This RFC adds a Thunderbolt-flavoured PCIe hotplug layer for QEMU.

The main goal is hotplug and hot-unplug of real and virtual devices in
guests where direct hotplug for those devices is not supported or not
usable.  macOS is the main validation target for this path: the guest can
observe endpoints through a guest-visible Thunderbolt layer even when direct
PCIe hotplug for the same endpoint is not usable.

In v2, the series is split so the modelling part is explicit:

* patch 1 adds the Thunderbolt root port and thunderbolt-bus.  This is the
  core Thunderbolt PCIe model: endpoint devices remain normal QEMU PCI
  devices, while the root port controls guest-visible link presence and PCIe
  hotplug signalling.
* patch 2 adds the Thunderbolt PCIe-to-PCI bridge for tunnelled PCI
  endpoints.
* patch 3 adds a separate thunderbolt-vga endpoint for display hotplug tests.
  The regular QEMU VGA device remains untouched and non-hotpluggable.
* patch 4 adds qtest coverage.
* patch 5 adds documentation and MAINTAINERS coverage.

I added myself as reviewer for the Thunderbolt PCIe files.  The files still
match the existing PCI area through their paths, so PCI maintainership is
inherited by normal get_maintainer.pl matching.

Runtime validation performed so far:

* macOS guest booted from QCOW2 overlays over immutable base disk images.
* Thunderbolt VGA hotplug/removal while the standard boot VGA remains.
* NVMe hotplug/removal with guest driver instance checks.
* USB controller hotplug, USB storage mount/write/eject, controller removal,
  replug, and data persistence verification.

Build/test validation:

* ninja -C build qemu-system-x86_64 tests/qtest/display-vga-test
* meson test -C build qtest-x86_64/display-vga-test --print-errorlogs
* scripts/checkpatch.pl --no-tree --no-signoff on the full diff:
  0 errors, with one expected warning for MIT inheritance in thunderbolt-vga.c.

Changes since v1:

* Split the single large patch into five patches.
* Clarified that the modelling piece is the Thunderbolt root port/bus.
* Added a MAINTAINERS entry with Dmitry R as reviewer.

Dmitry R (5):
  hw/pci-bridge: add Thunderbolt root port
  hw/pci-bridge: add Thunderbolt PCIe-to-PCI bridge
  hw/display: add Thunderbolt VGA endpoint
  tests/qtest: cover Thunderbolt VGA hotplug
  docs: document Thunderbolt PCIe hotplug layer

 MAINTAINERS                                 |   7 +
 README.md                                   |  91 ++++
 docs/specs/index.rst                        |   1 +
 docs/specs/thunderbolt-pcie-hotplug.rst     |  99 ++++
 hw/display/meson.build                      |   1 +
 hw/display/thunderbolt-vga.c                | 275 ++++++++++
 hw/pci-bridge/meson.build                   |   2 +
 hw/pci-bridge/thunderbolt_pcie_pci_bridge.c | 156 ++++++
 hw/pci-bridge/thunderbolt_root_port.c       | 565 ++++++++++++++++++++
 tests/qtest/display-vga-test.c              |  68 +++
 10 files changed, 1265 insertions(+)
 create mode 100644 README.md
 create mode 100644 docs/specs/thunderbolt-pcie-hotplug.rst
 create mode 100644 hw/display/thunderbolt-vga.c
 create mode 100644 hw/pci-bridge/thunderbolt_pcie_pci_bridge.c
 create mode 100644 hw/pci-bridge/thunderbolt_root_port.c

-- 
2.47.3