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(a)redhat.com&gt; --- 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