Description: search the forums for "dram" or "iommu" and you will fine a flurry of recent activity caused by failed boots or other issues once users update to 4.13. I had the issue myself, though my system technically booted. I got some systemd error messages about a device, it took a few tries for my encrypted root password prompt to appear, dbus failed, as did loading iwlwifi, which led me to google around on my phone to figure out what happened.

One quote, from a forum moderator, says: "IOMMU has been enabled by default, and it's not as "defaultable" as initially hoped."[1] I don't know how he/she found this out, but I don't read up on linux kernel known issues and tend to find that system-breaking things will be given a heads up on the arch home page. Historically, this has been handled awesomely; something goes awry, and I find the issue and solution on archlinux.org.

Steps to reproduce: upgrade as normal with pacman -Syu and find a lot of scary messages and broken functionality.

Some example requests for help in the forums:
- [1] https://bbs.archlinux.org/viewtopic.php?id=230362
- [2] https://bbs.archlinux.org/viewtopic.php?id=230396
- [3] https://bbs.archlinux.org/viewtopic.php?id=230460
- [4] https://bbs.archlinux.org/viewtopic.php?id=230115
- [5] https://bbs.archlinux.org/viewtopic.php?id=230156

I'm posting as a bug here because:
1) this seems widespread enough to make the arch news page
2) putting something in a highly visible place will hopefully save work in the forums
3) whatever the definitive fix/workaround should be is better than having users try lesser/inferior solutions

Thanks for your consideration.