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2.18. kernel-xen

Xen is a high-performance and secure open-source virtualization framework. The virtualization allows users to run guest operating systems in virtual machines on top of a host operating system.
  • The Xen hypervisor will not start when booting from an iSCSI disk. To work around this issue, disable the Xen hypervisor's EDD feature with the "edd=off" kernel parameter. For example:
    kernel /xen.gz edd=off
  • With certain hardware, blktap may not function as expected, resulting in slow disk I/O causing the guest to operate slowly also. To work around this issue, guests should be installed using a physical disk (i.e. a real partition or a logical volume). (BZ#545692)
  • When booting paravirtualized guests that support gigabyte page tables (i.e. a Fedora 11 guest) on Red Hat Enterprise Linux 5.7 Xen, the domain may fail to start if more than 2047MB of memory is configured for the domain. To work around this issue, pass the "nogbpages" parameter on the guest kernel command-line. (BZ#502826)
  • Boot parameters are required to enable SR/IOV Virtual Function devices. SR/IOV Virtual Function devices can only be accessed if the parameter pci_pt_e820_access=on is added to the boot stanza in the /boot/grub/grub.conf file. For example:
    title Red Hat Enterprise Linux Server (2.6.18-152.el5xen)
    root (hd0,1)
    kernel /xen.gz-2.6.18-152.el5 com1=115200,8n1 console=com1 iommu=1
    module /vmlinuz-2.6.18-152.el5xen ro root=LABEL=/ console=ttyS0,115200
    This enables the MMCONF access method for the PCI configuration space, a requirement for VF device support
  • Diskette drive media will not be accessible when using the virtualized kernel. To work around this, use a USB-attached diskette drive instead.
    Note that diskette drive media works well with other non-virtualized kernels. (BZ#401081)
  • Fully virtualized guests cannot correct for time lost due to the domain being paused and unpaused. Being able to correctly track the time across pause and unpause events is one of the advantages of paravirtualized kernels. This issue is being addressed upstream with replaceable timers, so fully virtualized guests will have paravirtualized timers. Currently, this code is under development upstream and should be available in later versions of Red Hat Enterprise Linux. (BZ#422531)
The following known issue applies to the Intel 64 and AMD64 architectures:
  • Upgrading a host (dom0) system to Red Hat Enterprise Linux 5.7 may render existing Red Hat Enterprise Linux 5.4 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 5.4 guest in single CPU mode and upgrade its kernel to the latest version (for Red Hat Enterprise Linux 5.4.z). (BZ#253087, BZ#251013)
The following known issues apply to the Itanium architecture:
  • 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. (BZ#249076)
  • 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:
    • Speed in bits/second
    • Number of data bits
    • Parity
    • io_base address
    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. (BZ#433771)
  • Virtualization does not work on some architectures that use Non-Uniform Memory Access (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 and does not work with kernel-xen, deselect the Virtualization option during installation.