[Bug 222937] [byhve] Severe RAM corruption after PciPassThrough-guest shutdown

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222937

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A commit references this bug:

Author: jhb
Date: Fri Oct 27 14:57:15 UTC 2017
New revision: 325039
URL: https://svnweb.freebsd.org/changeset/base/325039

Log:
  Rework pass through changes in r305485 to be safer.

  Specifically, devices that do not support PCI-e FLR and were not
  gracefully shutdown by the guest OS could continue to issue DMA
  requests after the VM was terminated.  The changes in r305485 meant
  that those DMA requests were completed against the host's memory which
  could result in random memory corruption.  Instead, leave ppt devices
  that are not attached to a VM disabled in the IOMMU and only restore
  the devices to the host domain if the ppt(4) driver is detached from a
  device.

  As an added safety belt, disable busmastering for a pass-through device
  when before adding it to the host domain during ppt(4) detach.

  PR:           222937
  Tested by:    Harry Schmalzbauer <freebsd at omnilan.de>
  Reviewed by:  grehan
  MFC after:    1 week
  Differential Revision:        https://reviews.freebsd.org/D12661

Changes:
  head/sys/amd64/vmm/io/iommu.c
  head/sys/amd64/vmm/io/ppt.c

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