1.1. Hardware prerequisites

1.1. Hardware prerequisites

Hardware requirements for para-virtualization and full virtualization

The following list is the recommended RAM and disk space for each para-virtualized or fully virtualized guest

It is advised to have at least one processing core or hyper-thread for each virtual machine.

You system will also require hardware virtualization extensions to use fully virtualized guest operating systems. The steps to identify whether your system has virtualization extensions can be found at Hardware virtualization extensions.

Hardware virtualization extensions

Full virtualization requires CPUs with hardware virtualization extensions. This section describes how to identify hardware virtualization extensions and enable them in your BIOS if they are disabled. If hardware virtualization extensions are not present you can only use para-virtualization with Red Hat Virtualization.

The virtualization extensions can not be disabled in the BIOS for AMD-V capable processors installed in a Rev 2 socket. The Intel® VT extensions can be disabled in the BIOS. Certain laptop vendors have disabled the Intel® VT extensions by default in their CPUs.

These instructions enable Intel® VT virtualization extensions If they are disabled in BIOS:

  1. Run the xm dmesg | grep VMX command. The output should display as follows:

    (XEN) VMXON is done
    (XEN) VMXON is done
  2. Run the cat /proc/cpuinfo | grep vmx command to verify the CPU flags have been set. The output should be similar to the following. Note vmx in the output:

    flags   : fpu tsc msr pae mce cx8 apic mtrr mca cmov pat pse36 clflush dts acpi mmx fxsr 
    sse sse2 ss ht  tm syscall lm constant_tsc pni monitor ds_cpl vmx est tm2 cx16 xtpr lahf_lm

Do not proceed if the output of xm dmesg | grep VMX is not VMXON is done for each CPU. Please visit the BIOS if other messages are reported.

The following commands verify that the virtualization extensions are enabled on AMD-V architectures:

  1. Run the xm dmesg | grep SVM command. The output should look like the following:

    (XEN) AMD SVM Extension is enabled for cpu 0
    (XEN) AMD SVM Extension is enabled for cpu 1
  2. Run the cat /proc/cpuinfo | grep svm to verify the CPU flags have been set. The output should be similar to the following. Note svm in the output:

    flags   :  fpu tsc msr pae mce cx8 apic mtrr mca cmov pat pse36 clflush mmx fxsr sse sse2 
    ht syscall nx mmxext fxsr_opt lm 3dnowext 3dnow pni cx16 lahf_lm cmp_legacy svm cr8legacy ts 
    fid vid ttp tm stc

Enabling Intel® VT and AMD-V virtualization hardware extensions

To run unmodified guest operating systems (also known as fully virtualized or HVM guests) you require a Intel® VT or AMD-V capable system. The steps below will provide an overview about how you can verify whether the virtualization extensions have been enabled, and that Red Hat Enterprise Linux 5 can use them:

First, verify the Intel® VT or AMD-V capabilities are enabled via the BIOS. The BIOS settings for Intel® VT or AMD-V are usually in the Chipset or Processor menus. However, they can sometimes be hidden under obscure menus, such as Security Settings or other non standard menus.

For Intel® VT architectures perform these steps to make sure Intel Virtualization Technology is enabled. Some menu items may have slightly different names:

  1. Reboot the computer and open the system's BIOS menu. This can usually be done by pressing delete or Alt + F4.

  2. Select Restore Defaults, and then select Save & Exit.

  3. Power off the machine and disconnect the power supply.

  4. Power on the machine and open the BIOS Setup Utility. Open the Processor section and enable Intel®Virtualization Technology or AMD-V. The values may also be called Virtualization Extensions on some machines. Select Save & Exit.

  5. Power off the machine and disconnect the power supply.

  6. The vmx or svm instructions should now be enabled.


After resetting or updating the BIOS you must reboot the system for the updated settings to take effect.

Note: This documentation is provided {and copyrighted} by Red Hat®, Inc. and is released via the Open Publication License. The copyright holder has added the further requirement that Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder. The CentOS project redistributes these original works (in their unmodified form) as a reference for CentOS-5 because CentOS-5 is built from publicly available, open source SRPMS. The documentation is unmodified to be compliant with upstream distribution policy. Neither CentOS-5 nor the CentOS Project are in any way affiliated with or sponsored by Red Hat®, Inc.