What processors do/do not support PAE?

Solution 1:

You should be pretty safe assuming PAE for any Pentium II or Athlon or newer, although some Pentium M's (marketed as Centrino)--namely, those with a 400 MHz bus--do not support PAE.

http://en.wikipedia.org/wiki/Physical_Address_Extension

PAE is provided by Intel Pentium Pro (and above) CPUs - including all later Pentium-series processors except the 400 MHz bus versions of the Pentium M, as well as by other processors such as the AMD Athlon and later AMD processor models with similar or more advanced versions of the same architecture.

http://pacoup.com/2009/05/27/pae-vs-64-bit-what-manufacturers-dont-want-you-to-know/comment-page-1

this option is compatible with any Intel Pentium Pro, Pentium II, III, 4, Core, Core 2, Core i7 and + processor, along with every recent AMD processors and Athlon series.

You should basically list these CPUs in the system requirements for your Linux distribution, or provide an alternate distro that has PAE disabled by default.

Solution 2:

I'm looking at how much risk there is in setting PAE as the default mode in a linux distribution.

I'll answer from a different perspective -- how much end user benefit there is in PAE today. In my experience, PAE is a technology that even quite tech-savvy users don't know about or don't care about (or both)... For most sysadmins today, if they need more than ~3.5GB RAM, they'll immediately reach for a 64-bit OS.

So IMHO regardless of how little incompatibility risk PAE has, you should not enable it. PAE's use case has been completely taken over by 64bit addressing now.