======================================== PCI addresses in domain XML and guest OS ======================================== .. contents:: When discussing PCI addresses, it's important to understand 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 ::

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 ::

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 ::

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 ::

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. zPCI addresses ============== For s390x machines, PCI addresses are handled yet differently. No topology information is relayed in the PCI addresses; instead, the ``fid`` and ``uid`` elements of the ``zpci`` device convey information. In the simplest case, the following XML snippet ::

will result in the following in a Linux guest:: 0001:00:00.0 Ethernet controller: Red Hat, Inc. Virtio network device Note that the PCI bridge is not visible in the guest; s390x always has a flat topology. Neither are any changes in the PCI address visible in the guest; replacing the PCI address for the virtio-net device with ::

will result in the exactly same view in the guest, as the addresses there are generated from the information provided via the ``zpci`` element (in fact, from the ``uid``). Therefore, replacing the virtio-net device definition with the following XML snippet ::

will yield the following result in a Linux guest:: 0007:00:00.0 Ethernet controller: Red Hat, Inc. Virtio network device