CentOS 6
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Nested KVM
Configure nested KVM. It's possible to install KVM and create virtual machines as nested KVM on KVM host.
[1] The Kernel of CentOS 6 is 2.6.32 and it does not support Nested KVM, so it's necessarry to update to supported Kernel for Nested KVM first.
[root@dlp ~]#
yum -y install centos-release-xen
[root@dlp ~]#
sed -i -e "s/enabled=1/enabled=0/g" /etc/yum.repos.d/CentOS-Xen.repo
[root@dlp ~]#
yum --enablerepo=Xen4CentOS -y update kernel
[root@dlp ~]#
vi /boot/grub/grub.conf
# add at the end of the line of kernel like follows

kernel /vmlinuz-3.4.61-9.el6.centos.alt.x86_64 ro root=/dev/mapper/VolGroup-lv_root
rd_NO_LUKS LANG=en_US.UTF-8 rd_NO_MD rd_LVM_LV=VolGroup/lv_swap SYSFONT=latarcyrheb-sun16
crashkernel=auto console=ttyS0,115200n8 rd_LVM_LV=VolGroup/lv_root
[root@dlp ~]#
# confirm status ( it's OK if it is "Y" )

[root@dlp ~]#
cat /sys/module/kvm_intel/parameters/nested

[2] Make sure the CPU model on your computer.
[root@dlp ~]#
virsh capabilities | egrep "/model|/vendor"

# CPU Model

# CPU Vendor

[3] Edit the configuration of a virtual machine you'd like to nest like follows. It's OK to configure nested KVM and you can create virtual machines on the virtual machine nested.
[root@dlp ~]#
virsh edit www
# edit a virtual machine "www"
# add following lines

<cpu mode='custom' match='exact'>
# CPU model

  <model fallback='allow'>SandyBridge</model>
# CPU vendor

  <feature policy='require' name='vmx'/>