binutils/binutils_2.22-8_changelog