Using virtiofs with libvirt/virt-install

Recently we switched our 9p filesystem usage in CoreOS Assembler to use virtiofs.

This is the technology behind a lot of new lightweight container VM technology like kata-containers and libkrun, but can also be easily used with libvirt.

Running as non-root using qemu:///session

Currently the virtiofs integration doesn’t work as non-root via a qemu:///session connection. There is an oustanding RFE for this upstream and downstream in RHEL that can be followed for updates.

virtiosfs with virt-install

I typically use virt-install to automate creation of my libvirt virtual machines. Using virt-install we can use the --filesystem= and --memorybacking= parameters to get what we want. The --memorybacking= parameter will add shared memory to the instance, which is required for virtiofs.

Here is an example virt-install command to bring up a VM with a virtiofs filesystem:

sudo virt-install --import --name virtiofs-tester                           \
    --accelerate --ram 4096 --vcpus 2 --autoconsole text                    \
    --os-variant fedora-unknown --network bridge=virbr0,model=virtio        \
    --disk size=10,backing_store=$PWD/Fedora-Cloud-Base-38-1.6.x86_64.qcow2 \
    --cloud-init=ssh-key=$PWD/                                    \
    --filesystem=/var/b/shared/,var-b-shared,driver.type=virtiofs           \

NOTE: In the future when this is supported in a non-root user session you should be able to remove the sudo from the command.

This will create libvirt XML with the following two key elements:

    <access mode="shared"/>
    <source type="memfd"/>


    <filesystem type="mount">
      <source dir="/var/b/shared/"/>
      <target dir="var-b-shared"/>
      <driver type="virtiofs"/>

NOTE: var-b-shared here is just a string that will be used to do the mount inside the guest. It can be a string representing where the user wants the filesystem to be mounted but it doesn’t have to be.

Now inside the guest you can mount the filesystem with:

[dustymabe@media ~]$ ssh root@
Warning: Permanently added '' (ED25519) to the list of known hosts.
[root@localhost ~]# 
[root@localhost ~]# sudo mkdir -p /var/b/shared
[root@localhost ~]# sudo mount -t virtiofs var-b-shared /var/b/shared/


In order to make the mount persistent across reboots add it to the /fstab via something like:

[root@localhost ~]# sudo tee -a /etc/fstab <<EOF 
var-b-shared /var/b/shared/ virtiofs defaults 0 2

And you’re done.