Xen 4.8 with Intel Integrated Graphics - DMA Error (PTE Read access not set) errors

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Xen 4.8 with Intel Integrated Graphics - DMA Error (PTE Read access not set) errors

Stephen Oberholtzer
Xen 4.8.4-pre
Dom0 = Debian Testing (Buster)

After upgrading my system (new motherboard+CPU, new Xen version), I got floods of DMA errors in Xen and Dom0.

This appears in `xl dmesg`:
(XEN) [VT-D]DMAR:[DMA Read] Request device [0000:00:02.0] fault addr 3b63651000, iommu reg = ffff82c000201000
(XEN) [VT-D]DMAR: reason 06 - PTE Read access is not set
(XEN) [VT-D]DMAR:[DMA Read] Request device [0000:00:02.0] fault addr 3b63651000, iommu reg = ffff82c000201000
(XEN) [VT-D]DMAR: reason 06 - PTE Read access is not set
(XEN) [VT-D]DMAR:[DMA Read] Request device [0000:00:02.0] fault addr 3b63651000, iommu reg = ffff82c000201000
(XEN) [VT-D]DMAR: reason 06 - PTE Read access is not set
(XEN) [VT-D]DMAR:[DMA Read] Request device [0000:00:02.0] fault addr 3b63651000, iommu reg = ffff82c000201000
(XEN) [VT-D]DMAR: reason 06 - PTE Read access is not set
(XEN) [VT-D]DMAR:[DMA Read] Request device [0000:00:02.0] fault addr 3b63651000, iommu reg = ffff82c000201000
(XEN) [VT-D]DMAR: reason 06 - PTE Read access is not set
(XEN) printk: 742 messages suppressed.
(XEN) [VT-D]DMAR:[DMA Read] Request device [0000:00:02.0] fault addr 3b63651000, iommu reg = ffff82c000201000
(XEN) printk: 749 messages suppressed.

This appears in `dmesg` on Dom0:

[   31.298250] [drm:gen8_irq_handler [i915]] *ERROR* Fault errors on pipe A: 0x00000080

(This exact message is generated about 100 times per second.)

Based on my searches, I added `iommu=no-igfx` to the Xen command line, and the problem seems to have gone away.   
According to https://xenbits.xen.org/docs/unstable/misc/xen-command-line.html, if `no-igfx` fixes anything, I should file a bug reporting the problem -- but should I report it to Xen, or my BIOS vendor?  I strongly suspect the latter, but I don't know what I should tell them.
--
-- Stevie-O
Real programmers use COPY CON PROGRAM.EXE


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