Perhaps surprisingly, most libvirt guests support only limited PCI device hotplug out of the box, or even none at all.
The reason for this apparent limitation is the fact that each hotplugged PCI device might require additional PCI controllers to be added to the guest, and libvirt has no way of knowing in advance how many devices will be hotplugged during the guest's lifetime, thus making it impossible to automatically provide the right amount of PCI controllers: any arbitrary number would end up being too big for some users, and too small for others.
Ultimately, the user is the only one who knows how much the guest will need to grow dynamically, so the responsibility of planning a suitable PCI topology in advance falls on them.
This document aims at providing all the information needed to successfully plan the PCI topology of a guest. Note that the details can vary a lot between architectures and even machine types, hence the way it's organized.
This is a PCI Express native machine type. The default PCI topology looks like
<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='0x10'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/> </controller>
and supports hotplugging a single PCI Express device, either emulated or assigned from the host.
Slots on the pcie-root controller do not support
hotplug, so the device will be hotplugged into the
pcie-root-port controller. If you plan to hotplug
more than a single PCI Express device, you should add a suitable
number of pcie-root-port controllers when defining
the guest: for example, add
<controller type='pci' model='pcie-root-port'/> <controller type='pci' model='pcie-root-port'/> <controller type='pci' model='pcie-root-port'/>
if you expect to hotplug up to three PCI Express devices, either emulated or assigned from the host. That's all the information you need to provide: libvirt will fill in the remaining details automatically.
If you expect to hotplug legacy PCI devices, then you will need specialized controllers, since all those mentioned above are intended for PCI Express devices only: add
<controller type='pci' model='dmi-to-pci-bridge'/> <controller type='pci' model='pci-bridge'/>
and you'll be able to hotplug up to 31 legacy PCI devices, either emulated or assigned from the host.
This is a legacy PCI native machine type. The default PCI topology looks like
<controller type='pci' index='0' model='pci-root'/>
where each of the 31 slots on the pci-root
controller is hotplug capable and can accept a legacy PCI
device, either emulated or assigned from the guest.
The default PCI topology for the pseries machine
type looks like
<controller type='pci' index='0' model='pci-root'> <model name='spapr-pci-host-bridge'/> <target index='0'/> </controller>
The 31 slots on a pci-root controller are all
hotplug capable and, despite the name suggesting otherwise,
starting with QEMU 2.9 all of them can accept PCI Express
devices in addition to legacy PCI devices; however,
libvirt will only place emulated devices on the default
pci-root controller.
In order to take advantage of improved error reporting and
recovering capabilities, PCI devices assigned from the
host need to be isolated by placing each on a separate
pci-root controller, which has to be prepared
in advance for hotplug to work: for example, add
<controller type='pci' model='pci-root'/> <controller type='pci' model='pci-root'/> <controller type='pci' model='pci-root'/>
if you expect to hotplug up to three PCI devices assigned from the host.
This machine type mostly behaves the same as the q35 machine type, so you can just refer to that section for information.
The only difference worth mentioning is that using legacy
PCI for mach-virt guests is extremely uncommon,
so you'll probably never need to add controllers other than
pcie-root-port.