[Bug 108326] New: [BXT] BUG / system hang when reading i915 debugfs entries with VT-d/IOMMU enabled

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108326

            Bug ID: 108326
           Summary: [BXT] BUG / system hang  when reading i915 debugfs
                    entries with VT-d/IOMMU enabled
           Product: DRI
           Version: DRI git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: eero.t.tamminen at intel.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org
     i915 platform: BXT
     i915 features: GPU hang

Setup:
- BXT / APL HW (e.g. J3455, J4205, A3960)
- git version of drm-tip kernel
- VT-d enabled in BIOS (normally it's enabled by default)
- IOMMU not disabled on kernel command line (no "intel_iommu=igfx_off" option)

Use-case:
- cd /sys/kernel/debug/dri/0/
- head *

Expected outcome:
- entries shown, like happens when VT-d / IOMMU is disabled

Actual outcome:
- System hangs after following console output (and sometimes backtrace):
[   49.565898] BUG: scheduling while atomic: migration/0/11/0x00000002
[   49.572983] Preemption disabled at:

Notes:
* I haven't seen this on any other (GEN7-GEN9) HW, only on BXT/APL
* Most distros seems to mount debugfs as user readable, so this is a local DOS
(security) issue
* I don't think this is a regression as it has been there at least for a year
(when I filed an internal ticket about this, which was now closed with a
request to file this to FDO instead)

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