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20.23. Adding Multifunction PCI Devices to KVM Guest Virtual Machines

To add a multi-function PCI device to a KVM guest virtual machine:
  1. Run the virsh edit guestname command to edit the XML configuration file for the guest virtual machine.
  2. In the <address> element, add a multifunction='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>
    Copy to Clipboard Toggle word wrap
    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 as function='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>
    Copy to Clipboard Toggle word wrap
  3. Run the lspci command. The output from the KVM guest virtual machine shows the virtio block device: 
    $ lspci

00:05.0 SCSI storage controller: Red Hat, Inc Virtio block device
00:05.1 SCSI storage controller: Red Hat, Inc Virtio block device
    Copy to Clipboard Toggle word wrap

    Note

    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.
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