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authorLinus Torvalds <torvalds@linux-foundation.org>2019-09-27 15:54:24 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2019-09-27 15:54:24 -0700
commit8f744bdee4fefb17fac052c7418b830de2b59ac8 (patch)
treeaab0dfba240bf48ed67c32b60a9edb372c7afee9 /Documentation
parent9977b1a71488742606376c09e19e0074e4403cdf (diff)
parenta62a8ef9d97da23762a588592c8b8eb50a8deb6a (diff)
Merge tag 'virtio-fs-5.4' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse
Pull fuse virtio-fs support from Miklos Szeredi: "Virtio-fs allows exporting directory trees on the host and mounting them in guest(s). This isn't actually a new filesystem, but a glue layer between the fuse filesystem and a virtio based back-end. It's similar in functionality to the existing virtio-9p solution, but significantly faster in benchmarks and has better POSIX compliance. Further permformance improvements can be achieved by sharing the page cache between host and guest, allowing for faster I/O and reduced memory use. Kata Containers have been including the out-of-tree virtio-fs (with the shared page cache patches as well) since version 1.7 as an experimental feature. They have been active in development and plan to switch from virtio-9p to virtio-fs as their default solution. There has been interest from other sources as well. The userspace infrastructure is slated to be merged into qemu once the kernel part hits mainline. This was developed by Vivek Goyal, Dave Gilbert and Stefan Hajnoczi" * tag 'virtio-fs-5.4' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse: virtio-fs: add virtiofs filesystem virtio-fs: add Documentation/filesystems/virtiofs.rst fuse: reserve values for mapping protocol
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/filesystems/index.rst10
-rw-r--r--Documentation/filesystems/virtiofs.rst60
2 files changed, 70 insertions, 0 deletions
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index fd2bcf99cda0..2c3a9f761205 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -37,3 +37,13 @@ filesystem implementations.
journalling
fscrypt
fsverity
+
+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.