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

# Sharing files with Virtio-FS

Contents

- Virtio-FS

- Host setup

- Guest setup

- Optional parameters

- Externally-launched virtiofsd

# 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 and libvirt 6.9.0, 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>

# Externally-launched virtiofsd

Libvirtd can also connect the vhost-user-fs device to a virtiofsd daemon launched outside of libvirtd. In that case socket permissions, the mount tag and all the virtiofsd options are out of libvirtd's control and need to be set by the application running virtiofsd.

    <filesystem type='mount'/>
      <driver type='virtiofs' queue='1024'/>
      <source socket='/var/virtiofsd.sock'/>
    </filesystem>