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

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

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On Fri, Feb 13, 2026 at 12:31:47AM -0500, Demi Marie Obenour wrote:
> On 2/12/26 20:23, Marek Marczykowski-Górecki wrote:
> > On Thu, Feb 12, 2026 at 04:11:50PM +0100, Roger Pau Monné wrote:
> >> On Tue, Feb 10, 2026 at 07:06:20PM +0100, Marek Marczykowski-Górecki wrote:
> >>> 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.
> >>
> >> Not sure it's going to help a lot, but does using a PVH dom0 make any
> >> difference?
> > 
> > Ok, now with the correct Xen version, it's better with PVH dom0. At
> > least on the login screen and few applications (from both dom0 and domU)
> > I don't see the glitches anymore. I can't do a full test, because PCI
> > passthrough doesn't seem to work with PVH dom0 on Xen 4.19 - and I need
> > it to start most VMs.
> > 
> > So, if the above test is representative, it's only about PV dom0.
> 
> This reminds me of a problem that I helped debug a while back,
> and which also affected Qubes OS.  That one was due to some part
> of Linux core MM clobbering the high bit of the PAT part of PTEs.
> As a result, a PTE that should have referenced entry X in the PAT
> wound up referencing entry (X & 3).
> 
> On native Linux, the bug was mostly harmless, which is why nobody
> found it earlier.  However, on Xen PV, the different PAT caused memory
> to be cached that should not be.  The resulting symptoms were very
> similar to what Marek described, though a bit less dramatic.
> 
> Can you try removing the Qubes downstream patch that changes the PAT?

I'd assume removing such patch would make things only worse (it either
shouldn't matter, or if the driver assumes Linux version, changing it
would actually break stuff).
Anyway, removing the patch didn't change anything.

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

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