virtiofs: Share Host Folders with KVM VMs

Sometimes a VM needs files from the host: source code in a dev lab, media for a server, or shared data between VMs on one host. virtiofs is a shared filesystem designed for VMs that is much faster and more POSIX-compliant than older 9p sharing or NFS over a virtual network.

Before you start

A KVM host with recent libvirt and QEMU, and the virtiofsd daemon (packaged as virtiofsd on many distributions).
A Linux guest with kernel 5.4 or newer, or Windows with virtio-win drivers and WinFsp.
For sharing between hosts, use NFS or Ceph instead; see NFS shared storage.

Enable shared memory for the VM

virtiofs requires the VM's memory to be shareable with the daemon. In the VM XML (virsh edit vmname):

<memoryBacking>
  <source type='memfd'/>
  <access mode='shared'/>
</memoryBacking>

Add the filesystem device

<filesystem type='mount' accessmode='passthrough'>
  <driver type='virtiofs'/>
  <source dir='/srv/shared/projects'/>
  <target dir='projects'/>
</filesystem>

target dir is a tag, not a path. Shut down and start the VM. virt-manager can add this under Add Hardware → Filesystem with driver virtiofs.

Mount in a Linux guest

sudo mkdir -p /mnt/projects && sudo mount -t virtiofs projects /mnt/projects

Persist it in /etc/fstab:

projects  /mnt/projects  virtiofs  defaults,nofail  0  0

Windows guests

Install virtio-win drivers (including the virtiofs driver) and WinFsp.
Start the VirtIO-FS service; the share appears as a drive letter.

See Windows VMs on KVM for virtio-win.

Security notes

The guest can read and write the shared directory with the daemon's permissions; share only what is needed.
Do not share sensitive host paths (like / or /etc).
For untrusted guests, prefer network file sharing with authentication or no sharing at all.

Limitations

Live migration of VMs using virtiofs has limited support depending on versions.
Shared memory backing prevents some memory features such as ballooning in certain setups.

Troubleshooting

VM fails to start, "virtiofs requires shared memory": add the memoryBacking block.
"virtiofsd not found": install the package or set the binary path in the driver element.
Permission denied in guest: host directory ownership or SELinux label; check with ls -Z.

Frequently asked questions

virtiofs or 9p?

virtiofs is faster and more compatible; 9p is older and simpler.

Can two VMs share the same folder?

Yes, each with its own virtiofs device, but concurrent writes need application-level care.

Does Proxmox support virtiofs?

Recent Proxmox versions offer virtiofs directory mappings for VMs.

Next steps

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