Nested Virtualization in Xen

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Introduction

As the virtualization becomes popular, there are more and more requirements for nested virtualization, which provides the capability to enable the virtual machines to start second level of virtual machines. For example, with nested virtualization, cloud vendors are able to migrate their own cloud to others without any modification.

Currently, we enabled the nested virtualization on Xen. We can boot up all popular VMMs as normal virtual machine (Xen, KVM, VMware Workstation, VMware ESX, Hyper-V, etc.) and all those VMMs are able to boot its own VM. Besides, with adding several critical nested features, we made a good progress in performance that the second level guest is almost same with first level guest: Performance optimization (virtual EPT, VMCS shadowing, etc.) has boosted Xen nested virtualization performance by 30%-500%.

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Abbreviation

Abbreviation	Description
L0	Xen hypervisor
L1	First level VM which is able to start second level VM
L2	The second level VM which boot up by L1
XEN_on_XEN	L1 XEN based on L0 XEN
VMware_ESX_on_XEN	L1 VMware ESX Server on L0 XEN
VMware_Workstaion_on_XEN	L1 VMware Workstation on L0 XEN
Hyperv_on_XEN	L1 WIN8 with Hyper-v feature enabled on L0 XEN
XP_mode	L1 WIN7 VirtualPC XP_mode on L0 XEN

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How to use nested

Booting Xen:

Nested feature is added to Xen long time ago. But it is really ready to use only from last year. So it’s better to use the latest Xen,

    git clone git://xenbits.xen.org/xen.git xen.git

NB: When I write this Wiki, there are still two patches are missing in upstream Xen if you want to boot up VMware. You can get those patches from:

http://www.gossamer-threads.com/lists/xen/devel/316994

http://www.gossamer-threads.com/lists/xen/devel/316993

Booting Dom0:

No special requirement for Dom0. Just use the stable one is enough.

QEMU:

Both Qemu-xen and Qemu traditional are OK.

L1 configuration:

To use nested virtualization, you need to add the following two lines in your guest configuration file:

    hap=1
    nestedhvm=1

NB: We don’t want to support L1 guest with shadow mode, since the performance is very poor. So I recommend you to enable the EPT(hap = 1) in the L1 guest. Besides, there is a known issue that nested is failing to work with L1 shadow mode.

To boot L1 VMware-ESX, VMware-Workstation, Win8-hyperv, you needs to mask CPU bit.

    cpuid = ['0x1:ecx=0xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx']

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Current status

Testing environment:

	Version
L0 XEN	git commit:58f5bcaf05621810f06bf5b3592e2ae87475053d
L1 XEN	git commit:e423b5cd60ff95ba3680e2e4a8440d4d19b2b13e
L1 KVM	3.12.-rc2
L1 VMware Workstation	10.0
L1 VMware ESX	5.1
L1 Hyperv	Win8 with hyperv feature enabled

L1 configuration:

    hap=1
    nestedhvm=1
    cpuid = ['0x1:ecx=0xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx']

Result:

L1\L2	XP_x86	XP_x64	RHEL6U4_x86	RHEL6U4_x64	WIN7_x86	WIN7_x64	WIN8_x86	WIN8_x64
XEN	V	V	V	V	V	V	V	V
KVM	V	V	V	V	V	V	V	V
VMware-ESX	V	V	V	V	V	V	V	V
VMware-Workstation	V*	V	V	V	V*	V	V	V
Hyperv	V	V	V	V	V	V	V	V

V means guest boots well

V* means it works before, but buggy with latest Xen

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Know Issues

Nested fails to work with L1 shadow mode

Virtual Box fails to boot on top of Xen (L1 panic while booting L2)

Hyper-v screen flush issue:

Current the screen flush is not correct after booting hyper-v. Two workarouds:

using remote desktop to connect guest

or set full update mode in Qemu.

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Not Tested

L1 stress test, performance test

L2 stress test performance test

L2 save restore, live migration

L1 save restore, live migration