Hypervisor issues with PTE

[Michael Richardson <mcr%sandelman.ca@localhost>]

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 <fvdl%netbsd.org@localhost>
> 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.