In https://github.com/QubesOS/qubes-issues/issues/7481, a user reported
that Xorg locked up when resizing a VM window.  While I do not have the
same hardware the user does and thus cannot reproduce the bug, the stack
trace seems to indicate a deadlock between xen_gntdev and i915.  It
appears that gnttab_unmap_refs_sync() is waiting for i915 to free the
pages, while i915 is waiting for the MMU notifier that called
gnttab_unmap_refs_sync() to return.  Result: deadlock.

The problem appears to be that a mapped grant in PV mode will stay in
the “invalidating” state until it is freed.  While MMU notifiers are
allowed to sleep, it appears that they cannot wait for the page to be
freed, as is happening here.  That said, I am not very familiar with
this code, so my diagnosis might be incorrect.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab