.. SPDX-License-Identifier: GPL-2.0

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virtiofs: virtio-fs host<->guest shared file system
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- Copyright (C) 2019 Red Hat, Inc.

Introduction
============
The virtiofs file system for Linux implements a driver for the paravirtualized
VIRTIO "virtio-fs" device for guest<->host file system sharing.  It allows a
guest to mount a directory that has been exported on the host.

Guests often require access to files residing on the host or remote systems.
Use cases include making files available to new guests during installation,
booting from a root file system located on the host, persistent storage for
stateless or ephemeral guests, and sharing a directory between guests.

Although it is possible to use existing network file systems for some of these
tasks, they require configuration steps that are hard to automate and they
expose the storage network to the guest.  The virtio-fs device was designed to
solve these problems by providing file system access without networking.

Furthermore the virtio-fs device takes advantage of the co-location of the
guest and host to increase performance and provide semantics that are not
possible with network file systems.

Usage
=====
Mount file system with tag ``myfs`` on ``/mnt``:

.. code-block:: sh

  guest# mount -t virtiofs myfs /mnt

Please see https://virtio-fs.gitlab.io/ for details on how to configure QEMU
and the virtiofsd daemon.

Internals
=========
Since the virtio-fs device uses the FUSE protocol for file system requests, the
virtiofs file system for Linux is integrated closely with the FUSE file system
client.  The guest acts as the FUSE client while the host acts as the FUSE
server.  The /dev/fuse interface between the kernel and userspace is replaced
with the virtio-fs device interface.

FUSE requests are placed into a virtqueue and processed by the host.  The
response portion of the buffer is filled in by the host and the guest handles
the request completion.

Mapping /dev/fuse to virtqueues requires solving differences in semantics
between /dev/fuse and virtqueues.  Each time the /dev/fuse device is read, the
FUSE client may choose which request to transfer, making it possible to
prioritize certain requests over others.  Virtqueues have queue semantics and
it is not possible to change the order of requests that have been enqueued.
This is especially important if the virtqueue becomes full since it is then
impossible to add high priority requests.  In order to address this difference,
the virtio-fs device uses a "hiprio" virtqueue specifically for requests that
have priority over normal requests.