[Bug 100162] [HSW] X server hang when toggling fullscreen in media player

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=100162

--- Comment #14 from Thomas Lindroth <thomas.lindroth at gmail.com> ---
I tried setting intel_iommu=on,igfx_off but as expected this broke IOMMU in
kvm. I don't know why that happens. As I understand it igfx_off should only
disable the IOMMU dedicated to the igpu without changing anything else.

With igfx_off I got errors like these when trying to start a VM with kvm:
  DMAR: DRHD: handling fault status reg 3
  DMAR: DMAR:[DMA Read] Request device [04:00.1] fault addr 1eac00000 
  DMAR:[fault reason 12] non-zero reserved fields in PTE

device [04:00.1] is one of the devices I assign to the VM. "non-zero reserved
fields in PTE" is an odd error. It makes me think there is some corruption of
the IOMMU pagetables caused by the igfx_off option.

While I was testing igfx_off I got lucky and did get a hang. The hang looked
the same as all my other hangs. "WARNING: CPU: 0 PID: 3133 at
/usr/src/linux-4.4.89/drivers/gpu/drm/i915/inte
l_display.c:3965 intel_crtc_wait_for_pending_flips+0x1dd/0x230()". Since the
hangs happen even with igfx_off I guess the problem is not IOMMU related.

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