"intel_iommu=on iommu=on,pt" causes hangs on HP DL380 G7

Issue

• This same config, with VT-d enabled in the BIOS, works fine and doesn't hang when run under RHEL6.

• Under RHEL5, the hang occurs when udev starts and is sometimes accompanied by an error message:

Starting udev: Uhhuh. NMI received for unknown reason f1.
You probably have a hardware problem with your RAM chips
Dazed and confused, but trying to continue

• That's what we get with 2.6.18-238.21.1.el5, but with newer RHEL5 kernels (2.6.18-308.1.1.el5), the system freezes much earlier, during cciss initialization:

cciss 0000:05:00.0: cciss: Trying to put board into performant mode
cciss 0000:05:00.0: Placing controller into performant mode
32bit 0000:05:00.0 uses non-identity mapping
64bit 0000:05:00.0 uses identity mapping
(...hang...)