pciback —
Xen backend paravirtualized PCI pass-through driver
The pciback driver is the backend part of the PCI
  pass-through functionality that can be used by the Xen dom0 to export
  pci(4) devices to a guest domain.
  To export a PCI device to a guest domain, the device has to be attached to
  pciback in the dom0.
When the guest domain is NetBSD, the
    device attached to the pciback driver will attach to
    a xpci(4) bus inside the guest
    domain.
To attach a device to the pciback driver, follow these
  steps:
  - look for the device PCI ID, via
      pcictl(8).
- edit boot.cfg(5) and add
      the PCI ID to the list of PCI IDs that you want to attach to
      pciback, in bus:device.function notation. The list
      is passed to dom0 module via thepciback.hideparameter:pciback.hide=(bus:dev.fun)(bus:dev.func)(...)
 See also boot(8).
- reboot dom0.
- add the PCI ID to the list of PCI devices in the domain configuration
      file:
    pci = ['bus:dev.fun',
      '...']
 
- start the guest domain.
Thepciback driver first appeared in
  NetBSD 5.1.
Currently, to attach a device to the pciback backend,
  this procedure has to be performed at
  boot(8) time. In the future, it
  will be possible to do it without requiring a dom0 reboot.
As PCI passthrough offers the possibility for guest domains to send arbitrary
  PCI commands to a physical device, this has direct impact on the overall
  stability and security of the system. For example, in case of erroneous or
  malicious commands, the device could overwrite physical memory portions, via
  DMA.