By default, KVM virtual machines created in Red Hat Enterprise Linux 5.4 have a virtual Realtek 8139 (rtl8139) network interface controller (NIC). The rtl8139 virtual NIC works fine in most environments, but may suffer from performance degradation issues on some networks for example, a 10 GigE (10 Gigabit Ethernet) network.

One workaround for this issue is switch to a different type of virtual NIC, for example, Intel PRO/1000 (e1000) or virtio (a virtual I/O driver for Linux that can talk to the hypervisor).

To switch to e1000: Alternatively, if you're having trouble installing the OS on the virtual machine because of the rtl8139 NIC (for example, because you're installing the OS over the network), you can create a virtual machine from scratch with an e1000 NIC. This method requires you to have at least one virtual machine already created (possibly installed from CD or DVD) to use as a template.

Currently, KVM cannot disable virtualization extensions on a CPU while it is being taken down. Consequently, suspending a host running KVM-based virtual machines may cause the host to crash. (BZ#509809)

The KSM module shipped in this release is a different version from the KSM module found on the latest upstream kernel versions. Newer features, such as exporting statistics on the /sys filesystem, that are implemented upstream are not in the version shipped in this release.

The mute button in the audio control panel on a Windows virtual machine does not mute the sound. BZ#482570

Hot-unplugging of PCI devices is not supported in this release. This feature will be introduced in a future update. (BZ#510679)

When migrating KVM guests between hosts, the NX CPU feature setting on both source and destination must match. Migrating a guest between a host with the NX feature disabled (i.e. disabled in the BIOS settings) and a host with the NX feature enabled may cause the guest to crash. (BZ#516029)

the application binary interface (ABI) between the KVM userspace (e.g. qemu-kvm) and the KVM kernel modules may change in future updates. Using the latest upstream qemu-kvm package is unsupported due to ABI differences. (BZ#515549)

Devices using the qlge driver cannot be assigned to a KVM guest using KVM's PCI Device Driver assignment. (BZ#507689)

the use of the qcow2 disk image format with KVM is considered a Technology Preview. (BZ#517880)

Hotplugging emulated devices after migration may result in the virtual machine crashing after a reboot or the devices no longer being visible. (BZ#507191)

Windows 2003 32-bit guests with more than 4GB of RAM may crash on reboot with the default qemu-kvm CPU settings. To work around this issue, configure a different CPU model on the management interface. (BZ#516762)

The KVM modules from the kmod-kvm package do not support kernels prior to version 2.6.18-159.el5. Error messages similar to the following will be returned if attempting to install these modules on older kernels:

FATAL: Error inserting kvm_intel
(/lib/modules/2.6.18-155.el5/weak-updates/kmod-kvm/kvm-intel.ko): Unknown
symbol in module, or unknown parameter (see dmesg)

(BZ#509361)

the kvm package has incorrect dependencies related to the libgcrypt package. Consequently, if the libgcrypt package installed on a system is earlier than version 1.4.4, the qemu-kvm process may refuse to start, returning a libgcrypt initialization error message. To work around this issue, update libgcrypt to the version provided by Red Hat Enterprise Linux 5.4. (BZ#503118)

The KVM modules available in the kmod-kvm package are loaded automatically at boot time if the kmod-kvm package is installed. To make these KVM modules available after installing the kmod-kvm package the system either needs to be rebooted or the modules can be loaded manually by running the /etc/sysconfig/modules/kvm.modules script. (BZ#501543)

Some Linux-based guests that use virtio virtual block devices may abort during installation, returning the error message: unhandled vm exit: 0x31 vcpu_id 0 To work around this issue, consider utilizing a different interface (other than virtio) for the guest virtual disk. (BZ#518081)

RHEL5.x virtualization relies on etherboot for remote booting. Etherboot is an implementation of the pxe standard, but lacks some features that are present in the new gpxe boot technology which is not shipped with RHEL. It is possible to use the gpxe roms with RHEL 5.4. As an example, gpxe roms can be used to interpret requests generated by Microsoft RIS or WDS. All components present in RHEL5.4 are capable of booting gpxe roms. The roms can be obtained directly from http://rom-o-matic.net/, or other sources like the Fedora Project. BZ#509208

The Preboot eXecution Environment (PXE) boot ROMs included with KVM are from the Etherboot project. Consequently, some bug fixes or features that are present on the newer gPXE project are not available on Etherboot. For example, Virtual Machines (VMs) cannot boot using Microsoft based PXE (ie. Remote Installation Services (RIS) or Windows Deployment Services (WDS)). (BZ#497692)

The following QEMU / KVM features are currently disabled and not supported: (BZ#512837)