20.23. Adding Multifunction PCI Devices to KVM Guest Virtual Machines
To add a multi-function PCI device to a KVM guest virtual machine:
- Run the
virsh edit guestnamecommand to edit the XML configuration file for the guest virtual machine. - In the
<address>element, add amultifunction='on'attribute. This enables the use of other functions for the particular multifunction PCI device.<disk type='file' device='disk'> <driver name='qemu' type='raw' cache='none'/> <source file='/var/lib/libvirt/images/rhel62-1.img'/> <target dev='vda' bus='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0' multifunction='on'/> </disk>
For a PCI device with two functions, amend the XML configuration file to include a second device with the same slot number as the first device and a different function number, such asfunction='0x1'. For Example:<disk type='file' device='disk'> <driver name='qemu' type='raw' cache='none'/> <source file='/var/lib/libvirt/images/rhel62-1.img'/> <target dev='vda' bus='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0' multifunction='on'/> </disk> <disk type='file' device='disk'> <driver name='qemu' type='raw' cache='none'/> <source file='/var/lib/libvirt/images/rhel62-2.img'/> <target dev='vdb' bus='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x1'/> </disk>
- Run the
lspcicommand. The output from the KVM guest virtual machine shows the virtio block device:$
lspci00:05.0 SCSI storage controller: Red Hat, Inc Virtio block device 00:05.1 SCSI storage controller: Red Hat, Inc Virtio block deviceNote
The SeaBIOS application runs in real mode for compatibility with BIOS interfaces. This limits the amount of memory available. As a consequence, SeaBIOS is only able to handle a limited number of disks. Currently, the supported number of disks is:- virtio-scsi — 64
- virtio-blk — 4
- ahci/sata — 24 (4 controllers with all 6 ports connected)
- usb-storage — 4
As a workaround for this problem, when attaching a large number of disks to your virtual machine, make sure that your system disk has a small pci slot number, so SeaBIOS sees it first when scanning the pci bus. It is also recommended to use the virtio-scsi device instead of virtio-blk as the per-disk memory overhead is smaller.
