port-xen/47669: X fails to start on Xen DOM0 (amd64)

[Pierre Pronchery <khorben%defora.org@localhost>]

>Number:         47669
>Category:       port-xen
>Synopsis:       X fails to find the video RAM when running on an amd64 DOM0
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    port-xen-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Mar 19 16:30:00 +0000 2013
>Originator:     Pierre Pronchery
>Release:        NetBSD 6.1_RC2
>Organization:
>Environment:
NetBSD 6.1_RC2 (XEN3_DOM0) #6: Sun Mar 17 23:54:06 CET 2013 
khorben%kwarx.defora.lan@localhost:/home/amd64/obj.6/sys/arch/amd64/compile/XEN3_DOM0
 amd64

Architecture: x86_64
Machine: amd64
>Description:
When starting modular Xorg on a 6.1_RC2 amd64 DOM0, it fails with the
following message:
(EE) RADEONHD(0): No Video RAM detected.

I haven't experienced this on earlier versions of the netbsd-6 branch;
however, I have recently raised the amount of RAM allocated to the DOM0
from 2048 to 4096MB, and I suspect this might well be related.

I can test with a lower amount when I get physical access to this
computer again, probably this evening.

This might be helpful:
modular-xorg-server-1.6.5nb14 Modular X11 server from modular X.org
modular-xorg-drivers-1.4 Modular Xorg driver meta-package
xf86-video-radeonhd-1.3.0nb2 Modular X.org driver for Radeon HD series

The machine has a total of 6 GB of RAM.  The video card is a Sapphire
ATI/AMD RadeonHD 4870, with 512 MB of video RAM IIRC. It works otherwise
fine with a GENERIC kernel.

>How-To-Repeat:
Run modular Xorg from pkgsrc 2012Q4 on a 6.1_RC2 amd64 XEN3_DOM0 kernel,
with 4096 MB of memory allocated to the DOM0.

>Fix:
Unknown at the moment.