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docs: Add pci-addresses.rst

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This document describes the relationship between PCI addresses as
seen in the domain XML and by the guest OS, which is a topic that
people get confused by time and time again.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>
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Andrea Bolognani 2020-04-14 19:37:09 +02:00
parent a7db0b757d
commit 2923e7a3dd
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element with no other attributes as an explicit request to
assign a PCI address for the device rather than some other
type of address that may also be appropriate for that same
device (e.g. virtio-mmio).
device (e.g. virtio-mmio).<br/>
The relationship between the PCI addresses configured in the domain
XML and those seen by the guest OS can sometime seem confusing: a
separate document describes <a href="pci-addresses.html">how PCI
addresses work</a> in more detail.
</dd>
<dt><code>drive</code></dt>
<dd>Drive addresses have the following additional

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========================================
PCI addresses in domain XML and guest OS
========================================
.. contents::
When discussing PCI addresses, it's important to understand the the
relationship between the addresses that can be seen in the domain XML
and those that are visible inside the guest OS.
Simple cases
============
When the PCI topology of the VM is very simple, the PCI addresses
will usually match.
For example, the domain XML snippet
::
<controller type='pci' index='0' model='pcie-root'/>
<controller type='pci' index='1' model='pcie-root-port'>
<model name='pcie-root-port'/>
<target chassis='1' port='0x8'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/>
</controller>
<interface type='network'>
<source network='default'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</interface>
will result in the PCI topology
::
0000:00:00.0 Host bridge: Intel Corporation 82G33/G31/P35/P31 Express DRAM Controller
0000:00:01.0 PCI bridge: Red Hat, Inc. QEMU PCIe Root port
0000:01:00.0 Ethernet controller: Red Hat, Inc. Virtio network device (rev 01)
showing up in the guest OS.
The PCI address of the ``virtio-net`` adapter, ``0000:01:00.0``, is
the same in both cases, so there's no confusion.
More complex cases
==================
In more complex cases, the PCI address visible in the domain XML will
correlate to the one seen by the guest OS in a less obvious way.
pcie-expander-bus
-----------------
This fairly uncommon device, which can be used with ``x86_64/q35``
guests, will help illustrate one such scenario.
For example, the domain XML snippet
::
<controller type='pci' index='0' model='pcie-root'/>
<controller type='pci' index='1' model='pcie-expander-bus'>
<model name='pxb-pcie'/>
<target busNr='254'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/>
</controller>
<controller type='pci' index='2' model='pcie-root-port'>
<model name='pcie-root-port'/>
<target chassis='2' port='0x0'/>
<address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</controller>
<interface type='network'>
<source network='default'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x02' slot='0x00' function='0x0'/>
</interface>
will result in the PCI topology
::
0000:00:00.0 Host bridge: Intel Corporation 82G33/G31/P35/P31 Express DRAM Controller
0000:00:01.0 Host bridge: Red Hat, Inc. QEMU PCIe Expander bridge
0000:fe:00.0 PCI bridge: Red Hat, Inc. QEMU PCIe Root port
0000:ff:00.0 Ethernet controller: Red Hat, Inc. Virtio network device (rev 01)
showing up in the guest OS.
This time the addresses don't match: this is because the ``busNr``
property for the ``pcie-expander-bus`` controller causes it to show
up as bus 254 (``0xfe`` in hexadecimal) instead of bus 1 as one might
expect based on its ``index`` property.
How can the domain XML shown above work at all, then? Surely the
``pcie-root-port`` controller and the ``virtio-net`` adapter should
use ``bus=0xfe`` and ``bus=0xff`` respectively for the configuration
to be accepted by libvirt?
As it turns out, that's not the case. The reason for this is that
QEMU, and consequently libvirt, uses the ``bus`` property of a
device's PCI address only to match it with the PCI controller that
has the same ``index`` property, and not to set the actual PCI
address, which is decided by the guest OS.
So, by looking at the XML snippet above, we can see that the
``virtio-net`` adapter plugs into the ``pcie-root-port`` controller,
which plugs into the ``pcie-expander-bus`` controller, which plugs
into ``pcie-root``: the guest OS sees the same topology, but assigns
different PCI addresses to some of its component.
The takeaway is that the *relationship* between controllers are the
very same whether you look at the domain XML or at the guest OS, but
the *actual PCI addresses* are not guaranteed to match and in fact,
except for the very simplest cases, they usually will not.
spapr-pci-host-bridge
---------------------
This device, which is unique to ``ppc64/pseries`` guests, will help
illustrate another scenario.
For example, the domain XML snippet
::
<controller type='pci' index='0' model='pci-root'>
<model name='spapr-pci-host-bridge'/>
<target index='0'/>
</controller>
<controller type='pci' index='1' model='pci-root'>
<model name='spapr-pci-host-bridge'/>
<target index='1'/>
</controller>
<interface type='network'>
<source network='default'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x01' slot='0x01' function='0x0'/>
</interface>
will result in the PCI topology
::
0001:00:01.0 Ethernet controller: Red Hat, Inc. Virtio network device
showing up in the guest OS. Note that the two
``spapr-pci-host-bridge`` controllers are not listed.
This time, in addition to the bus not matching just like in the
previous example, the interesting part is that the domain doesn't
match either: this is because each ``spapr-pci-host-bridge``
controller creates a separate PCI domain.
Once again, while the PCI addresses seen in the domain XML and those
seen by the guest OS do not match, the relationships between the
various devices are preserved.
Device assignment
=================
When using VFIO to assign host devices to a guest, an additional
caveat to keep in mind that the guest OS will base its decisions upon
the *target address* (guest side) rather than the *source address*
(host side).
For example, the domain XML snippet
::
<hostdev mode='subsystem' type='pci' managed='yes'>
<driver name='vfio'/>
<source>
<address domain='0x0001' bus='0x08' slot='0x00' function='0x0'/>
</source>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/>
</hostdev>
will result in the device showing up as ``0000:00:01.0`` in the
guest OS rather than as ``0001:08:00.1``, which is the address of the
device on the host.
Of course, all the rules and behaviors described above still apply.