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1.. SPDX-License-Identifier: GPL-2.02 3.. _virtiofs_index:4 5===================================================6virtiofs: virtio-fs host<->guest shared file system7===================================================8 9- Copyright (C) 2019 Red Hat, Inc.10 11Introduction12============13The virtiofs file system for Linux implements a driver for the paravirtualized14VIRTIO "virtio-fs" device for guest<->host file system sharing. It allows a15guest to mount a directory that has been exported on the host.16 17Guests often require access to files residing on the host or remote systems.18Use cases include making files available to new guests during installation,19booting from a root file system located on the host, persistent storage for20stateless or ephemeral guests, and sharing a directory between guests.21 22Although it is possible to use existing network file systems for some of these23tasks, they require configuration steps that are hard to automate and they24expose the storage network to the guest. The virtio-fs device was designed to25solve these problems by providing file system access without networking.26 27Furthermore the virtio-fs device takes advantage of the co-location of the28guest and host to increase performance and provide semantics that are not29possible with network file systems.30 31Usage32=====33Mount file system with tag ``myfs`` on ``/mnt``:34 35.. code-block:: sh36 37 guest# mount -t virtiofs myfs /mnt38 39Please see https://virtio-fs.gitlab.io/ for details on how to configure QEMU40and the virtiofsd daemon.41 42Mount options43-------------44 45virtiofs supports general VFS mount options, for example, remount,46ro, rw, context, etc. It also supports FUSE mount options.47 48atime behavior49^^^^^^^^^^^^^^50 51The atime-related mount options, for example, noatime, strictatime,52are ignored. The atime behavior for virtiofs is the same as the53underlying filesystem of the directory that has been exported54on the host.55 56Internals57=========58Since the virtio-fs device uses the FUSE protocol for file system requests, the59virtiofs file system for Linux is integrated closely with the FUSE file system60client. The guest acts as the FUSE client while the host acts as the FUSE61server. The /dev/fuse interface between the kernel and userspace is replaced62with the virtio-fs device interface.63 64FUSE requests are placed into a virtqueue and processed by the host. The65response portion of the buffer is filled in by the host and the guest handles66the request completion.67 68Mapping /dev/fuse to virtqueues requires solving differences in semantics69between /dev/fuse and virtqueues. Each time the /dev/fuse device is read, the70FUSE client may choose which request to transfer, making it possible to71prioritize certain requests over others. Virtqueues have queue semantics and72it is not possible to change the order of requests that have been enqueued.73This is especially important if the virtqueue becomes full since it is then74impossible to add high priority requests. In order to address this difference,75the virtio-fs device uses a "hiprio" virtqueue specifically for requests that76have priority over normal requests.77