Installation guide

Chapter 15. PCI passthrough
This chapter covers using PCI passthrough with Xen and KVM hypervisors.
KVM and Xen hypervisors support attaching PCI devices on the host system to 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 devices are limited by the virtualized system architecture. Out of the 32 available PCI devices for
a guest 4 are not removable. This means there are up to 28 PCI slots available for additional devices
per guest. Each PCI device can have up to 8 functions; some PCI devices have multiple functions
and only use one slot. Para-virtualized network, para-virtualized disk devices, or other PCI devices
using VT-d all use slots or functions. The exact number of devices available is difficult to calculate
due to the number of available devices. Each guest can use up to 32 PCI devices with each device
having up to 8 functions.
The VT-d or AMD IOMMU extensions must be enabled in BIOS.
Pro ced u re 15.1. Prep arin g an In t el syst em f o r PC I p asst hro u g h
1. En ab le t h e In t el VT- d exten sio n s
The Intel VT-d extensions provides hardware support for directly assigning a physical
devices to guest. The main benefit of the feature is to improve the performance as native for
device access.
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. Act ivat e In t el VT - d in t h e kern el
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.
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Red Hat Enterprise Linux Server (2.6.18-190.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-190.el5 ro root=/dev/VolGroup00/LogVol00
intel_iommu=on
initrd /initrd-2.6.18-190.el5.img
3. Read y to u se
Reboot the system to enable the changes. Your system is now PCI passthrough capable.
Pro ced u re 15.2. Prep arin g an AMD syst em fo r PC I passt h ro u g h
Red Hat En t erp rise Lin ux 5 Virt ualizat ion Guid e
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