i915 dma faults on Xen

[Jason Andryuk <jandryuk@xxxxxxxxx>]

Hi,

Bug opened at https://gitlab.freedesktop.org/drm/intel/-/issues/2576

I'm seeing DMA faults for the i915 graphics hardware on a Dell
Latitude 5500. These were captured when I plugged into a Dell
Thunderbolt dock with two DisplayPort monitors attached.  Xen 4.12.4
staging and Linux 5.4.70 (and some earlier versions).

Oct 14 18:41:49.056490 kernel:[   85.570347] [drm:gen8_de_irq_handler
[i915]] *ERROR* Fault errors on pipe A: 0x00000080
Oct 14 18:41:49.056494 kernel:[   85.570395] [drm:gen8_de_irq_handler
[i915]] *ERROR* Fault errors on pipe A: 0x00000080
Oct 14 18:41:49.056589 VM hypervisor: (XEN) [VT-D]DMAR:[DMA Read]
Request device [0000:00:02.0] fault addr 39b5845000, iommu reg =
ffff82c00021d000
Oct 14 18:41:49.056594 VM hypervisor: (XEN) [VT-D]DMAR: reason 06 -
PTE Read access is not set
Oct 14 18:41:49.056784 kernel:[   85.570668] [drm:gen8_de_irq_handler
[i915]] *ERROR* Fault errors on pipe A: 0x00000080
Oct 14 18:41:49.056789 kernel:[   85.570687] [drm:gen8_de_irq_handler
[i915]] *ERROR* Fault errors on pipe A: 0x00000080
Oct 14 18:41:49.056885 VM hypervisor: (XEN) [VT-D]DMAR:[DMA Read]
Request device [0000:00:02.0] fault addr 4238d0a000, iommu reg =
ffff82c00021d000
Oct 14 18:41:49.056890 VM hypervisor: (XEN) [VT-D]DMAR: reason 06 -
PTE Read access is not set

They repeat. In the log attached to
https://gitlab.freedesktop.org/drm/intel/-/issues/2576, they start at
"Oct 14 18:41:49.056589" and continue until I unplug the dock around
"Oct 14 18:41:54.801802".

I've also seen similar messages when attaching the laptop's HDMI port
to a 4k monitor. The eDP display by itself seems okay.

I tried Fedora 31 & 32 live images with intel_iommu=on, so no Xen, and
didn't see any errors

This is a kernel & xen log with drm.debug=0x1e. It also includes some
application (glass) logging when it changes resolutions which seems to
set off the DMA faults. 5500-igfx-messages-kern-xen-glass

Running xen with iommu=no-igfx disables the iommu for the i915
graphics and no faults are reported. However, that breaks some other
devices (Dell Latitude 7200 and 5580) giving a black screen with:

Oct 10 13:24:37.022117 kernel:[   14.884759] i915 0000:00:02.0: Failed
to idle engines, declaring wedged!
Oct 10 13:24:37.022118 kernel:[   14.964794] i915 0000:00:02.0: Failed
to initialize GPU, declaring it wedged!

Any suggestions welcome.

Thanks,
Jason