content: add virtio file system device

The virtio file system device transports Linux FUSE requests between a
FUSE daemon running on the host and the FUSE driver inside the guest.

The actual FUSE request definitions are not duplicated in the virtio
specification, similar to how virtio-scsi does not document SCSI
command details.  FUSE request definitions are available here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/fuse.h

This patch documents the core virtio file system device, which is
functional but lacks the DAX feature introduced in the next patch.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Fixes: https://github.com/oasis-tcs/virtio-spec/issues/49

3 files changed, 229 insertions, 0 deletions

diff --git a/content.tex b/content.tex
index 37a2190..679391e 100644
--- a/content.tex
+++ b/content.tex
@@ -5682,6 +5682,7 @@ descriptor for the \field{sense_len}, \field{residual},
 \input{virtio-input.tex}
 \input{virtio-crypto.tex}
 \input{virtio-vsock.tex}
+\input{virtio-fs.tex}
 
 \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
 
diff --git a/introduction.tex b/introduction.tex
index c96acf9..40f16f8 100644
--- a/introduction.tex
+++ b/introduction.tex
@@ -60,6 +60,9 @@ Levels'', BCP 14, RFC 2119, March 1997. \newline\url{http://www.ietf.org/rfc/rfc
 	\phantomsection\label{intro:SCSI MMC}\textbf{[SCSI MMC]} &
         SCSI Multimedia Commands,
         \newline\url{http://www.t10.org/cgi-bin/ac.pl?t=f&f=mmc6r00.pdf}\\
+	\phantomsection\label{intro:FUSE}\textbf{[FUSE]} &
+	Linux FUSE interface,
+	\newline\url{https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/fuse.h}\\
 
 \end{longtable}
 
diff --git a/virtio-fs.tex b/virtio-fs.tex
new file mode 100644
index 0000000..1ae17f8
--- /dev/null
+++ b/virtio-fs.tex
@@ -0,0 +1,225 @@
+\section{File System Device}\label{sec:Device Types / File System Device}
+
+The virtio file system device provides file system access.  The device either
+directly manages a file system or it acts as a gateway to a remote file system.
+The details of how the device implementation accesses files are hidden by the
+device interface, allowing for a range of use cases.
+
+Unlike block-level storage devices such as virtio block and SCSI, the virtio
+file system device provides file-level access to data.  The device interface is
+based on the Linux Filesystem in Userspace (FUSE) protocol.  This consists of
+requests for file system traversal and access the files and directories within
+it.  The protocol details are defined by \hyperref[intro:FUSE]{FUSE}.
+
+The device acts as the FUSE file system daemon and the driver acts as the FUSE
+client mounting the file system.  The virtio file system device provides the
+mechanism for transporting FUSE requests, much like /dev/fuse in a traditional
+FUSE application.
+
+This section relies on definitions from \hyperref[intro:FUSE]{FUSE}.
+
+\subsection{Device ID}\label{sec:Device Types / File System Device / Device ID}
+  26
+
+\subsection{Virtqueues}\label{sec:Device Types / File System Device / Virtqueues}
+
+\begin{description}
+\item[0] hiprio
+\item[1\ldots n] request queues
+\end{description}
+
+\subsection{Feature bits}\label{sec:Device Types / File System Device / Feature bits}
+
+There are currently no feature bits defined.
+
+\subsection{Device configuration layout}\label{sec:Device Types / File System Device / Device configuration layout}
+
+All fields of this configuration are always available.
+
+\begin{lstlisting}
+struct virtio_fs_config {
+        char tag[36];
+        le32 num_request_queues;
+};
+\end{lstlisting}
+
+\begin{description}
+\item[\field{tag}] is the name associated with this file system.  The tag is
+    encoded in UTF-8 and padded with NUL bytes if shorter than the
+    available space.  This field is not NUL-terminated if the encoded bytes
+    take up the entire field.
+\item[\field{num_request_queues}] is the total number of request virtqueues
+    exposed by the device.  Each virtqueue offers identical functionality and
+    there are no ordering guarantees between requests made available on
+    different queues.  Use of multiple queues is intended to increase
+    performance.
+\end{description}
+
+\drivernormative{\subsubsection}{Device configuration layout}{Device Types / File System Device / Device configuration layout}
+
+The driver MUST NOT write to device configuration fields.
+
+The driver MAY use from one up to \field{num_request_queues} request virtqueues.
+
+\devicenormative{\subsubsection}{Device configuration layout}{Device Types / File System Device / Device configuration layout}
+
+The device MUST set \field{num_request_queues} to 1 or greater.
+
+\subsection{Device Initialization}\label{Device Types / File System Device / Device Initialization}
+
+On initialization the driver first discovers the device's virtqueues.  The FUSE
+session is started by sending a FUSE\_INIT request as defined by the FUSE
+protocol on one request virtqueue.  All virtqueues provide access to the same
+FUSE session and therefore only one FUSE\_INIT request is required regardless
+of the number of available virtqueues.
+
+\subsection{Device Operation}\label{sec:Device Types / File System Device / Device Operation}
+
+Device operation consists of operating the virtqueues to facilitate file system
+access.
+
+The FUSE request types are as follows:
+\begin{itemize}
+\item Normal requests are made available by the driver on request queues and
+      are used by the device.
+\item High priority requests (FUSE\_INTERRUPT, FUSE\_FORGET, and
+      FUSE\_BATCH\_FORGET) are made available by the driver on the hiprio queue
+      so the device is able to process them even if the request queues are
+      full.
+\end{itemize}
+
+Note that FUSE notification requests are not supported.
+
+\subsubsection{Device Operation: Request Queues}\label{sec:Device Types / File System Device / Device Operation / Device Operation: Request Queues}
+
+The driver enqueues normal requests on an arbitrary request queue. High
+priority requests are not placed on request queues.  The device processes
+requests in any order.  The driver is responsible for ensuring that ordering
+constraints are met by making available a dependent request only after its
+prerequisite request has been used.
+
+Requests have the following format with endianness chosen by the driver in the
+FUSE\_INIT request used to initiate the session as detailed below:
+
+\begin{lstlisting}
+struct virtio_fs_req {
+        // Device-readable part
+        struct fuse_in_header in;
+        u8 datain[];
+
+        // Device-writable part
+        struct fuse_out_header out;
+        u8 dataout[];
+};
+\end{lstlisting}
+
+Note that the words "in" and "out" follow the FUSE meaning and do not indicate
+the direction of data transfer under VIRTIO.  "In" means input to a request and
+"out" means output from processing a request.
+
+\field{in} is the common header for all types of FUSE requests.
+
+\field{datain} consists of request-specific data, if any.  This is identical to
+the data read from the /dev/fuse device by a FUSE daemon.
+
+\field{out} is the completion header common to all types of FUSE requests.
+
+\field{dataout} consists of request-specific data, if any.  This is identical
+to the data written to the /dev/fuse device by a FUSE daemon.
+
+For example, the full layout of a FUSE\_READ request is as follows:
+
+\begin{lstlisting}
+struct virtio_fs_read_req {
+        // Device-readable part
+        struct fuse_in_header in;
+        union {
+                struct fuse_read_in readin;
+                u8 datain[sizeof(struct fuse_read_in)];
+        };
+
+        // Device-writable part
+        struct fuse_out_header out;
+        u8 dataout[out.len - sizeof(struct fuse_out_header)];
+};
+\end{lstlisting}
+
+The FUSE protocol documented in \hyperref[intro:FUSE]{FUSE} specifies the set
+of request types and their contents.
+
+The endianness of the FUSE protocol session is detectable by inspecting the
+uint32\_t \field{in.opcode} field of the FUSE\_INIT request sent by the driver
+to the device.  This allows the device to determine whether the session is
+little-endian or big-endian.  The next FUSE\_INIT message terminates the
+current session and starts a new session with the possibility of changing
+endianness.
+
+\subsubsection{Device Operation: High Priority Queue}\label{sec:Device Types / File System Device / Device Operation / Device Operation: High Priority Queue}
+
+The hiprio queue follows the same request format as the request queues.  This
+queue only contains FUSE\_INTERRUPT, FUSE\_FORGET, and FUSE\_BATCH\_FORGET
+requests.
+
+Interrupt and forget requests have a higher priority than normal requests.  The
+separate hiprio queue is used for these requests to ensure they can be
+delivered even when all request queues are full.
+
+\devicenormative{\paragraph}{Device Operation: High Priority Queue}{Device Types / File System Device / Device Operation / Device Operation: High Priority Queue}
+
+The device MUST NOT pause processing of the hiprio queue due to activity on a
+normal request queue.
+
+The device MAY process request queues concurrently with the hiprio queue.
+
+\drivernormative{\paragraph}{Device Operation: High Priority Queue}{Device Types / File System Device / Device Operation / Device Operation: High Priority Queue}
+
+The driver MUST submit FUSE\_INTERRUPT, FUSE\_FORGET, and FUSE\_BATCH\_FORGET requests solely on the hiprio queue.
+
+The driver MUST not submit normal requests on the hiprio queue.
+
+The driver MUST anticipate that request queues are processed concurrently with the hiprio queue.
+
+\subsubsection{Security Considerations}\label{sec:Device Types / File System Device / Security Considerations}
+
+The device provides access to a file system containing files owned by one or
+more POSIX user ids and group ids.  The device has no secure way of
+differentiating between users originating requests via the driver.  Therefore
+the device accepts the POSIX user ids and group ids provided by the driver and
+security is enforced by the driver rather than the device.  It is nevertheless
+possible for devices to implement POSIX user id and group id mapping or
+whitelisting to control the ownership and access available to the driver.
+
+File systems containing special files including device nodes and setuid
+executable files pose a security concern.  These properties are defined by the
+file type and mode, which are set by the driver when creating new files or by
+changes at a later time.  These special files present a security risk when the
+file system is shared with another machine.  A setuid executable or a device
+node placed by a malicious machine make it possible for unprivileged users on
+other machines to elevate their privileges through the shared file system.
+This issue can be solved on some operating systems using mount options that
+ignore special files.  It is also possible for devices to implement
+restrictions on special files by refusing their creation.
+
+When the device provides shared access to a file system between multiple
+machines, symlink race conditions, exhausting file system capacity, and
+overwriting or deleting files used by others are factors to consider.  These
+issues have a long history in multi-user operating systems and also apply to
+virtio-fs.  They are typically managed at the file system administration level
+by providing shared access only to mutually trusted users.
+
+\subsubsection{Live migration considerations}\label{sec:Device Types / File System Device / Live Migration Considerations}
+
+When a driver is migrated to a new device it is necessary to consider the FUSE
+session and its state.  The continuity of FUSE inode numbers (also known as
+nodeids) and fh values is necessary so the driver can continue operation
+without disruption.
+
+It is possible to maintain the FUSE session across live migration either by
+transferring the state or by redirecting requests from the new device to the
+old device where the state resides.  The details of how to achieve this are
+implementation-dependent and are not visible at the device interface level.
+
+Maintaining version and feature information negotiated by FUSE\_INIT is
+necessary so that no FUSE protocol feature changes are visible to the driver
+across live migration.  The FUSE\_INIT information forms part of the FUSE
+session state that needs to be transferred during live migration.