Graphical glitches (not refreshing?) with Linux's xe driver + Xen 4.19

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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Hi,

Recently I started testing compatibility with Intel Lunar Lake. This is
the first one that uses "xe" instead of "i915" Linux driver for iGPU.
I test it with Qubes OS 4.3, which uses Xen 4.19.4 and PV dom0 running
Linux 6.17.9 in this test.
The result is rather sad:
https://openqa.qubes-os.org/tests/166691#step/simple_gui_apps/30
or
https://openqa.qubes-os.org/tests/166700#step/desktop_linux_manager_config/42

The black windows are mapped from a domU (using grant tables). But if
you watch the video (in the "logs&assets" tab), login window of dom0 is
affected too (background is rendered significantly later than its text
and widgets). 

I didn't found any relevant errors in kernel (or Xen) messages. Just in
case, full dumps:
https://openqa.qubes-os.org/tests/166700/file/desktop_linux_manager_config-journalctl.log
https://openqa.qubes-os.org/tests/166700/file/desktop_linux_manager_config-xl-dmesg.log

Some user reported similar issue with Intel Arc Pro B50, which also uses
"xe" driver:
https://github.com/QubesOS/qubes-issues/issues/10661

I tested also Linux 6.19-rc8, no improvements.

My previous tests of the xe driver on older systems (Meteor Lake, Raptor
Lake) resulted in similar behavior, while i915 works fine there. But there
I needed to use force_probe parameter, and blamed the issue on xe not
officially supporting those platforms. I wanted to compare with i915 on
LNL, but i915 doesn't support that model at all (and if I force it, I
get kernel panic in i915_pci_probe...).

Interestingly, it appears to work just fine on Qubes OS 4.2 (Xen 4.17.6,
same Linux dom0 version). I did not tested with Xen 4.21.

In both cases, our Xen has the patch to use Linux's PAT layout applied.
That's a workaround for
https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/7648, which is
fixed now, but apparently similar bug is still present in a few other
drivers (including binary nvidia one...).

The same Linux kernel started without Xen works just fine.

Any ideas?

Having good and a bad version, I can try to bisect this one, but given
it requires rebuilding Xen toolstack each time too (due to bisection
across releases), I'd rather avoid it if possible...

I'm CC-ing also intel-xe list, maybe somebody there have some ideas how
to debug this. 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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