I noticed that, late July / early August, Eric Delcamp posted a message 
to this forum about getting a "HYPERVISOR_mmu_update failed" panic when 
booting INSTALL_XEN3_DOMU on a Linux dom0, with a recent Xen. (see 
http://mail-index.netbsd.org/port-xen/2006/07/26/0007.html)

I found that message, because I got the message myself when using the 
latest development Xen sources (with a few mods), running the 
OpenSolaris dom0, and then booting INSTALL_XEN3_DOMU in -current.

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.

- Frank