Hypervisor issues with PTE

Re: http://mail-index.netbsd.org/port-xen/2006/09/12/0001.
html
and http://mail-index.netbsd.org/port-xen/2006/07/26/0007.
html

Frank van der Linden wrote:

 > From: Frank van der Linden <fvdlnetbsd.org>
 > List: port-xen
 > Date: 09/12/2006 21:06:56
 >
 > Anyway, this is just a note that I tracked down this
problem. When
 > compiled with debugging options on, Xen uses an unused
PTE bit to track
 > granted table pages, in an effort to catch cleanup
problems early.
 > However, the NetBSD pmap already uses all of those
bits. So, at the
 > first attempt to enter a tracked (pv_list) mapping,
Xen refuses, because
 > the same bit it is using for debugging purposes is set
in the PTE. The
 > definition (_PAGE_GNTTAB) is even marked as "has
to be disabled for *BSD").

 > I guess there are Xen compiles out there that have
debug on by default,
 > and with the current Xen sources, NetBSD (and *BSD in
general), won't
 > boot on that.

Is the use of all PTE bits an absolute in NetBSD (we can
never fix this),
or is there some bit of work that has not yet been done?

I ask because Fedora Core certainly ships with hypervisor
debugging on,
and it would be nice to be able to run NetBSD guests on
"stock" Xen-capable systems.

If we can never fix this in NetBSD --- that's fine, I'll
build my own hypervisor
kernel. But, it will become a major PITA as Xen becomes more
popular.