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@xxxxxxxxxx>
---
docs/formatdomain.html.in | 6 +-
docs/pci-addresses.rst | 184 ++++++++++++++++++++++++++++++++++++++
2 files changed, 189 insertions(+), 1 deletion(-)
create mode 100644 docs/pci-addresses.rst
diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 6f43976815..0077666862 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -4286,7 +4286,11 @@
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
diff --git a/docs/pci-addresses.rst b/docs/pci-addresses.rst
new file mode 100644
index 0000000000..96c6466899
--- /dev/null
+++ b/docs/pci-addresses.rst
@@ -0,0 +1,184 @@
+========================================
+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* rather than the *source address*.
+
+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``.
+
+Of course, all the rules and behaviors described above still apply.
--
2.25.2
