6. Virtualization

Running 16 cores or more using AMD Rev F processors no longer results in system resets when performing fully-virtualized guest installations.

On an AMD NPT system used as a PAE host, guests can now have more than 4GB of memory.

Note that nested paging can only translate 32-bit guest virtual addresses. This is because of a hardware feature that exists only in 32-bit physical address extensions (PAE).

When entering the second stage of a Windows™ Server 2003 installation, you no longer need to manually edit /etc/xen/[name of guest machine] to continue. The current user interface now allows you to change media on CD-ROMs attached to the guest.

The Virtual Machine Manager (virt-manager) included in this release now allows users to specify kernel boot parameters to the paravirtualized guest installer.

During the lifetime of dom0, you can now create guests (i.e. xm create) more than 32,750 times.

When using virt-manager to add disks to an existing guest, duplicate entries are no longer created in the guest's /etc/xen/[domain name] configuration file.

Paravirtualized guests can only have a maximum of 16 disk devices.

Repeated live migration of paravirtualized guests between two hosts may cause one host to panic. If a host is rebooted after migrating a guest out of the system and before migrating the same guest back, the panic will not occur.

If a system configured for kdump encounters a kernel panic while an IDE device is performing I/O, the system may be unable to successfully boot into the kdump environment. This occurs if the IDE device is controlled by a device driver other than libata, and is caused by a bug in the IDE/ATA driver stack.

To work around this, use the kdump command-line argument hd[X]=noprobe for storage devices and hd[X]=cdrom for optical drives, where [X] is the device identifier. Either command-line argument should be added to KDUMP_COMMANDLINE_APPEND in /etc/sysconfig/kdump.

Migrating paravirtualized guests through xm migrate [domain] [dom0 IP address] does not work.

When installing Red Hat Enterprise Linux 5 on a fully virtualized SMP guest, the installation may freeze. This can occur when the host (dom0) is running Red Hat Enterprise Linux 5.2.

To prevent this, set the guest to use a single processor using the install. You can do this by using the --vcpus=1 option in virt-install. Once the installation is completed, you can set the guest to SMP by modifying the allocated vcpus in virt-manager.

The hypervisor can now support up to 512GB of memory. To ensure that the system properly detects and uses this much memory, boot the virtualized kernel with the parameter xenheap_megabytes=64.

For systems that use Non-Uniform Memory Access (NUMA), Red Hat recommends that you use the parameter dom0_mem=512m. Note that this can be set to a maximum of 120GB (dom0_mem=120gb).

You can now run paravirtualized 32-bit guests on 64-bit hosts. This capability is now included as a technology preview. Note that the ability to save, restore, and migrate paravirtualized 32-bit guests on 64-bit hosts is not functional, and as such should not be attempted.

Migrating paravirtualized guests through xm migrate [domain] [dom0 IP address] does not work.

Creating a guest more than 1,000 times will cause dom0 to reboot suddenly. Red Hat recommends that you pre-empt this by rebooting dom0 before the domain ID of any guest reaches 1000.

Installing the Virtualization feature may cause a

time went backwards

warning on HP systems with model numbers xw9300 and xw9400.

To work around this issue for xw9400 machines, configure the BIOS settings to enable the HPET timer. Note that this option is not available on xw9300 machines.

Installing Red Hat Enterprise Linux 3.9 on a fully virtualized guest may be extremely slow. In addition, booting up the guest after installation may result in hda: lost interrupt errors.

To avoid this bootup error, configure the guest to use the SMP kernel.

Upgrading a host (dom0) system to Red Hat Enterprise Linux 5.2 may render existing Red Hat Enterprise Linux 4.5 SMP paravirtualized guests unbootable. This is more likely to occur when the host system has more than 4GB of RAM.

To work around this, boot each Red Hat Enterprise Linux 4.5 guest in single CPU mode and upgrade its kernel to the latest version (for Red Hat Enterprise Linux 4.5.z).

When attempting to create a fully-virtualized guest, the hypervisor no longer hangs due to memory shortage when a large amount of memory is allocated to the guest. This version of the hypervisor automatically takes care of allocating needed memory for hypervisor overhead.

Virtualization on this architecture can now support guests with RAM beyond 65,434 MB.

By default, the Itanium dom0 virtualized kernel can now boot with up to 4 CPUs and a maximum system memory of 4GB. In previous releases, the Itanium dom0 virtualized kernel only booted with 1 CPU and a maximum system memory of 500MB by default.

In addition to this, the Itanium dom0 virtualized kernel allocates as much system memory as it can (up to the 4GB maximum by default, or up to the maximum amount specified with the dom0_mem= parameter) while avoiding memory starvation on the hypervisor.

Migrating paravirtualized guests through xm migrate [domain] [dom0 IP address] does not work.

Creating a guest more than 1,000 times will cause dom0 to reboot suddenly. Red Hat recommends that you pre-empt this by rebooting dom0 before the domain ID of any guest reaches 1000.

If you encounter a guest installation failure, Red Hat recommends that you restart the xend daemon before attempting to install a new guest.

On some Itanium systems configured for console output to VGA, the dom0 virtualized kernel may fail to boot. This is because the virtualized kernel failed to properly detect the default console device from the Extensible Firmware Interface (EFI) settings.

When this occurs, add the boot parameter console=tty to the kernel boot options in /boot/efi/elilo.conf.

On some Itanium systems (such as the Hitachi Cold Fusion 3e), the serial port cannot be detected in dom0 when VGA is enabled by the EFI Maintenance Manager. As such, you need to supply the following serial port information to the dom0 kernel:

These details must be specified in the append= line of the dom0 kernel in /boot/efi/elilo.conf. For example:

append="com1=19200,8n1,0x3f8—quiet rhgb console=tty0 console=ttyS0,19200n8"

In this example, com1 is the serial port, 19200 is the speed (in bits/second), 8n1 specifies the number of data bits/parity settings, and 0x3f8 is the io_base address.

Virtualization does not work on architectures that use NUMA. As such, installing the virtualized kernel on systems that use NUMA will result in a boot failure.

Some installation numbers install the virtualized kernel by default. If you have such an installation number and your system uses NUMA (or cannot disable NUMA), deselect the Virtualization option during installation.

Currently, live migration of fully virtualized guests is not supported on this architecture. In addition, kexec and kdump are also not supported for virtualization on this architecture.