Frank van der Linden wrote:
> 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
Yes, Xen debug builds use the bit reserved by pmap to mark wired pages 
for its own purposes.
Maybe we could check if the hypervisor is OK to run BSD and panic with 
the appropriate message if not. I'm not sure we can see in the 
hypervisor start info struct that it was built with -DDEBUG, but a least 
we can try to set the bit on some page early and check the result (like 
in pmap_bootstrap()).

-- Mathieu