Could not enable shadow paging for domain?

[Tom Horsley]

Playing with Xen, Fedora Core 6, and 8-way Opteron system at work
with 8 gig of memory:

Xen seems to stop being able to create domains after it has been
up for a while (doing things like testing lots of guest installs
or even just dom0 for non-xen related testing).

Any attempt at this point to create a new domain winds up
with error (12, 'Cannot allocate memory'), and the
xend-debug.log shows ERROR: Could not enable shadow paging for domain.

Rebooting the machine (the real one :-) fixes the problem until
it has been up for a while again...

This sound familiar to anyone? Should I file a bug (what would
be a good component to use? kernel-xen?).