Creating a Shared Folder Between Host and VM

How to create a shared folder between a VM and the Host using virtiofs in a KVM/QEMU environment.

[01] Environment

Component	Version / Notes
Host OS	Ubuntu 24.04
Hypervisor	KVM + QEMU
Manager	virt-manager, virsh, libvirt
Filesystem	virtiofs

Why virtiofs? virtiofs is a shared filesystem protocol designed specifically for virtual machines. Compared to older approaches like VirtIO-9P, it offers better performance (closer to native I/O), proper POSIX semantics, and active upstream support in modern Linux kernels.

[02] Host-Side Setup

Create the directory that will be shared with the VM and set its permissions:

# Create the directory
sudo mkdir vm-shared

# Set permissions (read/write for all users)
sudo chmod 777 vm-shared

chmod 777 is convenient for a local lab environment, but in production or multi-user systems, restrict permissions to only the users who need access.

[03] virt-manager Setup

3-1. Open Virtual Machine Details

Two ways to open the VM details panel:

virt-manager menu → Edit → Virtual Machine Details
VM window menu → View → Details

3-2. Enable Shared Memory

Navigate to Memory → Details → check Enable shared memory.

Shared memory must be enabled before virtiofs will work. This allows the host and guest to share a memory region used for fast data transfer.

3-3. Add Filesystem Hardware

Click Add Hardware at the bottom of the details panel
Select Filesystem
Under Details, fill in:

Field	Value
Source path	Host directory to expose (use Browse → Local Directory)
Target path	Mount tag name used inside the VM (e.g., `vm-shared`)

Click Apply

The target path is not a real directory path inside the VM — it is a tag name that you reference when running the mount command.

[04] VM-Side Setup

4-1. Create the Mount Point

Inside the VM, create the directory where the shared folder will be mounted:

# Create the mount point directory
sudo mkdir /root/vm-shared

# Set permissions
sudo chmod 777 /root/vm-shared

4-2. Mount the Shared Folder

# mount -t virtiofs <target_tag> <mount_point>
sudo mount -t virtiofs vm-shared /root/vm-shared

Replace vm-shared with the Target path (tag) you set in virt-manager Step 3-3.

4-3. Make the Mount Persistent (Optional)

To automatically mount the shared folder on every VM boot, add an entry to /etc/fstab:

# /etc/fstab entry
vm-shared  /root/vm-shared  virtiofs  defaults  0  0

Then verify:

# Test the fstab entry without rebooting
sudo mount -a

# Confirm it is mounted
df -h | grep vm-shared

[05] Troubleshooting

Symptom	Likely Cause	Fix
`mount: unknown filesystem type 'virtiofs'`	Guest kernel too old	Upgrade kernel to 5.4+ or install `linux-modules-extra`
Mount succeeds but files don’t appear	Wrong target tag	Verify tag in virt-manager matches `mount` command
Permission denied on files	UID mismatch between host and guest	Align UID/GID or use ACLs
Shared memory checkbox greyed out	VM is running	Shut down the VM first, then enable shared memory

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