Tag Archives: intel

Nested Virtualization — KVM, Intel, with VMCS Shadowing

[Previous installments on Nested Virtualization with KVM and Intel.]

This is part of some recent testing that I’ve been doing with upstream KVM (for 3.10.1). The threads linked here has initial tests bench-marking kernel compile (with make defconfig, a default config file) times in L2. And some minimal guestfish appliance start-up timings in L1.

Some details:

  • Setup information to test with VMCS (Virtual Machine Control Structure) Shadowing. In brief, VMCS Shadowing — a processor specific feature — as described upstream, can reduce the overhead of nested virtualization by reducing the number of VMExits from L1 to L0.
  • Simple scripts used to create L1 and L2.
  • Libvirt XMLs of L1, L2 guests, for reference.

The gritty details of reasons for VMExits are described in Intel architecture manuals, Volume 3b, APPENDIX 1.


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Nested Virtualization with KVM Intel

Some context: In regular virtualization, your physical linux host is the hypervisor, and runs multiple operating systems. Nested Virtualization let’s you run a guest inside a regular guest(essentially a Guest hypervisor).For AMD there is nested-support available since a while, and some people reported success w/ nesting KVM guests. For Intel arch., there is support available recently, an year-ish, and some in progress work, so thought I’d give it a whirl when Adam Young started discussion about it in context of openstack project.

Some of the common use-cases for that are being discussed for nested-virtualization
– For instance, a cloud user gets a beefy, Regualar Guest(which she completely controls). Now, this user can turn regular guest into a hypervisor, and can cheerfully run/manage multiple guests for developing or testing w/o the hassle and intervention of the cloud provider.
– Possibility of having a many instances of virtualization setup (hypervisor and its guests) on one single Bare metal.
– Ability to debug and test hypervisor software

I have immediate access to a moderately beefy Intel hardware, and rest of the post is based on Intel’s CPU virt extensions. Before proceeding, let’s settle on some terminology for clarity:

  • Physical Host (Host hypervisor/Bare metal)
    • Config: Intel(R) Xeon(R) CPU(4 cores/socket); 10GB Memory; CPU Freq – 2GHz; Running latest Fedora-16(Minimal foot-print, @core only with Virt pkgs;x86_64; kernel-3.1.8-2.fc16.x86_64
  • Regualr Guest (Or Guest Hypervisor)
    • Config: 4GB Memory; 4vCPU; 20GB Raw disk image with cache =’none’ to have decent I/O; Minimal, @core F16; And same virt-packages as Physical Host; x86_64
  • Nested Guest (Guest installed inside the Regular Guest)
    • Config: 2GB Memory; 1vCPU; Minimal(@core only) F16; x86_64

Enabling Nesting on the Physical Host

Node Info of the Physical Host.

# virsh nodeinfo
CPU model:           x86_64
CPU(s):              4
CPU frequency:       1994 MHz
CPU socket(s):       1
Core(s) per socket:  4
Thread(s) per core:  1
NUMA cell(s):        1
Memory size:         10242864 kB

Let us first ensure kvm_intel kernel module has nesting enabled. By default, it’s disabled for Intel arch[ but enabled for AMD — SVM (secure virtual machine) extensions arch.]

# modinfo kvm_intel | grep -i nested
parm:           nested:bool

And, we need to pass this kvm-intel.nested=1 on kernel commandline while rebooting the host to enable nesting for the Intel KVM kernel module. Which can be verified after boot by doing:

# cat /sys/module/kvm_intel/parameters/nested 
# systool -m kvm_intel -v   | grep -i nested
    nested              = "Y"

Or alternatively, Adam Young identified that nesting can be enabled by adding this directive options kvm-intel nested=y to the end of /etc/modprobe.d/dist.conf file and reboot the host so it persists.

Set up the Regular Guest(or Guest hypervisor)
Install a regular guest using virt-install or oz tool or any other preferred way. I made a quick script here. And ensure to have cache=’none’ in the disk attribute of the Guest Hypervisor’s xml file. (observation: Install via virt-install tool didn’t seem have this option picked by default.) Here is the ‘drive’ attribute libvirt xml snippet:

    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source file='/var/lib/libvirt/images/regular-guest.img'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>

Now, let’s try to enable Intel VMX(Virtual Machine Extensions) in the regular guest’s CPU. We can do it by running the below on the Physical host(aka Host Hypervisor), and adding the ‘cpu’ attribute to the regular-guest’s libvirt xml file, and start the guest.

# virsh  capabilities | virsh cpu-baseline /dev/stdin 
<cpu match='exact'>
  <feature policy='require' name='dca'/>
  <feature policy='require' name='xtpr'/>
  <feature policy='require' name='tm2'/>
  <feature policy='require' name='vmx'/>
  <feature policy='require' name='ds_cpl'/>
  <feature policy='require' name='monitor'/>
  <feature policy='require' name='pbe'/>
  <feature policy='require' name='tm'/>
  <feature policy='require' name='ht'/>
  <feature policy='require' name='ss'/>
  <feature policy='require' name='acpi'/>
  <feature policy='require' name='ds'/>
  <feature policy='require' name='vme'/>

The o/p of the above cmd has a variety of options. Since we need only vmx extensions, I tried the simple way by adding to the regular-guest’s libvirt xml(virsh edit ..) and started it.

<cpu match='exact'>
 <feature policy='require' name='vmx'/>

Thanks to Jiri Denemark for the above hint. Also note that, there is a very detailed and informative post from Dan P Berrange on host/guest CPU models in libvirt.

As we enabled vmx in the guest-hypervisor, let’s confirm that vmx is exposed in the emulated CPU by ensuring qemu-kvm is invoked with -cpu core2duo,+vmx :

[root@physical-host ~]# ps -ef | grep qemu-kvm
qemu     17102     1  4 22:29 ?        00:00:34 /usr/bin/qemu-kvm -S -M pc-0.14 
-cpu core2duo,+vmx -enable-kvm -m 3072
-smp 3,sockets=3,cores=1,threads=1 -name f16test1 
-uuid f6219dbd-f515-f3c8-a7e8-832b99a24b5d -nographic -nodefconfig 
-nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/f16test1.monitor,server,nowait 
-mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown
-drive file=/export/vmimgs/f16test1.img,if=none,id=drive-virtio-disk0,format=raw,cache=none
-device virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1
-netdev tap,fd=21,id=hostnet0 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:e6:cc:4e,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -usb -device usb-tablet,id=input0 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5

Now, let’s attempt to create a nested guest

Here comes the more interesting part, the nested-guest config. will be 2G RAM; 1vcpu; 8GB virtual disk. And let’s invoke a virt-install cmdline with a minimal kickstart install:

[root@regular-guest ~]# virt-install --connect=qemu:///system \
    --network=bridge:virbr0 \
    --initrd-inject=/root/fed.ks \
   --extra-args=ks=file:/fed.ks console=tty0 console=ttyS0,115200 serial rd_NO_PLYMOUTH \
    --name=nested-guest --disk path=/var/lib/libvirt/images/nested-guest.img,size=6 \
    --ram 2048 \
    --vcpus=1 \
    --check-cpu \
    --hvm \

Starting install...
Retrieving file .treeinfo...                                                                                                 | 1.7 kB     00:00 ... 
Retrieving file vmlinuz...                                                                                                   | 7.9 MB     00:08 ... 
Retrieving file initrd.img...                               28% [==============                                   ] 647 kB/s |  38 MB     02:25 ETA 

virt-install proceeds fine(to a certain extent), doing all regular things like getting access to network, create devices, create file-systems, dep checks performed, and finally package install proceeds:

Welcome to Fedora for x86_64

     ┌─────────────────────┤ Package Installation ├──────────────────────┐
     │                                                                   │
     │                                                                   │
     │                                 24%                               │
     │                                                                   │
     │                   Packages completed: 52 of 390                   │
     │                                                                   │
     │ Installing glibc-common-2.14.90-14.x86_64 (112 MB)                │
     │ Common binaries and locale data for glibc                         │
     │                                                                   │
     │                                                                   │
     │                                                                   │

And now, it’s stuck like that for ever. Doesn’t budge, trying to install pkgs for eternity. Let’s try to see what’s the state of the guest in a seperate terminal

[root@regular-guest ~]# virsh list
 Id Name                 State
  1 nested-guest         paused

[root@regular-guest ~]# 
[root@regular-guest ~]#  virsh domstate nested-guest --reason
paused (unknown)

[root@regular-guest ~]# 

So our nested-guest seems to be paused, And package install on the nested-guest’s serial console is still hung. I gave up at this point. Need to try if I can get any helpful info w/ virt-dmesg tool aor any other ways to debug this further.

Just to note, there is enough disk space and memory on the ‘regular-guest’, so that case is ruled out here. And, I tried to destroy the broken nested-guest, and attempted to create a fresh one(repeated twice). Still no dice.

So not much luck yet with Intel arch, I’d have to try on an AMD machine.

UPDATE(on Intel arch): After trying a couple of times, I was finally able to ssh to the nested guest, but, after a reboot, the nested-guest loses the IP rendering it inaccessible.(Info: the regular-guest has a bridged IP, and nested-guest has a NATed IP) . And I couldn’t login via serial-console, as it’s broken due to a regression(which has a workaround). Also, refer to comments below for further discussion on NATed networking caveats.
UPDATE2: The correct syntax to be added to /etc/modprobe.conf/dist.conf is options kvm-intel nested=y


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