Chapter 13. KVM Para-virtualized Drivers

Para-virtualized drivers are available for virtualized Windows guests running on KVM hosts. These para-virtualized drivers are included in the virtio-win package. The virtio-win package supports block (storage) devices and network interface controllers.

As with the KVM module, the virtio-win drivers package is only available on hosts running Red Hat Enterprise Linux 5.4 and newer.

Para-virtualized drivers enhance the performance of fully virtualized guests. With the para-virtualized drivers guest I/O latency decreases and throughput increases to near bare-metal levels. It is recommended to use the para-virtualized drivers for fully virtualized guests running I/O heavy tasks and applications.

The KVM para-virtualized drivers are automatically loaded and installed on the following versions of Red Hat Enterprise Linux:

Red Hat Enterprise Linux 4.8 and newer
Red Hat Enterprise Linux 5.3 and newer
Red Hat Enterprise Linux 6.

Those Red Hat Enterprise Linux versions detect and install the drivers so additional installation steps are not required.

Important

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 following Microsoft Windows versions have supported KVM para-virtualized drivers:

Windows XP (32-bit only)
Windows Server 2003 (32-bit and 64-bit versions)
Windows Server 2008 (32-bit and 64-bit versions)
Windows 7 (32-bit and 64-bit versions)

13.1. Installing the KVM Windows para-virtualized drivers

This section covers the installation process for the KVM Windows para-virtualized drivers. The KVM para-virtualized drivers can be loaded during the Windows installation or installed after the guest is installed.

You can install the para-virtualized drivers on your guest by one of the following methods:

hosting the installation files on a network accessible to the guest,
using a virtualized CD-ROM device of the driver installation disk .iso file, or
using a virtualized floppy device to install the drivers during boot time (for Windows guests).

This guide describes installation from the para-virtualized installer disk as a virtualized CD-ROM device.

Download the drivers
The virtio-win package contains the para-virtualized block and network drivers for all supported Windows guests.
If the Red Hat Enterprise Linux Supplementary channel entitlements are not enabled for the system, the download will not be available. Enable the Red Hat Enterprise Linux Supplementary channel to access the virtio-win package.
Download the virtio-win package with the yum command.
```
# yum install virtio-win
```
The drivers are also available on the Red Hat Enterprise Linux Supplementary disc or from Microsoft (windowsservercatalog.com). Note that the Red Hat Enterprise Virtualization Hypervisor and Red Hat Enterprise Linux are created on the same code base so the drivers for the same version (for example, 5.5) are supported for both environments.
The virtio-win package installs a CD-ROM image, virtio-win.iso, in the /usr/share/virtio-win/ directory.
Install the para-virtualized drivers
It is recommended to install the drivers on the guest before attaching or modifying a device to use the para-virtualized drivers.
For block devices storing root file systems or other block devices required for booting the guest, the drivers must be installed before the device is modified. If the drivers are not installed on the guest and the driver is set to the virtio driver the guest will not boot.

Installing drivers with a virtualized CD-ROM

This procedure covers installing the para-virtualized drivers with a virtualized CD-ROM after Windows is installed.

Follow Procedure 13.1, “Using virt-manager to mount a CD-ROM image for a Windows guest” to add a CD-ROM image with virt-manager and then install the drivers.

Procedure 13.1. Using virt-manager to mount a CD-ROM image for a Windows guest

Open virt-manager and the guest
Open virt-manager, select your guest from the list by double clicking the guest name.
Open the hardware tab
Click the Add Hardware button in the Hardware tab.
Select the device type
This opens a wizard for adding the new device. Select Storage from the dropdown menu.
Click the Forward button to proceed.
Select the ISO file
Choose the File (disk image) option and set the file location of the para-virtualized drivers .iso image file. The location file is named /usr/share/virtio-win/virtio-win.iso.
If the drivers are stored on a physical CD-ROM, use the Normal Disk Partition option.
Set the Device type to IDE cdrom and click Forward to proceed.
Disc assigned
The disk has been assigned and is available for the guest once the guest is started. Click Finish to close the wizard or back if you made a mistake.
Reboot
Reboot or start the guest to add the new device. Virtualized IDE devices require a restart before they can be recognized by guests.

Once the CD-ROM with the drivers is attached and the guest has started, proceed with Procedure 13.2, “Windows installation”.

Procedure 13.2. Windows installation

Open My Computer
On the Windows guest, open My Computer and select the CD-ROM drive.
Select the correct installation files
There are four files available on the disc. Select the drivers you require for your guest's architecture:
- the para-virtualized block device driver (RHEV-Block.msi for 32-bit guests or RHEV-Block64.msi for 64-bit guests),
- the para-virtualized network device driver (RHEV-Network.msi for 32-bit guests or RHEV-Block64.msi for 64-bit guests),
- or both the block and network device drivers.
Double click the installation files to install the drivers.
Install the block device driver
1. Start the block device driver installation
  Double click RHEV-Block.msi or RHEV-Block64.msi.
  Press Next to continue.
2. Confirm the exception
  Windows may prompt for a security exception.
  Press Yes if it is correct.
3. Finish
  Press Finish to complete the installation.
Install the network device driver
1. Start the network device driver installation
  Double click RHEV-Network.msi or RHEV-Network64.msi.
  Press Next to continue.
2. Performance setting
  This screen configures advanced TCP settings for the network driver. TCP timestamps and TCP window scaling can be enabled or disabled. The default is, 1, for window scaling to be enabled.
  TCP window scaling is covered by IETF RFC 1323. The RFC defines a method of increasing the receive window size to a size greater than the default maximum of 65,535 bytes up to a new maximum of 1 gigabyte (1,073,741,824 bytes). TCP window scaling allows networks to transfer at closer to theoretical network bandwidth limits. Larger receive windows may not be supported by some networking hardware or operating systems.
  TCP timestamps are also defined by IETF RFC 1323. TCP timestamps are used to better calculate Return Travel Time estimates by embedding timing information is embedded in packets. TCP timestamps help the system to adapt to changing traffic levels and avoid congestion issues on busy networks.
  Value Action
  0 Disable TCP timestamps and window scaling.
  1 Enable TCP window scaling.
  2 Enable TCP timestamps.
  3 Enable TCP timestamps and window scaling.
  Press Next to continue.
3. Confirm the exception
  Windows may prompt for a security exception.
  Press Yes if it is correct.
4. Finish
  Press Finish to complete the installation.
Reboot
Reboot the guest to complete the driver installation.