[Bug 299302] Crash and reboot after startx

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=299302

--- Comment #3 from marek <man130117@outlook.com> ---
I have asked Claude for help and this what he found:

  Further kgdb analysis: vm_phys_fictitious_tree.rbh_root == NULL, i.e. no
fictitious ranges are registered. The faulting vm_page is the vm_page_array
entry for PFN 0xc0004 (PA 0xC0004000, inside the i915 GTT aperture BAR
0xC0000000–0xCFFFFFFF, in a hole between vm_phys_segs); the whole struct is
uninitialized (random phys_addr, object, ref_count). So the aperture was never
registered via vm_phys_fictitious_reg_range() (or was unregistered). On amd64's
dense vm_page_array, PHYS_TO_VM_PAGE() returns this hole entry, and
lkpi_vmf_insert_pfn_prot_locked()'s NULL check (D53412) can't detect it.
Triggered by xf86-video-intel (SNA GTT mmaps) on Ivy Bridge (HD 4000) with
drm-612-kmod-6.12.110.1501000_3; modesetting+glamor avoids it.

Then - following Claude's advice - I uninstalled `xf86-video-intel` driver and
changed `/usr/local/etc/X11/xorg.conf.d/20-driver.conf` from:

  Section "Device"
          Identifier "Card0"
          Driver     "intel"
  EndSection

to:

  Section "Device"
          Identifier "Intel"
          Driver     "modesetting"
          Option     "AccelMethod" "glamor"
  EndSection

and now it works as it should (GNOME starts, mouse and keyboard work and no
crashes)! 

For details see the attached 'kgdb-session.txt' and '299302 - Crash and reboot
after startx.odt'.

I'm not a FreeBSD expert, but I've been using it since version 7 and I've never
seen it crash this badly! What's going on?

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