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Chapter 12. PCI passthrough

This chapter covers using PCI passthrough with KVM.
Certain hardware platforms allow virtualized guests to directly access various hardware devices and components. This process in virtualization is known as passthrough. Passthrough is known as device assignment in some of the KVM documentation and the KVM code.
The KVM hypervisor supports attaching PCI devices on the host system to virtualized guests. PCI passthrough allows guests to have exclusive access to PCI devices for a range of tasks. PCI passthrough allows PCI devices to appear and behave as if they were physically attached to the guest operating system. PCI passthrough can improve the I/O performance of devices attached to virtualized guests.
Almost all PCI and PCI Express devices that support passthrough, except for graphics cards, can be directly attached to virtualized guests with PCI passthrough.
PCI passthrough is only available on hardware platforms supporting either Intel VT-d or AMD IOMMU. These Intel VT-d or AMD IOMMU extensions must be enabled in BIOS for PCI passthrough to function.
Red Hat Enterprise Linux 6.0 and newer supports hot plugging PCI passthrough devices into virtualized guests.
Out of the 32 available PCI devices for a guest 4 are not removable. This means there are only 28 PCI slots available for additional devices per guest. Every para-virtualized network or block device uses one slot. Each guest can use up to 28 additional devices made up of any combination of para-virtualized network, para-virtualized disk devices, or other PCI devices using VT-d.
Procedure 12.1. Preparing an Intel system for PCI passthrough
  1. Enable the Intel VT-d extensions

    The Intel VT-d extensions provides hardware support for directly assigning a physical devices to guest.
    The VT-d extensions are required for PCI passthrough with Red Hat Enterprise Linux. The extensions must be enabled in the BIOS. Some system manufacturers disable these extensions by default.
    These extensions are often called various terms in BIOS which differ from manufacturer to manufacturer. Consult your system manufacturer's documentation.
  2. Activate Intel VT-d in the kernel

    Activate Intel VT-d in the kernel by appending the intel_iommu=on parameter to the kernel line of the kernel line in the /boot/grub/grub.conf file.
    The example below is a modified grub.conf file with Intel VT-d activated.
    title Red Hat Enterprise Linux Server (2.6.32-36.x86-645)
            root (hd0,0)
            kernel /vmlinuz-2.6.32-36.x86-64 ro root=/dev/VolGroup00/LogVol00 rhgb quiet intel_iommu=on
            initrd /initrd-2.6.32-36.x86-64.img
  3. Ready to use

    Reboot the system to enable the changes. Your system is now PCI passthrough capable.
Procedure 12.2. Preparing an AMD system for PCI passthrough
  • Enable AMD IOMMU extensions

    The AMD IOMMU extensions are required for PCI passthrough with Red Hat Enterprise Linux. The extensions must be enabled in the BIOS. Some system manufacturers disable these extensions by default.
AMD systems only require that the IOMMU is enabled in the BIOS. The system is ready for PCI passthrough once the IOMMU is enabled.

12.1. Adding a PCI device with virsh

These steps cover adding a PCI device to a virtualized guest on a KVM hypervisor using hardware-assisted PCI passthrough.
This example uses a USB controller device with the PCI identifier code, pci_8086_3a6c, and a fully virtualized guest named win2k3.
  1. Identify the device

    Identify the PCI device designated for passthrough to the guest. The virsh nodedev-list command lists all devices attached to the system. The --tree option is useful for identifying devices attached to the PCI device (for example, disk controllers and USB controllers).
    # virsh nodedev-list --tree
    For a list of only PCI devices, run the following command:
    # virsh nodedev-list | grep pci
    In the output from this command, each PCI device is identified by a string, as shown in the following example output:

    Tip: determining the PCI device

    Comparing lspci output to lspci -n (which turns off name resolution) output can assist in deriving which device has which device identifier code.
    Record the PCI device number; the number is needed in other steps.
  2. Information on the domain, bus and function are available from output of the virsh nodedev-dumpxml command:
    # virsh nodedev-dumpxml pci_8086_3a6c
      <capability type='pci'>
        <id='0x3a6c'>82801JD/DO (ICH10 Family) USB2 EHCI Controller #2</product>
        <vendor id='0x8086'>Intel Corporation</vendor>
  3. Detach the device from the system. Attached devices cannot be used and may cause various errors if connected to a guest without detaching first.
    # virsh nodedev-dettach pci_8086_3a6c 
    Device pci_8086_3a6c dettached
  4. Convert slot and function values to hexadecimal values (from decimal) to get the PCI bus addresses. Append "0x" to the beginning of the output to tell the computer that the value is a hexadecimal number.
    For example, if bus = 0, slot = 26 and function = 7 run the following:
    $ printf %x 0
    $ printf %x 26
    $ printf %x 7
    The values to use:
  5. Run virsh edit (or virsh attach device) and added a device entry in the <devices> section to attach the PCI device to the guest.
    # virsh edit win2k3
    <hostdev mode='subsystem' type='pci' managed='yes'>
          <address domain='0x0000' bus='0x00' slot='0x1a' function='0x7'/>
  6. Once the guest system is configured to use the PCI address, the host system must be configured to stop using the device. The ehci driver is loaded by default for the USB PCI controller.
    $ readlink /sys/bus/pci/devices/0000\:00\:1a.7/driver
  7. Detach the device:
    $ virsh nodedev-dettach pci_8086_3a6c
  8. Verify it is now under the control of pci_stub:
    $ readlink /sys/bus/pci/devices/0000\:00\:1d.7/driver
  9. Set a sebool to allow the management of the PCI device from the guest:
    $ setsebool -P virt_manage_sysfs 1
  10. Start the guest system :
    # virsh start win2k3
The PCI device should now be successfully attached to the guest and accessible to the guest operating system.

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