Generic PCI Express to PCI Bridge
================================

Description
===========
PCIE-to-PCI bridge is a new method for legacy PCI
hierarchies creation on Q35 machines.

Previously Intel DMI-to-PCI bridge was used for this purpose.
But due to its strict limitations - no support of hot-plug,
no cross-platform and cross-architecture support - a new generic
PCIE-to-PCI bridge should now be used for any legacy PCI device usage
with PCI Express machine.

This generic PCIE-PCI bridge is a cross-platform device,
can be hot-plugged into appropriate root port (requires additional actions,
see 'PCIE-PCI bridge hot-plug' section),
and supports devices hot-plug into the bridge itself
(with some limitations, see below).

Hot-plug of legacy PCI devices into the bridge
is provided by bridge's built-in Standard hot-plug Controller.
Though it still has some limitations, see below.

PCIE-PCI bridge hot-plug
=======================
Guest OSes require extra efforts to enable PCIE-PCI bridge hot-plug.
Motivation - now on init any PCI Express root port which doesn't have
any device plugged in, has no free buses reserved to provide any of them
to a hot-plugged devices in future.

To solve this problem we reserve additional buses on a firmware level.
Currently only SeaBIOS is supported.
The way of bus number to reserve delivery is special
Red Hat vendor-specific PCI capability, added to the root port
that is planned to have PCIE-PCI bridge hot-plugged in.

Capability layout (defined in include/hw/pci/pci_bridge.h):

    uint8_t id;     Standard PCI capability header field
    uint8_t next;   Standard PCI capability header field
    uint8_t len;    Standard PCI vendor-specific capability header field

    uint8_t type;   Red Hat vendor-specific capability type
                    List of currently existing types:
                        RESOURCE_RESERVE = 1


    uint32_t bus_res;   Minimum number of buses to reserve

    uint64_t io;           IO space to reserve
    uint32_t mem           Non-prefetchable memory to reserve

    At most one of the following two fields may be set to a value
    different from -1:
    uint32_t mem_pref_32;  Prefetchable memory to reserve (32-bit MMIO)
    uint64_t mem_pref_64;  Prefetchable memory to reserve (64-bit MMIO)

If any reservation field is -1 then this kind of reservation is not
needed and must be ignored by firmware.

At the moment this capability is used only in QEMU generic PCIe root port
(-device pcie-root-port). Capability construction function takes all reservation
fields values from corresponding device properties. By default all of them are
set to -1 to leave root port's default behavior unchanged.

Usage
=====
A detailed command line would be:

[qemu-bin + storage options] \
-m 2G \
-device pcie-root-port,bus=pcie.0,id=rp1 \
-device pcie-root-port,bus=pcie.0,id=rp2 \
-device pcie-root-port,bus=pcie.0,id=rp3,bus-reserve=1 \
-device pcie-pci-bridge,id=br1,bus=rp1 \
-device pcie-pci-bridge,id=br2,bus=rp2 \
-device e1000,bus=br1,addr=8

Then in monitor it's OK to execute next commands:
device_add pcie-pci-bridge,id=br3,bus=rp3 \
device_add e1000,bus=br2,addr=1 \
device_add e1000,bus=br3,addr=1

Here you have:
 (1) Cold-plugged:
    - Root ports: 1 QEMU generic root port with the capability mentioned above,
                  2 QEMU generic root ports without this capability;
    - 2 PCIE-PCI bridges plugged into 2 different root ports;
    - e1000 plugged into the first bridge.
 (2) Hot-plugged:
    - PCIE-PCI bridge, plugged into QEMU generic root port;
    - 2 e1000 cards, one plugged into the cold-plugged PCIE-PCI bridge,
                     another plugged into the hot-plugged bridge.

Limitations
===========
The PCIE-PCI bridge can be hot-plugged only into pcie-root-port that
has proper 'bus-reserve' property value to provide secondary bus for the
hot-plugged bridge.

Windows 7 and older versions don't support hot-plug devices into the PCIE-PCI bridge.
To enable device hot-plug into the bridge on Linux there're 3 ways:
1) Build shpchp module with this patch http://www.spinics.net/lists/linux-pci/msg63052.html
2) Use kernel 4.14+ where the patch mentioned above is already merged.
3) Set 'msi' property to off - this forces the bridge to use legacy INTx,
    which allows the bridge to notify the OS about hot-plug event without having
    BUSMASTER set.

Implementation
==============
The PCIE-PCI bridge is based on PCI-PCI bridge, but also accumulates PCI Express
features as a PCI Express device (is_express=1).

