libvirt: Sharing files with Virtio-FS
Description: libvirt, virtualization, virtualization API

# Sharing files with Virtio-FS

Contents

- Sharing files with Virtio-FS

- Virtio-FS

- Host setup

- Guest setup

- Optional parameters

# Virtio-FS

Virtio-FS is a shared file system that lets virtual machines access a directory tree on the host. Unlike existing approaches, it is designed to offer local file system semantics and performance.

See https://virtio-fs.gitlab.io/

# Host setup

The host-side virtiofsd daemon, like other vhost-user backed devices, requires shared memory between the host and the guest. As of QEMU 4.2, this requires specifying a NUMA topology for the guest and explicitly specifying a memory backend. Multiple options are available:

Either of the following:

- Use file-backed memory

  Configure the directory where the files backing the memory will be stored with the memory_backing_dir option in /etc/libvirt/qemu.conf

      # This directory is used for memoryBacking source if configured as file.
      # NOTE: big files will be stored here
      memory_backing_dir = "/dev/shm/"

- Use hugepage-backed memory

  Make sure there are enough huge pages allocated for the requested guest memory. For example, for one guest with 2 GiB of RAM backed by 2 MiB hugepages:

      # virsh allocpages 2M 1024

# Guest setup

1. Specify the NUMA topology

   in the domain XML of the guest. For the simplest one-node topology for a guest with 2GiB of RAM and 8 vCPUs:

       <domain>
         ...
         <cpu ...>
           <numa>
             <cell id='0' cpus='0-7' memory='2' unit='GiB' memAccess='shared'/>
           </numa>
         </cpu>
        ...
       </domain>

   Note that the CPU element might already be specified and only one is allowed.

2. Specify the memory backend

   Either of the following:

   - File-backed memory

         <domain>
           ...
           <memoryBacking>
             <access mode='shared'/>
           </memoryBacking>
           ...
         </domain>

     This will create a file in the directory specified in qemu.conf

   - Hugepage-backed memory

         <domain>
           ...
           <memoryBacking>
             <hugepages>
               <page size='2' unit='M'/>
             </hugepages>
             <access mode='shared'/>
           </memoryBacking>
           ...
         </domain>

3. Add the vhost-user-fs QEMU device via the filesystem element

       <domain>
         ...
         <devices>
           ...
           <filesystem type='mount' accessmode='passthrough'>
             <driver type='virtiofs'/>
             <source dir='/path'/>
             <target dir='mount_tag'/>
           </filesystem>
           ...
         </devices>
       </domain>

   Note that despite its name, the target dir is actually a mount tag and does not have to correspond to the desired mount point in the guest.

   So far, passthrough is the only supported access mode and it requires running the virtiofsd daemon as root.

4. Boot the guest and mount the filesystem

       guest# mount -t virtiofs mount_tag /mnt/mount/path

   Note: this requires virtiofs support in the guest kernel (Linux v5.4 or later)

# Optional parameters

More optional elements can be specified

    <driver type='virtiofs' queue='1024'/>
    <binary path='/usr/libexec/virtiofsd' xattr='on'>
      <cache mode='always'/>
      <lock posix='on' flock='on'/>
    </binary>