Problems with Xorg-x11 7.4 e xorg-server 1.5.3-r6

When KDE starts, X crashes.

Configuration:
app-emulation/xen 3.4.1
app-emulation/xen-tools 3.4.1 (Xend daemon and tools)
sys-kernel/xen-sources  2.6.29-r4
x11-base/xorg-server 1.5.3-r6
x11-base/xorg-x11 7.4

Launched Xen virtual machine with debian.5-0.x86-64, kernel guest domU
2.6.29-r4
Then:
export DISPLAY="ip_of_the_host:1"

From the host system:
Xnest -ac :1

From the guest:
startkde

Launching konsole, from within virtual machine, X crashes.

Same test, using:
X -ac :1
instead of Xnest.
startkde from the guest

Result: the system stucks

It seems to be a problem of MTRR. As a matter of fact, launching a new X
window, we get:

error setting MTRR (base = 0xf9000000, size = 0x00e00000, type = 1)
Invalid argument (22)


Update Xorg to 1.6.3.901

Configuration:

X
All packets installed with X (xorg, x11, etc.) are from version 7.3+19

xen

Packet: xen-tools
State: installed
Automatically installed: no
Version: 3.9-4

Packet: xen-hypervisor-3.2-1-i386
State: installed
Automatically installed: yes
Version: 3.2.1-2

Packet: linux-image-2.6.26-2-xen-686
State: installed
Automatically installed: yes
Version: 2.6.26-17lenny2

Packet: xen-linux-system-2.6.26-2-xen-686
State: installed
Automatically installed: no
Version: 2.6.26-17lenny2