[Bug 229852] [PATCH] bhyve: IOMMU (Intel VTd) PCI passthrough attempt locks up some systems

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=229852

--- Comment #16 from commit-hook at freebsd.org ---
A commit references this bug:

Author: scottl
Date: Wed Jun 19 06:41:07 UTC 2019
New revision: 349184
URL: https://svnweb.freebsd.org/changeset/base/349184

Log:
  Implement VT-d capability detection on chipsets that have multiple
  translation units with differing capabilities

  From the author via Bugzilla:
  ---
  When an attempt is made to passthrough a PCI device to a bhyve VM
  (causing initialisation of IOMMU) on certain Intel chipsets using
  VT-d the PCI bus stops working entirely. This issue occurs on the
  E3-1275 v5 processor on C236 chipset and has also been encountered
  by others on the forums with different hardware in the Skylake
  series.

  The chipset has two VT-d translation units. The issue is caused by
  an attempt to use the VT-d device-IOTLB capability that is
  supported by only the first unit for devices attached to the
  second unit which lacks that capability. Only the capabilities of
  the first unit are checked and are assumed to be the same for all
  units.

  Attached is a patch to rectify this issue by determining which
  unit is responsible for the device being added to a domain and
  then checking that unit's device-IOTLB capability. In addition to
  this a few fixes have been made to other instances where the first
  unit's capabilities are assumed for all units for domains they
  share. In these cases a mutual set of capabilities is determined.
  The patch should hopefully fix any bugs for current/future
  hardware with multiple translation units supporting different
  capabilities.

  A description is on the forums at
  https://forums.freebsd.org/threads/pci-passthrough-bhyve-usb-xhci.65235
  The thread includes observations by other users of the bug
  occurring, and description as well as confirmation of the fix.
  I'd also like to thank Ordoban for their help.

  ---
  Personally tested on a Skylake laptop, Skylake Xeon server, and
  a Xeon-D-1541, passing through XHCI and NVMe functions.  Passthru
  is hit-or-miss to the point of being unusable without this
  patch.

  PR: 229852
  Submitted by: callum at aitchison.org
  MFC after: 1 week

Changes:
  head/sys/amd64/vmm/intel/vtd.c

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