Xen on arm Chromebook seems to cause no display on screen

[Chuck Zmudzinski <brchuckz@xxxxxxxxxxxx>]

Hello,

I am sending this again because the original message was not word
wrapped to around 72 characters. Sorry about that.

We are trying to boot Xen on a Samsung XE303C12 Chromebook aka "snow"
following the suggestions in the slide show presentiation here:

https://www.slideshare.net/xen_com_mgr/xpds16-porting-xen-on-arm-to-a-new-soc-julien-grall-arm

This device uses an exynos5250 SOC dual core 1.7 GHz with 2 MB RAM, it is
a Samsung armv7 chip with virtualization extensions.

In particular, we have it working fairly well both on the bare metal with
a recent 6.1.59 Linux LTS kernel and also with a recent 5.4.257 LTS
kernel with KVM, the older LTS kernel version is used to test KVM because
support for KVM on arm v7 was removed from Linux around kernel version
5.7. So we know we have the hypervisor mode enabled because we were able
to use it with KVM.

For Xen, we are using the latest Debian build of Xen 4.17 for the Debian
armhf architecture:

(XEN) Xen version 4.17.2-pre (Debian 4.17.1+2-gb773c48e36-1) 
(pkg-xen-devel@xxxxxxxxxxxxxxxxxxxxxxx) (arm-linux-gnueabihf-gcc (Debian 
12.2.0-14) 12.2.0) debug=n Thu May 18 19:26:30 UTC 2023

The Linux kernel is a custom build that adds the Xen config kernel
options (CONFIG_XEN_DOM0, etc) on top of a kernel that works well on the
same Chromebook model on the bare metal. I can provide the config options
of the kernel that was used if that is helpful.

Our method of booting is to have u-boot boot the Xen hypervisor and load
the device tree after adding the dom0 to the otherwise unaltered device
tree from the Linux kernel using u-boot fdt commands to add a /chosen
node, as described on the Xen wiki and in the pages linked from there. We
have also tried adding and loading an initrd.img using the device tree
/chosen node but that made no difference in our tests.

We actually have the Linux LTS kernel version 6.1.59 working as dom0 with
Xen using the same version of u-boot that we used for KVM, but with a big
problem.

The problem we see is that when booting the 6.1.59 kernel version as dom0
with Xen, the screen is totally dark and the only way to access the
system is remotely through ssh. Logs indicate most everything else is
working, such as the wifi card so we can access it remotely via ssh and a
USB optical mouse lights up when connected so USB is also working.
Obviously, the disk is also working. The Chromebook is configured to boot
from the device's SD card slot by turning on Chrome OS developer mode
options to enable booting from the SD card slot.

The mystery is that when booting the exact same 6.1.59 kernel on the bare
metal instead of booting it as dom0 on Xen, it boots up with full access
to the screen and we can interact with the system using the X.org windows
system. But booting as dom0 with Xen, the screen is totally dark and the
only access we have to the system is through the network via ssh. Also,
when booting the 5.4.257 kernel with KVM in hypervisor mode, the screen
works and we can interact with the system through the X.org windows
system.

We also have not yet done a thorough analysis of the differences in the
kernel boot logs when booting on the bare metal vs. booting as dom0 on
Xen, but nothing stood out in the logs as an obvious cause of this
problem after a quick look at the logs.

Any ideas why booting the same Linux kernel that results in a working
X.org display on the bare metal instead as dom0 on Xen would cause the
display to remain dark, but most other basic functions would work, such
as network, disk, and USB?