Heads-up: Requiring PAE for running Xen

[Jeremy Katz]

As we move forward with Xen enablement, there's a desire for
being able to access more than 4 gigs of RAM on 32-bit Xen hosts.  The
options for handling this are
1) Another kernel.  This is bad due to 
   a) we're running out of CD space already
   b) keeping things matched up between the HV and the guest kernels
   c) migration is worlds of pain with two types of kernels
2) Switch the 32-bit xen kernels to require PAE.  For most "current"
non-laptop hardware, this is a non-issue.  It does mean that xen won't
work a lot of earlier PentiumM laptops
3) Do nothing, tell people to use 64bit if they want more than 4 gigs of
RAM
4) Make the PAE code handled at runtime.  This is a pretty non-trivial
amount of work :)

Given these, we're looking at going with #2 and thus only having Xen
work on PAE-capable hardware in the development tree.  And we're
planning to try to execute this switchover the beginning of next week.
Note that this will not affect bare metal installs at all.

Jeremy