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Subject[PATCH v4 16/16] virtio-fs: add Documentation/filesystems/virtiofs.rst
Date
From: Stefan Hajnoczi <stefanha@redhat.com>

Add information about the new "virtiofs" file system.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
Documentation/filesystems/index.rst | 10 +++++
Documentation/filesystems/virtiofs.rst | 60 ++++++++++++++++++++++++++
MAINTAINERS | 11 +++++
3 files changed, 81 insertions(+)
create mode 100644 Documentation/filesystems/virtiofs.rst

diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 2de2fe2ab078..56e94bfc580f 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -32,3 +32,13 @@ filesystem implementations.

journalling
fscrypt
+
+Filesystems
+===========
+
+Documentation for filesystem implementations.
+
+.. toctree::
+ :maxdepth: 2
+
+ virtiofs
diff --git a/Documentation/filesystems/virtiofs.rst b/Documentation/filesystems/virtiofs.rst
new file mode 100644
index 000000000000..4f338e3cb3f7
--- /dev/null
+++ b/Documentation/filesystems/virtiofs.rst
@@ -0,0 +1,60 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================================
+virtiofs: virtio-fs host<->guest shared file system
+===================================================
+
+- 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.
diff --git a/MAINTAINERS b/MAINTAINERS
index 9cbcf167bdd0..459b3fa8e25e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17117,6 +17117,17 @@ S: Supported
F: drivers/s390/virtio/
F: arch/s390/include/uapi/asm/virtio-ccw.h

+VIRTIO FILE SYSTEM
+M: Stefan Hajnoczi <stefanha@redhat.com>
+M: Miklos Szeredi <miklos@szeredi.hu>
+L: virtualization@lists.linux-foundation.org
+L: linux-fsdevel@vger.kernel.org
+W: https://virtio-fs.gitlab.io/
+S: Supported
+F: fs/fuse/virtio_fs.c
+F: include/uapi/linux/virtio_fs.h
+F: Documentation/filesystems/virtiofs.rst
+
VIRTIO GPU DRIVER
M: David Airlie <airlied@linux.ie>
M: Gerd Hoffmann <kraxel@redhat.com>
--
2.21.0
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