Kernel module¶

This kernel module implements a basic interface to the IVSHMEM device for Looking Glass in VM → VM mode.

Additionally in VM → host mode, it can be used to generate a shared memory device on the host machine that supports dmabuf.

Prerequisites¶

The Linux kernel headers for your kernel version are required for building. Install them with apt-get

apt-get install linux-headers-$(uname -r)

Then switch to the module/ directory

cd module/

Using DKMS (recommended)¶

You can use the kernel’s DKMS feature to keep the module across upgrades. dkms must be installed.

apt-get install dkms

Installing¶

To install the module into DKMS, run

dkms install "."

Loading¶

For VM → VM, simply modprobe the module:

modprobe kvmfr

For VM → host with dmabuf, modprobe with the parameter static_size_mb:

modprobe kvmfr static_size_mb=32

Just like above, multiple devices can be created by separating the sizes with commas.

Compiling & loading (manual)¶

To compile the module manually, run make in the module directory.

Loading¶

For VM → VM mode, run:

insmod kvmfr.ko

For VM → host mode with dmabuf, instead of creating a shared memory file, load this module with the parameter static_size_mb. For example, a 32 MiB shared memory device can be created with:

insmod kvmfr.ko static_size_mb=32

Multiple devices can be created by separating the sizes with commas. For example, static_size_mb=128,64 would create two kvmfr devices: kvmfr0 would be 128 MB and kvmfr1 would be 64 MiB.

Note

If you have already loaded an older version of the module, unload it first. You can do this by rebooting, or with rmmod:

rmmod kvmfr.ko

Usage¶

The module will create the /dev/kvmfr0 node, which represents the KVMFR interface. To use the interface, you need permission to access it by either: creating a udev rule to ensure your user can read and write to it, or simply change its ownership manually, i.e.:

sudo chown user:user /dev/kvmfr0

As an example, you can create a new file in /etc/udev/rules.d/99-kvmfr.rules with the following contents:

SUBSYSTEM=="kvmfr", OWNER="user", GROUP="kvm", MODE="0660"

(replace user with your username)

Usage with Looking Glass is simple, you only need to specify the path to the device node, for example:

./looking-glass-client -f /dev/kvmfr0

You may also use a config file: ~/.looking-glass-client.ini, or /etc/looking-glass-client.ini.

[app]
shmFile=/dev/kvmfr0

VM → Host¶

In VM → host mode, use this device in place of the shared memory file.

QEMU¶

Add the following arguments to your qemu command line:

-device ivshmem-plain,id=shmem0,memdev=looking-glass
-object memory-backend-file,id=looking-glass,mem-path=/dev/kvmfr0,size=32M,share=yes

Note

The size argument must be the same size you passed to the static_size_mb argument when loading the kernel module.

libvirt¶

Starting with QEMU 6.2 and libvirt 7.9, JSON style QEMU configuration is the default syntax. Users running QEMU 6.2 or later and libvirt 7.9 or later, should use this XML block to configure their VM for kvmfr:

<qemu:commandline>
  <qemu:arg value='-device'/>
  <qemu:arg value='{"driver":"ivshmem-plain","id":"shmem0","memdev":"looking-glass"}'/>
  <qemu:arg value='-object'/>
  <qemu:arg value='{"qom-type":"memory-backend-file","id":"looking-glass","mem-path":"/dev/kvmfr0","size":33554432,"share":true}'/>
</qemu:commandline>

Note

The "size" tag represents the size of the shared memory device in bytes. Once you determine the proper size of the device as per Determining Memory, use the figure you got to calculate the size in bytes:

size_in_MB x 1024 x 1024 = size_in_bytes

If you are running QEMU older than 6.2 or libvirt older than 7.9, please use legacy syntax for IVSHMEM setup:

<qemu:commandline>
  <qemu:arg value='-device'/>
  <qemu:arg value='ivshmem-plain,id=shmem0,memdev=looking-glass'/>
  <qemu:arg value='-object'/>
  <qemu:arg value='memory-backend-file,id=looking-glass,mem-path=/dev/kvmfr0,size=32M,share=yes'/>
</qemu:commandline>

Note

Using the legacy syntax on QEMU 6.2/libvirt 7.9 may cause QEMU to abort with the following error message: “error: internal error: ... PCI: slot 1 function 0 not available for pcie-root-port, in use by ivshmem-plain”
Remember to add xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0' to the <domain> tag.

Running libvirt this way violates AppArmor and cgroups policies, which will block the VM from running. These policies must be amended to allow the VM to start:

For AppArmor, create /etc/apparmor.d/local/abstractions/libvirt-qemu if it doesn’t exist, and add the following:
```
# Looking Glass
/dev/kvmfr0 rw,
```
For cgroups, edit /etc/libvirt/qemu.conf, uncomment the cgroup_device_acl block, and add /dev/kvmfr0 to the list. Then restart libvirtd:
```
sudo systemctl restart libvirtd.service
```

systemd-modules-load¶

For convenience, you may load the KVMFR module when starting your computer. We can use the systemd-modules-load.service(8) service for this task.

Create the file /etc/modules-load.d/kvmfr.conf with the following contents:

# KVMFR Looking Glass module
kvmfr

This will now run the next time you start your machine.

If you are running in VM → host mode, you must additionally create another file /etc/modprobe.d/kvmfr.conf to properly set the size. It should have the following contents:

#KVMFR Looking Glass module
options kvmfr static_size_mb=32

Note

Don’t forget to adjust static_size_mb to your needs.