Boot hangs early unless using "iommu=off"

Description:

My PC hangs when booting from linux-6.9.7.arch1-1. The console displays:

Booting `Arch Linux' Loading Linux linux ... Loading initial ramdisk ...

Then goes no further.

Booting with "iommu=off" or "amd_iommu=off" works as expected.

Additional info:

This affects the following kernels:

linux-6.9.7.arch1-1 linux-6.9.6.arch1-1 linux-6.9.5.arch1-1 linux-6.9.4.arch1-1 linux-6.9.3.arch1-1 linux-6.9.2.arch1-1 linux-6.9.1.arch1-2 linux-6.9.1.arch1-1

linux-6.8.9.arch1-2 and earlier are unaffected.

Also see https://bbs.archlinux.org/viewtopic.php?id=297469

Steps to reproduce:

1. Boot PC using one of the affected kernels above
2. Watch in dismay