Sharing files with Virtio-FS

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

Almost all virtio devices (all that use virtqueues) require access to at least certain portions of guest RAM (possibly policed by DMA). In case of virtiofsd, much like in case of other vhost-user (see https://www.qemu.org/docs/master/interop/vhost-user.html) virtio devices that are realized by an userspace process, this in practice means that QEMU needs to allocate the backing memory for all the guest RAM as shared memory. As of QEMU 4.2, it is possible to explicitly specify a memory backend when specifying the NUMA topology. This method is however only viable for machine types that do support NUMA. As of QEMU 5.0.0, it is possible to specify the memory backend without NUMA (using the so called memobject interface).

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 (this step is only required for the NUMA case)

    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>