port-xen/53057: dom0 xorg (with vesa) crashes with more than 3GB dom0_memory

[coypu%sdf.org@localhost]

>Number:         53057
>Category:       port-xen
>Synopsis:       dom0 xorg (with vesa) crashes with more than 3GB dom0_memory
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-xen-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Feb 25 21:15:00 +0000 2018
>Originator:     coypu
>Release:        NetBSD 8.0_BETA
>Organization:
>Environment:
NetBSD planets 8.0_BETA NetBSD 8.0_BETA (XEN3_DOM0) #5: Sun Feb 25 22:37:39 IST 2018  fly@planets:/home/fly/obj8/sys/arch/amd64/compile/XEN3_DOM0 amd64

>Description:
starting Xorg gets garbage in the screen, killing it instantly reboots.
I can't convince netbsd/xen to give me core dumps or even the dmesg buffer, so no messages.

if I had to guess, this would be a probable reason:
"Your machine does not initialize mem_clusters; sparse_dumps disabled"
>How-To-Repeat:

>Fix: