Using QEMU Virtualization

QEMU is a free and open-source hypervisor that performs hardware virtualization. The virtual NVDIMM (vNVDIMM) feature was introduced in QEMU v2.6.0. Only the memory type (pmem) has been implemented. The block window type (blk) is an outstanding feature enhancement to be implemented in the future. Refer to the QEMU release notes for any changes.

Installing QEMU

QEMU is available as prebuilt binaries for most Linux Distributions, Mac OS, and Windows. Installation instructions can be found on the QEMU download page and shown below for several popular operating systems.

$ pacman -S qemu

Create a Guest VM

The storage of a vNVDIMM device in QEMU is provided by the memory backend (i.e. memory-backend-file and memory-backend-ram). The -object describes the location, size, and id of the memory-backend and -device maps the -object to the guest. A simple way to create a single vNVDIMM device at startup time is done via the following command line options:

 -machine pc,nvdimm
 -m $RAM_SIZE,slots=$N,maxmem=$MAX_SIZE
 -object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE
 -device nvdimm,id=nvdimm1,memdev=mem1


  • the nvdimm machine option enables vNVDIMM feature.

  • slots=$N should be equal to or larger than the total amount of normal RAM devices and vNVDIMM devices, e.g. $N should be >= 2 here.

    If the HotPlug feature is required, assign enough slots for the eventual total number of RAM and vNVDIMM devices.

  • maxmem=$MAX_SIZE should be equal to or larger than the total size

    of normal RAM devices and vNVDIMM devices, e.g. $MAX_SIZE should be

    >= $RAM_SIZE + $NVDIMM_SIZE here.

  • object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE creates a backend storage of size $NVDIMM_SIZE on a file $PATH. All

    accesses to the virtual NVDIMM device go to the file $PATH.

    • share=on/off controls the visibility of guest writes. If

      share=on, then guest writes will be applied to the backend

      file. If another guest uses the same backend file with option

      share=on, then above writes will be visible to it as well. If

      share=off, then guest writes won't be applied to the backend

      file and thus will be invisible to other guests.

  • device nvdimm,id=nvdimm1,memdev=mem1 creates a virtual NVDIMM

    device whose storage is provided by above memory backend device.

Multiple vNVDIMM devices can be created if multiple pairs of -object and -device are provided. See Example 1 below.

Creating a Guest with Two Emulated vNVDIMMs

The following example creates a Fedora 27 guest with two 4GiB vNVDIMMs, 4GiB of DDR Memory, 4 vCPUs, a VNC Server on port 0 for console access, and ssh traffic redirected from port 2222 on the host to port 22 in the guest for direct ssh access from a remote system.

# sudo qemu-img create -f raw /virtual-machines/qemu/fedora27.raw 20G
# sudo qemu-system-x86_64 -drive file=/virtual-machines/qemu/fedora27.raw,format=raw,index=0,media=disk \
  -m 4G,slots=4,maxmem=32G \
  -smp 4 \
  -machine pc,accel=kvm,nvdimm=on \
  -enable-kvm \
  -vnc :0 \
  -net nic \
  -net user,hostfwd=tcp::2222-:22 \
  -object memory-backend-file,id=mem1,share,mem-path=/virtual-machines/qemu/f27nvdimm0,size=4G \
  -device nvdimm,memdev=mem1,id=nv1,label-size=2M \
  -object memory-backend-file,id=mem2,share,mem-path=/virtual-machines/qemu/f27nvdimm1,size=4G \
  -device nvdimm,memdev=mem2,id=nv2,label-size=2M \

For a detailed description of the options shown above, and many others, refer to the QEMU User's Guide.

When creating a brand new QEMU guest with a blank OS disk image file, an ISO will need to be presented to the guest from which the OS can then be installed. A guest can access a local or remote ISO using:

Local ISO:

--drive media=cdrom,file=/downloads/Fedora-Server-dvd-x86_64-28-1.1.iso,readonly

Remote ISO:

--drive media=cdrom,file=,readonly

Open Firewall Ports

To access the guests remotely, the firewall on the host system needs to be opened to allow remote access for VNC and SSH. In the following examples, we open a range of ports to accommodate several guests. You only need to open the ports for the number of guests you plan to use.

$ sudo firewall-cmd --list-ports
$ sudo firewall-cmd --get-default-zone
$ sudo firewall-cmd --state
$ sudo firewall-cmd --zone=FedoraServer --add-port=5900-5910/tcp --permanent
$ sudo firewall-cmd --zone=FedoraServer --add-port=2220-2210/tcp --permanent
$ sudo firewall-cmd --reload
$ sudo systemctl restart firewalld

Connecting to the Guest VM using VNS and SSH

Use a VNC Viewer to access the console to complete the OS installation and access the guest. The default VNC port starts at 5900. The example used-vnc :0 which equates to port 5900.

Additionally you can connect to the guest once the operating system has been configured via SSH. Specify the username configured during the guest OS installation process and the hostname or IP address of the host system, eg:

# ssh user@hostname -p 2222


Labels contain metadata to describe the NVDIMM features and namespace configuration. Labels are stored on each NVDIMM in a reserved area called the label storage area. The exact location of the label storage area is NVDIMM-specific. QEMU v2.7.0 and later store labels at the end of backend storage. If a memory backend file, which was previously used as the backend of a vNVDIMM device without labels, is now used for a vNVDIMM device with label, the data in the label area at the end of file will be inaccessible to the guest. If any useful data (e.g. the meta-data of the file system) was stored there, the latter usage may result guest data corruption (e.g. breakage of guest file system).

QEMU v2.7.0 and later implement the label support for vNVDIMM devices. To enable label on vNVDIMM devices, users can simply add "label-size=$LBLSIZE" option to "-device nvdimm", e.g.

-device nvdimm,id=nvdimm1,memdev=mem1,label-size=256K


The minimal label size is 128KB. which is enough to store approximately 1000 labels. Labels are never overwritten in-place. New labels or updates to existing labels are written to new labels slots within the label storage area.

Label information can be accessed using the ndctl command utility which needs to be installed within the guest. See the NDCTL Users Guide for more details.

NVDIMM HotPlug Feature

QEMU v2.8.0 and later implement the hotplug support for dynamically adding vNVDIMM devices to a running guest. Similarly to the RAM hotplug, the vNVDIMM hotplug is accomplished by two monitor console commands "object_add" and "device_add".

When QEMU is running, a monitor console is provided for performing interactive operations on the guest. Using the commands available in the monitor console, it is possible to inspect the running operating system, change removable media, take screenshots or audio grabs and control several other aspects of the virtual machine.

If the guest uses VNC, as Example 1 showed with -vnc :0, connect to the guest using a VNC Viewer and access the monitor using Ctrl-Alt-2 (and Ctrl-Alt-1 to get back to the VM display). Alternatives include:

  • virsh qemu-monitor-command

  • Using telnet. The guest will need to be started with -qmp tcp:localhost:4444,server --monitor stdio. To connect, use telnet localhost 4444.

  • Using UNIX Sockets. Start the guest with -qmp unix:./qmp-sock,server --monitor stdio and connect using nc -U ./qmp-sock.

The QMP protocol used by the monitor is JSON based. When using the telnet or UNIX sockets, commands must be passed in a correctly formatted JSON strings. See the QMP protocol documentation for more information.

For example, the following commands add another 4GB vNVDIMM device to the guest using the qemu monitor interface. The (qemu) monitor prompt is accessed via a vnc connection to the host using the guest vnc port, then pressing Ctrl-Alt-2, as described above.

 (qemu) object_add memory-backend-file,id=mem3,share=on,mem-path=/virtual-machines/qemu/f27nvdimm2,size=4G
 (qemu) device_add nvdimm,id=nvdimm2,memdev=mem3

Each hotplugged vNVDIMM device consumes one memory slot. Users should always ensure the memory option "-m ...,slots=N" specifies enough number of slots, i.e.

N >= number of RAM devices + number of statically plugged vNVDIMM devices + number of hotplugged vNVDIMM devices

The similar is required for the memory option "-m ...,maxmem=M", i.e.

M >= size of RAM devices + size of statically plugged vNVDIMM devices + size of hotplugged vNVDIMM devices

More detailed information about the HotPlug feature can be found in the QEMU Memory HotPlug Documentation.

NVDIMM IO Alignment

QEMU uses mmap(2) to maps vNVDIMM backends and aligns the mapping address to the page size (getpagesize(2)) by default. However, some types of backends may require an alignment different than the page size. In that case, QEMU v2.12.0 and later provide 'align' option to memory-backend-file to allow users to specify the proper alignment.

For example, device dax require the 2 MB alignment, so we can use following QEMU command line options to use /dev/dax0.0 as the backend of vNVDIMM:

 -object memory-backend-file,id=mem1,share=on,mem-path=/dev/dax0.0,size=4G,align=2M
 -device nvdimm,id=nvdimm1,memdev=mem1

Guest Data Persistence

Though QEMU supports multiple types of vNVDIMM backends on Linux, the only backend that can guarantee the guest write persistence is:

A. DAX device (e.g., /dev/dax0.0) or B. DAX file (mounted with the -o dax option)

When using B (A file supporting direct mapping of persistent memory) as a backend, write persistence is guaranteed if the host kernel has support for the MAP_SYNC flag in the mmap system call (available since Linux 4.15 and on certain distro kernels) and additionally both 'pmem' and 'share' flags are set to 'on' on the backend.

If these conditions are not satisfied i.e. if either 'pmem' or 'share' are not set, if the backend file does not support DAX or if MAP_SYNC is not supported by the host kernel, write persistence is not guaranteed after a system crash. For compatibility reasons, these conditions are ignored if not satisfied. Currently, no way is provided to test for them. For more details, reference the mmap(2) man page:

When using other types of backends, it's suggested to set 'unarmed' option of '-device nvdimm' to 'on', which sets the unarmed flag of the guest NVDIMM region mapping structure. This unarmed flag indicates guest software that this vNVDIMM device contains a region that cannot accept persistent writes. In result, for example, the guest Linux NVDIMM driver, marks such vNVDIMM device as read-only.

NVDIMM Persistence

ACPI 6.2 Errata A added support for a new Platform Capabilities Structure which allows the platform to communicate what features it supports related to NVDIMM data persistence. Users can provide a persistence value to a guest via the optional "nvdimm-persistence" machine command line option:

-machine pc,accel=kvm,nvdimm,nvdimm-persistence=cpu

There are currently two valid values for this option:

  • "mem-ctrl" - The platform supports flushing dirty data from the memory controller to the NVDIMMs in the event of power loss.

  • "cpu" - The platform supports flushing dirty data from the CPU cache to the NVDIMMs in the event of power loss. This implies that the platform also supports flushing dirty data through the memory controller on power loss.

If the vNVDIMM backend is in host persistent memory that can be accessed in SNIA NVM Programming Model (e.g., Intel NVDIMM), it's suggested to set the 'pmem' option of memory-backend-file to 'on'. When 'pmem' is 'on' and QEMU is built with libpmem support (configured with --enable-libpmem), QEMU will take necessary operations to guarantee the persistence of its own writes to the vNVDIMM backend(e.g., in vNVDIMM label emulation and live migration). If 'pmem' is 'on' while there is no libpmem support, qemu will exit and report a "lack of libpmem support" message to ensure the persistence is available. For example, if we want to ensure the persistence for some backend file, use the QEMU command line:

-object memory-backend-file,id=nv_mem,mem-path=/XXX/yyy,size=4G,pmem=on


By default, qemu claims the guest machine supports only clflush. As any modern machine (starting with Skylake and Pinnacle Ridge) has clflushopt or clwb (Cannon Lake), you can significantly improve performance by passing a -cpu flag to qemu. Unless you require live migrating, -cpu host is a good choice.

Last updated