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PV vs. HVM

With the paravirtualized approach (PV), the guest OS has to be Xen-aware, so it can pass all the ring 0 operations to the hypervisor (HV), that has actual hardware access through device drivers.

In HVM (Hardware Virtual Machine) mode with Intel's VT-x ("Vanderpool") or SVM from AMD (AMD-V, "Pacifica") the HV can now run Xen-agnostic guests (e.g. Microsoft Windows) with virtualized hardware. The processor's virtualization extensions will be able to handle the guest's kernel mode calls.

In Linux, /proc/cpuinfo lists virtualization features[1]:

$ grep -E 'vmx|svm' /proc/cpuinfo
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat \
                  pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm syscall nx lm \
                  constant_tsc pni monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm


Before we can start our DomU domains, we have to configure xend, the node control daemon running in userspace.

$ grep ^\( /etc/xen/xend-config.sxp
(logfile /var/log/xen/xend.log)
(loglevel INFO)
(xend-relocation-server no)
(xend-relocation-hosts-allow '^localhost$ ^localhost\\.localdomain$')
(network-script network-bridge)
(vif-script vif-bridge)
(dom0-min-mem 128)
(dom0-cpus 0)
(vncpasswd )

These are pretty much the default settings, except for the network-script and vif-script options:

  • (network-script network-bridge), (vif-script vif-bridge) - The Dom0 creates a bridge interface for the VM, passing all traffic to/from the domU. This way we can access the VM from the outside world with a real IP address.
  • (network-script network-route), (vif-script vif-route) - Same effect as creating a bridge, but done in the IP layer.
  • (network-script network-nat), (vif-script vif-nat) - Network traffic is NAT'ed between the VM and the outside world. We don't want this :-\

Xend is much more powerful and if we ever get around to cover Xen management tools, we will have to explain the configuration too. Until then it's just:

$ grep ^\( /etc/xen/xend-config-xenapi.sxp
(xend-relocation-server yes)
(xend-relocation-hosts-allow '^localhost$ ^localhost\\.localdomain$')
(network-script network-bridge)
(vif-script vif-bridge)
(dom0-min-mem 196)
(dom0-cpus 0)
(vncpasswd )


Here our DomU will be fully virtualized and won't know anything about Xen. Let's see how a WindowsXP DomU looks like:

$ grep ^[a-z] /etc/xen/winxp.cfg 
kernel          = '/usr/lib/xen/boot/hvmloader'
builder         = 'hvm'
device_model    = '/usr/lib/xen/bin/qemu-dm'
memory          = '1024'
vcpus           = '2'
disk            = [ 'phy:/dev/vg01/winxp0,hda,w',
                    'phy:/dev/loop0,ioemu:hdc:cdrom,r' ]
name            = 'winxp'
dhcp            = 'dhcp'
vif             = [ 'bridge=eth0,mac=00:16:3e:38:d4:15' ]
on_poweroff     = 'destroy'
on_reboot       = 'restart'
on_crash        = 'restart'
boot            = 'c'
vnc             = 1
vncviewer       = 0
vnclisten       = ''
sdl             = 0

DomU: ParavirtOps

Our DomU is Xen-aware, so it's possible to host the kernel in the Dom0. The DomU could run the same kernel as the Dom0. Notice that we don't have to specify builder and device_model and can point to a real kernel too:

$ grep ^[a-z] /etc/xen/linux.cfg 
kernel          = '/mnt/xen/linux/vmlinux'
memory          = '256'
vcpus           = '2'
root            = '/dev/xvda1'
disk            = [ 'phy:/dev/vg01/linux0,xvda,w' ]
name            = 'linux'
vif             = [ 'bridge=eth0,mac=00:16:3e:38:d4:07' ]
on_poweroff     = 'destroy'
on_reboot       = 'restart'
on_crash        = 'restart'
extra           = 'console=hvc0 init=/sbin/init rootfstype=ext4 rootflags=nobarrier'

We're even passing boot parameters to the DomU, hey!



Now we can start (and control) our new virtual machine:

$ xm create /etc/xen/winxp.cfg
$ xm list
Name                 ID Mem(MiB) VCPUs State   Time(s)
Domain-0              0      239     1 r-----   3160.4
winxp                 2     1024     2 -b----     92.5
linux                 3      256     1 r-----    312.1

$ xm console linux
linux-domU$ uptime 
 23:59:36 up 10 min,  1 user,  load average: 0.79, 0.35, 0.12
linux-domU$ ^]

To connect to the Windows console, we can use vncviewer:

$ vncviewer

Otherwise Windows can be accessed via RDP over the network as usual.

Attaching disks to (running) domains, listing them, and detaching again:

$ xl block-attach linux 'format=raw, vdev=xvde, access=rw, target=/dev/sde'

$ xl block-list linux
Vdev  BE  handle state evt-ch ring-ref BE-path                       
51712 0   1      4     0      0        /local/domain/0/backend/vbd/3/51712
51808 0   1      4     -1     -1       /local/domain/0/backend/vbd/3/51808

$ readlink -f $(xenstore-read /local/domain/0/backend/vbd/3/51808/params)

$ xl block-detach linux xvde