Xen : Nested VM

These days I created Xen Hypervisor as a VM for studying. I choosed Ubuntu as a Dom0 distro, followed the Wiki.

To use HVM DomU, I have to set host cpu emulation explicitly because Dom0 VM needs Hardware-Assisted Virtualizaton of the host cpu.

# ensure host machine cpu support assisted virtualization
# in my case it's intel, in case amd that's smx
$ cat /proc/cpuinfo | grep vmx
flags ... vmx ...

# qemu command for start xen vm for nested vmx
# "-cpu host" or "qemu64,+vmx"
$ qemu-system-x86_64 -cpu host -m 4G -enable-kvm -drive -hda=/data/xen.img -boot order=c 
  \ -device virtio-net,netdev=tap0 -netdev tap,id=tap0,ifname=tap0,script=no,downscript=no
# in Xen VM, ensure HVM is supported
$ xl dmesg | grep -i hvm
(XEN) HVM: VMX enabled

Besides, in xen config file I have to comment out the following line otherwise the HVM doesn’t start.

# /etc/xen/ubuntu-hvm.cfg
# HVM doesn't start by the error such as 
# libxl: error: libxl_dm.c:2426:device_model_spawn_outcome: Domain 2:domain 2 device model: spawn failed (rc=-3)
# xen_version : 4.11.4-pre
sdl = 1
# /etc/xen/ubuntu-hvm.cfg 
# comment out the line
# sdl = 1


# create tap device and bridge for Xen VM
$ tunctl -t tap0
$ brctl addbr br0
$ brctl  addif br0 tap0

$ ip a add dev br0
$ ifconfig br0 up
$ ifconfig tap0 up
$ ip a show br0
$ ip l show master br0

# fire wall rules for ip forwarding and NAT from Xen 
# ensure ip fowarding is enabled
$ sysctl net.ipv4.ip_forward      
net.ipv4.ip_forward = 1
$ iptables -t filter -I FORWARD -i br0 -j ACCEPT
$ iptables -t filter -I FORWARD -o br0 -j ACCEPT
$ iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
# or
$ firewall-cmd --zone=public --add-masquerade --permanent


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