[qemu.git] / docs / pcie.txt

PCI EXPRESS GUIDELINES
======================

1. Introduction
================
The doc proposes best practices on how to use PCI Express/PCI device
in PCI Express based machines and explains the reasoning behind them.

The following presentations accompany this document:
 (1) Q35 overview.
     http://wiki.qemu.org/images/4/4e/Q35.pdf
 (2) A comparison between PCI and PCI Express technologies.
     http://wiki.qemu.org/images/f/f6/PCIvsPCIe.pdf

Note: The usage examples are not intended to replace the full
documentation, please use QEMU help to retrieve all options.

2. Device placement strategy
============================
QEMU does not have a clear socket-device matching mechanism
and allows any PCI/PCI Express device to be plugged into any
PCI/PCI Express slot.
Plugging a PCI device into a PCI Express slot might not always work and
is weird anyway since it cannot be done for "bare metal".
Plugging a PCI Express device into a PCI slot will hide the Extended
Configuration Space thus is also not recommended.

The recommendation is to separate the PCI Express and PCI hierarchies.
PCI Express devices should be plugged only into PCI Express Root Ports and
PCI Express Downstream ports.

2.1 Root Bus (pcie.0)
=====================
Place only the following kinds of devices directly on the Root Complex:
    (1) PCI Devices (e.g. network card, graphics card, IDE controller),
        not controllers. Place only legacy PCI devices on
        the Root Complex. These will be considered Integrated Endpoints.
        Note: Integrated Endpoints are not hot-pluggable.

        Although the PCI Express spec does not forbid PCI Express devices as
        Integrated Endpoints, existing hardware mostly integrates legacy PCI
        devices with the Root Complex. Guest OSes are suspected to behave
        strangely when PCI Express devices are integrated
        with the Root Complex.

    (2) PCI Express Root Ports (ioh3420), for starting exclusively PCI Express
        hierarchies.

    (3) DMI-PCI Bridges (i82801b11-bridge), for starting legacy PCI
        hierarchies.

    (4) Extra Root Complexes (pxb-pcie), if multiple PCI Express Root Buses
        are needed.

   pcie.0 bus
   ----------------------------------------------------------------------------
        |                |                    |                  |
   -----------   ------------------   ------------------   --------------
   | PCI Dev |   | PCIe Root Port |   | DMI-PCI Bridge |   |  pxb-pcie  |
   -----------   ------------------   ------------------   --------------

2.1.1 To plug a device into pcie.0 as a Root Complex Integrated Endpoint use:
          -device <dev>[,bus=pcie.0]
2.1.2 To expose a new PCI Express Root Bus use:
          -device pxb-pcie,id=pcie.1,bus_nr=x[,numa_node=y][,addr=z]
      Only PCI Express Root Ports and DMI-PCI bridges can be connected
      to the pcie.1 bus:
          -device ioh3420,id=root_port1[,bus=pcie.1][,chassis=x][,slot=y][,addr=z]                                     \
          -device i82801b11-bridge,id=dmi_pci_bridge1,bus=pcie.1


2.2 PCI Express only hierarchy
==============================
Always use PCI Express Root Ports to start PCI Express hierarchies.

A PCI Express Root bus supports up to 32 devices. Since each
PCI Express Root Port is a function and a multi-function
device may support up to 8 functions, the maximum possible
number of PCI Express Root Ports per PCI Express Root Bus is 256.

Prefer grouping PCI Express Root Ports into multi-function devices
to keep a simple flat hierarchy that is enough for most scenarios.
Only use PCI Express Switches (x3130-upstream, xio3130-downstream)
if there is no more room for PCI Express Root Ports.
Please see section 4. for further justifications.

Plug only PCI Express devices into PCI Express Ports.


   pcie.0 bus
   ----------------------------------------------------------------------------------
        |                 |                                    |
   -------------    -------------                        -------------
   | Root Port |    | Root Port |                        | Root Port |
   ------------     -------------                        -------------
         |                            -------------------------|------------------------
    ------------                      |                 -----------------              |
    | PCIe Dev |                      |    PCI Express  | Upstream Port |              |
    ------------                      |      Switch     -----------------              |
                                      |                  |            |                |
                                      |    -------------------    -------------------  |
                                      |    | Downstream Port |    | Downstream Port |  |
                                      |    -------------------    -------------------  |
                                      -------------|-----------------------|------------
                                             ------------
                                             | PCIe Dev |
                                             ------------

2.2.1 Plugging a PCI Express device into a PCI Express Root Port:
          -device ioh3420,id=root_port1,chassis=x,slot=y[,bus=pcie.0][,addr=z]  \
          -device <dev>,bus=root_port1
2.2.2 Using multi-function PCI Express Root Ports:
      -device ioh3420,id=root_port1,multifunction=on,chassis=x,addr=z.0[,slot=y][,bus=pcie.0] \
      -device ioh3420,id=root_port2,chassis=x1,addr=z.1[,slot=y1][,bus=pcie.0] \
      -device ioh3420,id=root_port3,chassis=x2,addr=z.2[,slot=y2][,bus=pcie.0] \
2.2.3 Plugging a PCI Express device into a Switch:
      -device ioh3420,id=root_port1,chassis=x,slot=y[,bus=pcie.0][,addr=z]  \
      -device x3130-upstream,id=upstream_port1,bus=root_port1[,addr=x]          \
      -device xio3130-downstream,id=downstream_port1,bus=upstream_port1,chassis=x1,slot=y1[,addr=z1]] \
      -device <dev>,bus=downstream_port1

Notes:
  - (slot, chassis) pair is mandatory and must be unique for each
    PCI Express Root Port. slot defaults to 0 when not specified.
  - 'addr' parameter can be 0 for all the examples above.


2.3 PCI only hierarchy
======================
Legacy PCI devices can be plugged into pcie.0 as Integrated Endpoints,
but, as mentioned in section 5, doing so means the legacy PCI
device in question will be incapable of hot-unplugging.
Besides that use DMI-PCI Bridges (i82801b11-bridge) in combination
with PCI-PCI Bridges (pci-bridge) to start PCI hierarchies.

Prefer flat hierarchies. For most scenarios a single DMI-PCI Bridge
(having 32 slots) and several PCI-PCI Bridges attached to it
(each supporting also 32 slots) will support hundreds of legacy devices.
The recommendation is to populate one PCI-PCI Bridge under the DMI-PCI Bridge
until is full and then plug a new PCI-PCI Bridge...

   pcie.0 bus
   ----------------------------------------------
        |                            |
   -----------               ------------------
   | PCI Dev |               | DMI-PCI BRIDGE |
   ----------                ------------------
                               |            |
                  ------------------    ------------------
                  | PCI-PCI Bridge |    | PCI-PCI Bridge |   ...
                  ------------------    ------------------
                                         |           |
                                  -----------     -----------
                                  | PCI Dev |     | PCI Dev |
                                  -----------     -----------

2.3.1 To plug a PCI device into pcie.0 as an Integrated Endpoint use:
      -device <dev>[,bus=pcie.0]
2.3.2 Plugging a PCI device into a PCI-PCI Bridge:
      -device i82801b11-bridge,id=dmi_pci_bridge1[,bus=pcie.0]                        \
      -device pci-bridge,id=pci_bridge1,bus=dmi_pci_bridge1[,chassis_nr=x][,addr=y]   \
      -device <dev>,bus=pci_bridge1[,addr=x]
      Note that 'addr' cannot be 0 unless shpc=off parameter is passed to
      the PCI Bridge.

3. IO space issues
===================
The PCI Express Root Ports and PCI Express Downstream ports are seen by
Firmware/Guest OS as PCI-PCI Bridges. As required by the PCI spec, each
such Port should be reserved a 4K IO range for, even though only one
(multifunction) device can be plugged into each Port. This results in
poor IO space utilization.

The firmware used by QEMU (SeaBIOS/OVMF) may try further optimizations
by not allocating IO space for each PCI Express Root / PCI Express
Downstream port if:
    (1) the port is empty, or
    (2) the device behind the port has no IO BARs.

The IO space is very limited, to 65536 byte-wide IO ports, and may even be
fragmented by fixed IO ports owned by platform devices resulting in at most
10 PCI Express Root Ports or PCI Express Downstream Ports per system
if devices with IO BARs are used in the PCI Express hierarchy. Using the
proposed device placing strategy solves this issue by using only
PCI Express devices within PCI Express hierarchy.

The PCI Express spec requires that PCI Express devices work properly
without using IO ports. The PCI hierarchy has no such limitations.


4. Bus numbers issues
======================
Each PCI domain can have up to only 256 buses and the QEMU PCI Express
machines do not support multiple PCI domains even if extra Root
Complexes (pxb-pcie) are used.

Each element of the PCI Express hierarchy (Root Complexes,
PCI Express Root Ports, PCI Express Downstream/Upstream ports)
uses one bus number. Since only one (multifunction) device
can be attached to a PCI Express Root Port or PCI Express Downstream
Port it is advised to plan in advance for the expected number of
devices to prevent bus number starvation.

Avoiding PCI Express Switches (and thereby striving for a 'flatter' PCI
Express hierarchy) enables the hierarchy to not spend bus numbers on
Upstream Ports.

The bus_nr properties of the pxb-pcie devices partition the 0..255 bus
number space. All bus numbers assigned to the buses recursively behind a
given pxb-pcie device's root bus must fit between the bus_nr property of
that pxb-pcie device, and the lowest of the higher bus_nr properties
that the command line sets for other pxb-pcie devices.


5. Hot-plug
============
The PCI Express root buses (pcie.0 and the buses exposed by pxb-pcie devices)
do not support hot-plug, so any devices plugged into Root Complexes
cannot be hot-plugged/hot-unplugged:
    (1) PCI Express Integrated Endpoints
    (2) PCI Express Root Ports
    (3) DMI-PCI Bridges
    (4) pxb-pcie

Be aware that PCI Express Downstream Ports can't be hot-plugged into
an existing PCI Express Upstream Port.

PCI devices can be hot-plugged into PCI-PCI Bridges. The PCI hot-plug is ACPI
based and can work side by side with the PCI Express native hot-plug.

PCI Express devices can be natively hot-plugged/hot-unplugged into/from
PCI Express Root Ports (and PCI Express Downstream Ports).

5.1 Planning for hot-plug:
    (1) PCI hierarchy
        Leave enough PCI-PCI Bridge slots empty or add one
        or more empty PCI-PCI Bridges to the DMI-PCI Bridge.

        For each such PCI-PCI Bridge the Guest Firmware is expected to reserve
        4K IO space and 2M MMIO range to be used for all devices behind it.

        Because of the hard IO limit of around 10 PCI Bridges (~ 40K space)
        per system don't use more than 9 PCI-PCI Bridges, leaving 4K for the
        Integrated Endpoints. (The PCI Express Hierarchy needs no IO space).

    (2) PCI Express hierarchy:
        Leave enough PCI Express Root Ports empty. Use multifunction
        PCI Express Root Ports (up to 8 ports per pcie.0 slot)
        on the Root Complex(es), for keeping the
        hierarchy as flat as possible, thereby saving PCI bus numbers.
        Don't use PCI Express Switches if you don't have
        to, each one of those uses an extra PCI bus (for its Upstream Port)
        that could be put to better use with another Root Port or Downstream
        Port, which may come handy for hot-plugging another device.


5.3 Hot-plug example:
Using HMP: (add -monitor stdio to QEMU command line)
  device_add <dev>,id=<id>,bus=<PCI Express Root Port Id/PCI Express Downstream Port Id/PCI-PCI Bridge Id/>


6. Device assignment
====================
Host devices are mostly PCI Express and should be plugged only into
PCI Express Root Ports or PCI Express Downstream Ports.
PCI-PCI Bridge slots can be used for legacy PCI host devices.

6.1 How to detect if a device is PCI Express:
  > lspci -s 03:00.0 -v (as root)

    03:00.0 Network controller: Intel Corporation Wireless 7260 (rev 83)
    Subsystem: Intel Corporation Dual Band Wireless-AC 7260
    Flags: bus master, fast devsel, latency 0, IRQ 50
    Memory at f0400000 (64-bit, non-prefetchable) [size=8K]
    Capabilities: [c8] Power Management version 3
    Capabilities: [d0] MSI: Enable+ Count=1/1 Maskable- 64bit+
    Capabilities: [40] Express Endpoint, MSI 00
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    Capabilities: [100] Advanced Error Reporting
    Capabilities: [140] Device Serial Number 7c-7a-91-ff-ff-90-db-20
    Capabilities: [14c] Latency Tolerance Reporting
    Capabilities: [154] Vendor Specific Information: ID=cafe Rev=1 Len=014 

If you can see the "Express Endpoint" capability in the
output, then the device is indeed PCI Express.


7. Virtio devices
=================
Virtio devices plugged into the PCI hierarchy or as Integrated Endpoints
will remain PCI and have transitional behaviour as default.
Transitional virtio devices work in both IO and MMIO modes depending on
the guest support. The Guest firmware will assign both IO and MMIO resources
to transitional virtio devices.

Virtio devices plugged into PCI Express ports are PCI Express devices and
have "1.0" behavior by default without IO support.
In both cases disable-legacy and disable-modern properties can be used
to override the behaviour.

Note that setting disable-legacy=off will enable legacy mode (enabling
legacy behavior) for PCI Express virtio devices causing them to
require IO space, which, given the limited available IO space, may quickly
lead to resource exhaustion, and is therefore strongly discouraged.


8. Conclusion
==============
The proposal offers a usage model that is easy to understand and follow
and at the same time overcomes the PCI Express architecture limitations.
Commit	Line	Data
453ac883 MA	1	PCI EXPRESS GUIDELINES
	2	======================
	3
	4	1. Introduction
	5	================
	6	The doc proposes best practices on how to use PCI Express/PCI device
	7	in PCI Express based machines and explains the reasoning behind them.
	8
	9	The following presentations accompany this document:
	10	(1) Q35 overview.
	11	http://wiki.qemu.org/images/4/4e/Q35.pdf
	12	(2) A comparison between PCI and PCI Express technologies.
	13	http://wiki.qemu.org/images/f/f6/PCIvsPCIe.pdf
	14
	15	Note: The usage examples are not intended to replace the full
	16	documentation, please use QEMU help to retrieve all options.
	17
	18	2. Device placement strategy
	19	============================
	20	QEMU does not have a clear socket-device matching mechanism
	21	and allows any PCI/PCI Express device to be plugged into any
	22	PCI/PCI Express slot.
	23	Plugging a PCI device into a PCI Express slot might not always work and
	24	is weird anyway since it cannot be done for "bare metal".
	25	Plugging a PCI Express device into a PCI slot will hide the Extended
	26	Configuration Space thus is also not recommended.
	27
	28	The recommendation is to separate the PCI Express and PCI hierarchies.
	29	PCI Express devices should be plugged only into PCI Express Root Ports and
	30	PCI Express Downstream ports.
	31
	32	2.1 Root Bus (pcie.0)
	33	=====================
	34	Place only the following kinds of devices directly on the Root Complex:
	35	(1) PCI Devices (e.g. network card, graphics card, IDE controller),
	36	not controllers. Place only legacy PCI devices on
	37	the Root Complex. These will be considered Integrated Endpoints.
	38	Note: Integrated Endpoints are not hot-pluggable.
	39
	40	Although the PCI Express spec does not forbid PCI Express devices as
	41	Integrated Endpoints, existing hardware mostly integrates legacy PCI
	42	devices with the Root Complex. Guest OSes are suspected to behave
	43	strangely when PCI Express devices are integrated
	44	with the Root Complex.
	45
	46	(2) PCI Express Root Ports (ioh3420), for starting exclusively PCI Express
	47	hierarchies.
	48
	49	(3) DMI-PCI Bridges (i82801b11-bridge), for starting legacy PCI
	50	hierarchies.
	51
	52	(4) Extra Root Complexes (pxb-pcie), if multiple PCI Express Root Buses
	53	are needed.
	54
	55	pcie.0 bus
	56	----------------------------------------------------------------------------
	57	\| \| \| \|
	58	----------- ------------------ ------------------ --------------
	59	\| PCI Dev \| \| PCIe Root Port \| \| DMI-PCI Bridge \| \| pxb-pcie \|
	60	----------- ------------------ ------------------ --------------
	61
	62	2.1.1 To plug a device into pcie.0 as a Root Complex Integrated Endpoint use:
	63	-device <dev>[,bus=pcie.0]
	64	2.1.2 To expose a new PCI Express Root Bus use:
65	-device pxb-pcie,id=pcie.1,bus_nr=x[,numa_node=y][,addr=z]
66	Only PCI Express Root Ports and DMI-PCI bridges can be connected
67	to the pcie.1 bus:
68	-device ioh3420,id=root_port1[,bus=pcie.1][,chassis=x][,slot=y][,addr=z] \
69	-device i82801b11-bridge,id=dmi_pci_bridge1,bus=pcie.1
70
71
72	2.2 PCI Express only hierarchy
73	==============================
74	Always use PCI Express Root Ports to start PCI Express hierarchies.
75
76	A PCI Express Root bus supports up to 32 devices. Since each
77	PCI Express Root Port is a function and a multi-function
78	device may support up to 8 functions, the maximum possible
79	number of PCI Express Root Ports per PCI Express Root Bus is 256.
80
81	Prefer grouping PCI Express Root Ports into multi-function devices
82	to keep a simple flat hierarchy that is enough for most scenarios.
83	Only use PCI Express Switches (x3130-upstream, xio3130-downstream)
84	if there is no more room for PCI Express Root Ports.
85	Please see section 4. for further justifications.
86
87	Plug only PCI Express devices into PCI Express Ports.
88
89
90	pcie.0 bus
91	----------------------------------------------------------------------------------
92	\| \| \|
93	------------- ------------- -------------
94	\| Root Port \| \| Root Port \| \| Root Port \|
95	------------ ------------- -------------
96	\| -------------------------\|------------------------
97	------------ \| ----------------- \|
98	\| PCIe Dev \| \| PCI Express \| Upstream Port \| \|
99	------------ \| Switch ----------------- \|
100	\| \| \| \|
101	\| ------------------- ------------------- \|
102	\| \| Downstream Port \| \| Downstream Port \| \|
103	\| ------------------- ------------------- \|
104	-------------\|-----------------------\|------------
105	------------
106	\| PCIe Dev \|
107	------------
108
109	2.2.1 Plugging a PCI Express device into a PCI Express Root Port:
110	-device ioh3420,id=root_port1,chassis=x,slot=y[,bus=pcie.0][,addr=z] \
111	-device <dev>,bus=root_port1
112	2.2.2 Using multi-function PCI Express Root Ports:
2e41dfe1 C	113	-device ioh3420,id=root_port1,multifunction=on,chassis=x,addr=z.0[,slot=y][,bus=pcie.0] \
	114	-device ioh3420,id=root_port2,chassis=x1,addr=z.1[,slot=y1][,bus=pcie.0] \
	115	-device ioh3420,id=root_port3,chassis=x2,addr=z.2[,slot=y2][,bus=pcie.0] \
	116	2.2.3 Plugging a PCI Express device into a Switch:
453ac883 MA	117	-device ioh3420,id=root_port1,chassis=x,slot=y[,bus=pcie.0][,addr=z] \
	118	-device x3130-upstream,id=upstream_port1,bus=root_port1[,addr=x] \
	119	-device xio3130-downstream,id=downstream_port1,bus=upstream_port1,chassis=x1,slot=y1[,addr=z1]] \
	120	-device <dev>,bus=downstream_port1
	121
	122	Notes:
2e41dfe1 C	123	- (slot, chassis) pair is mandatory and must be unique for each
2e41dfe1 C	124	PCI Express Root Port. slot defaults to 0 when not specified.
453ac883 MA	125	- 'addr' parameter can be 0 for all the examples above.
	126
	127
	128	2.3 PCI only hierarchy
	129	======================
	130	Legacy PCI devices can be plugged into pcie.0 as Integrated Endpoints,
	131	but, as mentioned in section 5, doing so means the legacy PCI
	132	device in question will be incapable of hot-unplugging.
	133	Besides that use DMI-PCI Bridges (i82801b11-bridge) in combination
	134	with PCI-PCI Bridges (pci-bridge) to start PCI hierarchies.
	135
	136	Prefer flat hierarchies. For most scenarios a single DMI-PCI Bridge
	137	(having 32 slots) and several PCI-PCI Bridges attached to it
	138	(each supporting also 32 slots) will support hundreds of legacy devices.
	139	The recommendation is to populate one PCI-PCI Bridge under the DMI-PCI Bridge
	140	until is full and then plug a new PCI-PCI Bridge...
	141
	142	pcie.0 bus
	143	----------------------------------------------
	144	\| \|
	145	----------- ------------------
	146	\| PCI Dev \| \| DMI-PCI BRIDGE \|
	147	---------- ------------------
	148	\| \|
	149	------------------ ------------------
	150	\| PCI-PCI Bridge \| \| PCI-PCI Bridge \| ...
	151	------------------ ------------------
	152	\| \|
	153	----------- -----------
	154	\| PCI Dev \| \| PCI Dev \|
	155	----------- -----------
	156
	157	2.3.1 To plug a PCI device into pcie.0 as an Integrated Endpoint use:
	158	-device <dev>[,bus=pcie.0]
	159	2.3.2 Plugging a PCI device into a PCI-PCI Bridge:
	160	-device i82801b11-bridge,id=dmi_pci_bridge1[,bus=pcie.0] \
	161	-device pci-bridge,id=pci_bridge1,bus=dmi_pci_bridge1[,chassis_nr=x][,addr=y] \
	162	-device <dev>,bus=pci_bridge1[,addr=x]
	163	Note that 'addr' cannot be 0 unless shpc=off parameter is passed to
	164	the PCI Bridge.
	165
	166	3. IO space issues
	167	===================
	168	The PCI Express Root Ports and PCI Express Downstream ports are seen by
	169	Firmware/Guest OS as PCI-PCI Bridges. As required by the PCI spec, each
	170	such Port should be reserved a 4K IO range for, even though only one
	171	(multifunction) device can be plugged into each Port. This results in
	172	poor IO space utilization.
	173
	174	The firmware used by QEMU (SeaBIOS/OVMF) may try further optimizations
	175	by not allocating IO space for each PCI Express Root / PCI Express
	176	Downstream port if:
	177	(1) the port is empty, or
	178	(2) the device behind the port has no IO BARs.
	179
	180	The IO space is very limited, to 65536 byte-wide IO ports, and may even be
	181	fragmented by fixed IO ports owned by platform devices resulting in at most
	182	10 PCI Express Root Ports or PCI Express Downstream Ports per system
	183	if devices with IO BARs are used in the PCI Express hierarchy. Using the
	184	proposed device placing strategy solves this issue by using only
	185	PCI Express devices within PCI Express hierarchy.
	186
	187	The PCI Express spec requires that PCI Express devices work properly
	188	without using IO ports. The PCI hierarchy has no such limitations.
189
190
191	4. Bus numbers issues
192	======================
193	Each PCI domain can have up to only 256 buses and the QEMU PCI Express
194	machines do not support multiple PCI domains even if extra Root
195	Complexes (pxb-pcie) are used.
196
197	Each element of the PCI Express hierarchy (Root Complexes,
198	PCI Express Root Ports, PCI Express Downstream/Upstream ports)
199	uses one bus number. Since only one (multifunction) device
200	can be attached to a PCI Express Root Port or PCI Express Downstream
201	Port it is advised to plan in advance for the expected number of
202	devices to prevent bus number starvation.
203
204	Avoiding PCI Express Switches (and thereby striving for a 'flatter' PCI
205	Express hierarchy) enables the hierarchy to not spend bus numbers on
206	Upstream Ports.
207
208	The bus_nr properties of the pxb-pcie devices partition the 0..255 bus
209	number space. All bus numbers assigned to the buses recursively behind a
210	given pxb-pcie device's root bus must fit between the bus_nr property of
211	that pxb-pcie device, and the lowest of the higher bus_nr properties
212	that the command line sets for other pxb-pcie devices.
213
214
215	5. Hot-plug
216	============
217	The PCI Express root buses (pcie.0 and the buses exposed by pxb-pcie devices)
218	do not support hot-plug, so any devices plugged into Root Complexes
219	cannot be hot-plugged/hot-unplugged:
220	(1) PCI Express Integrated Endpoints
221	(2) PCI Express Root Ports
222	(3) DMI-PCI Bridges
223	(4) pxb-pcie
224
225	Be aware that PCI Express Downstream Ports can't be hot-plugged into
226	an existing PCI Express Upstream Port.
227
228	PCI devices can be hot-plugged into PCI-PCI Bridges. The PCI hot-plug is ACPI
229	based and can work side by side with the PCI Express native hot-plug.
230
231	PCI Express devices can be natively hot-plugged/hot-unplugged into/from
232	PCI Express Root Ports (and PCI Express Downstream Ports).
233
234	5.1 Planning for hot-plug:
235	(1) PCI hierarchy
236	Leave enough PCI-PCI Bridge slots empty or add one
237	or more empty PCI-PCI Bridges to the DMI-PCI Bridge.
238
239	For each such PCI-PCI Bridge the Guest Firmware is expected to reserve
240	4K IO space and 2M MMIO range to be used for all devices behind it.
241
242	Because of the hard IO limit of around 10 PCI Bridges (~ 40K space)
243	per system don't use more than 9 PCI-PCI Bridges, leaving 4K for the
244	Integrated Endpoints. (The PCI Express Hierarchy needs no IO space).
245
246	(2) PCI Express hierarchy:
247	Leave enough PCI Express Root Ports empty. Use multifunction
248	PCI Express Root Ports (up to 8 ports per pcie.0 slot)
249	on the Root Complex(es), for keeping the
250	hierarchy as flat as possible, thereby saving PCI bus numbers.
251	Don't use PCI Express Switches if you don't have
252	to, each one of those uses an extra PCI bus (for its Upstream Port)
253	that could be put to better use with another Root Port or Downstream
254	Port, which may come handy for hot-plugging another device.
255
256
257	5.3 Hot-plug example:
258	Using HMP: (add -monitor stdio to QEMU command line)
259	device_add <dev>,id=<id>,bus=<PCI Express Root Port Id/PCI Express Downstream Port Id/PCI-PCI Bridge Id/>
260
261
262	6. Device assignment
263	====================
264	Host devices are mostly PCI Express and should be plugged only into
265	PCI Express Root Ports or PCI Express Downstream Ports.
266	PCI-PCI Bridge slots can be used for legacy PCI host devices.
267
268	6.1 How to detect if a device is PCI Express:
269	> lspci -s 03:00.0 -v (as root)
270
271	03:00.0 Network controller: Intel Corporation Wireless 7260 (rev 83)
272	Subsystem: Intel Corporation Dual Band Wireless-AC 7260
273	Flags: bus master, fast devsel, latency 0, IRQ 50
274	Memory at f0400000 (64-bit, non-prefetchable) [size=8K]
275	Capabilities: [c8] Power Management version 3
276	Capabilities: [d0] MSI: Enable+ Count=1/1 Maskable- 64bit+
277	Capabilities: [40] Express Endpoint, MSI 00
278	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
279	Capabilities: [100] Advanced Error Reporting
280	Capabilities: [140] Device Serial Number 7c-7a-91-ff-ff-90-db-20
281	Capabilities: [14c] Latency Tolerance Reporting
282	Capabilities: [154] Vendor Specific Information: ID=cafe Rev=1 Len=014
283
284	If you can see the "Express Endpoint" capability in the
285	output, then the device is indeed PCI Express.
286
287
288	7. Virtio devices
289	=================
290	Virtio devices plugged into the PCI hierarchy or as Integrated Endpoints
291	will remain PCI and have transitional behaviour as default.
292	Transitional virtio devices work in both IO and MMIO modes depending on
293	the guest support. The Guest firmware will assign both IO and MMIO resources
294	to transitional virtio devices.
295
296	Virtio devices plugged into PCI Express ports are PCI Express devices and
297	have "1.0" behavior by default without IO support.
298	In both cases disable-legacy and disable-modern properties can be used
299	to override the behaviour.
300
301	Note that setting disable-legacy=off will enable legacy mode (enabling
302	legacy behavior) for PCI Express virtio devices causing them to
303	require IO space, which, given the limited available IO space, may quickly
304	lead to resource exhaustion, and is therefore strongly discouraged.
305
306
307	8. Conclusion
308	==============
309	The proposal offers a usage model that is easy to understand and follow
310	and at the same time overcomes the PCI Express architecture limitations.