Tag Archives: qemu

QEMU agent for graceful shutdown of Windows guests under libvirt/qemu-kvm

libvirt sending an ACPI shutdown doesn't always prompt Windows guests to shutdown. That's why there is a QEMU guest agent (which is also handy for freezing/thawing guest file systems.) Installing QEMU guest agent will also cause libvirt to block on shutdown commands until the guest has terminated.

libvirt XML definition required

<channel type='unix'>
<target type='virtio' name='org.qemu.guest_agent.0'/>
<address type='virtio-serial' controller='0' bus='0' port='1'/>

You may also have to create /var/lib/libvirt/qemu/channel/target on the KVM host.

mkdir -p /var/lib/libvirt/qemu/channel/target
chown -R libvirt-qemu:kvm /var/lib/libvirt/qemu/channel/target

virtio-win drivers distributed by the Fedora Project contain the guest-agent and required virtioserial drivers for communication between guest and host and can be downloaded as an RPM package called virtio-win.

Once you have the RPM, you can either install it or convert it to a Debian package using alien. The ISO will be installed to /usr/share/virtio-win/virtio-win.iso and can be mounted in the guest.

virsh attach-disk GuestName /usr/share/virtio-win/virtio-win.iso hdc –type cdrom –mode readonly

Once you have restarted the guest with the XML definition changes, you can complete the following steps

  • Install virtioserial driver for PCI Simple Communication Controller under Device Manager, in the vioserial folder
  • Install the guest-agent located under the guest-agent folder for your architecture in the virtio-win ISO

libvirt 1.2.9 KVM qemu AppArmor support

If you want to use AppArmor support in libvirt 1.2.9, you will need to copy the following files over from the source:

cp libvirt-1.2.9/examples/apparmor/libvirt-lxc /etc/apparmor.d/abstractions
cp libvirt-1.2.9/examples/apparmor/TEMPLATE.lxc /etc/apparmor.d/libvirt
cp libvirt-1.2.9/examples/apparmor/TEMPLATE.qemu /etc/apparmor.d/libvirt
ln -s /etc/apparmor.d/libvirt/TEMPLATE.qemu /etc/apparmor.d/libvirt/TEMPLATE.kvm

This is due to the fact that libvirt looks for both TEMPLATE.lxc and TEMPLATE.qemu instead of just TEMPLATE to enable AppArmor support.

You may encounter the following error when trying to a KVM guest with AppArmor support enabled:

error: Failed to start domain test-domain
error: internal error: cannot load AppArmor profile 'libvirt-598505b5-1549-4164-97bd-d1d37fdd8995'

If you look at /var/log/libvirtd.log you may see the following error message:

2014-10-24 18:00:52.280+0000: 4468: error : virCommandWait:2533 : internal error: Child process (/usr/lib/libvirt/virt-aa-helper -p 0 -c -u libvirt-598505b5-1549-4164-97bd-d1d37fdd8995) unexpected exit status 1: virt-aa-helper: error: template does not exist
virt-aa-helper: error: could not create profile

2014-10-24 18:00:52.280+0000: 4468: error : AppArmorGenSecurityLabel:468 : internal error: cannot load AppArmor profile 'libvirt-598505b5-1549-4164-97bd-d1d37fdd8995'

This is due to the fact there is a bug in libvirt 1.2.9 which looks for /etc/apparmor.d/libvirt/TEMPLATE.kvm instead of /etc/apparmor.d/libvirt/TEMPLATE.qemu. This has been fixed via a patch which has not been integrated in a release upstream as of this post. You may symlink TEMPLATE.kvm to TEMPLATE.qemu to resolve this issue until it is resolved upstream.

Linux 3.17 KVM, qemu 2.1, libvirt 1.2.9 experiences (and how to cleanly disable TCP checksum offload in libvirt)

Update: This issue has been resolved in kernel 3.18.10 release. The below instructions are no longer required if your distribution has updated the kernel or backported the fix.

Due to latency issues that I was having with KVM and Windows 2008 R2 with Linux 3.10, I decided to update to Linux 3.17 series despite the TCP checksuming issue that I had been encountering (eg. virtio-net not working at all between guests due to the CHECKSUM_PARTIAL bug in 3.11 and above.)

I updated to Linux 3.17.1, and kept qemu at 2.0 (included in Ubuntu 14.04) and libvirt 1.2.2. Unfortunately, the TCP checksuming bug still exists. However, this resolved my Windows 2008 R2 latency issues. I am no longer seeing latency jumps to 1500ms or packet loss under load, this was using SRV-IO passthrough of a NIC.

Due to the issues I was experiencing with TCP checksuming, virtio-net and openvswitch I decided to update to libvirt 1.2.9 which includes new support for tuning guest network interfaces. This allows me to cleanly turn off TCP checksuming on an interface using the following interface definition (and thus allows all my guests to function properly):

<interface type='network'>
  <model type='virtio'/>
  <driver name='vhost'>
    <guest csum='off' tso4='off' tso6='off'/>

Additionally, my Sophos UTM 9 guest (which is my firewall) no longer halts cleanly so I tried updating to qemu 2.1 – but this did not solve the issue. I have decided to leave the newer releases in place, as they have improved performance with the Windows guests as well.

For those interested, pre-built packages for Ubuntu 14.04 amd64 are available here.

Intel E1G42ET (82576 controller) SR-IOV with Windows 2008 R2 guest

I've followed the Redhat Enterprise Linux 7 Using SR-IOV guide, with the following changes made for Ubuntu 14.04 and the fact that the Intel driver set (PROWinx64) doesn't install the drivers automatically.

Make sure to bring the network link state up before you start the virtual machine, or the network driver will report "Network cable unplugged" permanently. igbvf doesn't want to detach on Linux 3.10 on Ubuntu 14.04, so I have blacklisted the module.


blacklist igbvf


options igb max_vfs=7

Download the latest Intel Virtual Function drivers from the Intel site, extract PROWinx64 with your favourite archival program. Then run the following command:

pnputil -a PRO1000\Winx64\NDIS62\v1q62x64.inf

Then you can either go to Device Manager and scan for New hardware changes or restart the virtual machine. Your guest networking should now be working.

New KVM deployment bugs and recommendations (Ubuntu 14.04: qemu 2.0, libvirt 1.2.4, Linux 3.10)

New Linux KVM qemu deployment, running on Ubuntu 14.04 with Linux 3.10 kernel and openvswitch. Hardware setup is 2 SSD in RAID1, and 2 7200RPM HDD in RAID1 using mdadm. bcache is being used as the backing cache for the HDD.


  • hv_vapic ("vapic state='on'" in libvirt) causes Windows 2008 R2 and above VMs not to boot if CPU is an Intel IvyBridge or greater (check /sys/module/kvm_intel/parameters/enable_apicv) – Redhat Bugzilla
  • Linux 3.12 or greater (Ubuntu 14.04 ships with 3.13) have issues with virtio-net NIC and TSO (RX and TX checksuming) offloading – TCP sessions can't be established across virtual machines in certain situations (think a virtual machine as a firewall) – Debian Bugreport
  • Windows virtual machines still freeze up/high latency if you use virtio NIC, this is with the latest signed drivers available from the Fedora Project
  • Still have issues with "Russian roulette" of network interfaces with openvswitch – Blog post


Installed Packages

apt-get install haveged ntp sysstat irqbalance acpid
Linux KVM, openvswitch, virt-install, virt-top
apt-get install qemu-kvm libvirt-bin virtinst virt-top openvswitch-switch sysfsutils iotop gdisk iftop
apt-get install python-software-properties
add-apt-repository ppa:g2p/storage && apt-get update && apt-get install bcache-tools

Tuning memory, scheduler I/O subsystems for Linux KVM

Taken from RHEL 6 tuned (virtual-host)


Disable experimental virtio-net zero copy transmit

RHEL 7 has experimental_zcopytx disabled by default.

options vhost_net  experimental_zcopytx=0

Use virtio-blk for guests, and enable Multiqueue virtio-net (except Windows)

Linux KVM page describing Multiqueue

  <interface type='network'>
    <model type='virtio'/>
    <driver name='vhost' queues='4'/>

Where number of queues is equal to the number of virtual processors assigned to the virtual machine. Don't forget to enable the vhost_net kernel module, edit /etc/default/qemu-kvm and set VHOST_NET_ENABLED=1.

Make sure to enable Multiqueue support in the guest

ethtool -L eth0 combined 4

Use deadline scheduler, and enable transparent hugepages for KVM

GRUB_CMDLINE_LINUX_DEFAULT="elevator=deadline transparent_hugepage=always"

Don't forget to run update-grub to make the changes persistent.

For Windows guests, take advantage of Hyper-V enlightments and use e1000 Ethernet adapter

Linux KVM presentation on Hyper-V enlightenment (slightly outdated)

  • hv_vapic (for "supported processors") for Virtual APIC
  • hv_time (aka "hypervclock") for TSC invariant timestamps passed to guest
  • hv_relaxed to prevent BSOD under high load (when a timer can't be serviced when expected)
  • hv_spinlocks let's the guest know when a virtual processor is trying to acquire a lock on the same resource as another processor
    <relaxed state='on'/>
    <vapic state='on'/>
    <spinlocks state='on' retries='4096'/>
<clock offset='localtime'>
  <timer name='hypervclock' present='yes'/>
  <timer name='hpet' present='no'/>

Build and install longterm Linux 3.10 kernel for stability (and working openvswitch with virtio-net)

apt-get -y install build-essential
cd /usr/local/src
wget https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.10.44.tar.xz
tar -Jxf linux-3.10.44.tar.xz
cd linux-3.10.44
cp /boot/config-`uname -r` .config
make olddefconfig
make -j`nproc` INSTALL_MOD_STRIP=1 deb-pkg
dpkg -i ../*.deb
apt-mark hold linux-libc-dev

Time keeping is king on FreeBSD – TSC and "how not to have time go backwards in guest"

<clock offset='localtime'>
  <timer name='rtc' tickpolicy='catchup'/>
  <timer name='pit' tickpolicy='delay'/>
  <timer name='hpet' present='no'/>

I/O caching under QEMU KVM virtualization on Linux

Caching modes in QEMU

Mode Host page cache Guest disk write cache
none off on
writethrough on off
writeback on on
unsafe on ignored


  • device.virtio-disk0.config-wce=off (qemu) or config-wce=off (libvirt) prevents guest from setting the write cache
  • Use cache=none for local RAW storage, cache=writethrough for NFS/iSCSI backed storage