docs: Add pci-addresses.rst

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: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NLaine Stump <laine@redhat.com>

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

docs/pci-addresses.rst

+========================================
+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.