* [virtio-dev] [PATCH v4 1/2] content: add virtio file system device
2019-03-05 16:12 [virtio-dev] [PATCH v4 0/2] virtio-fs: add virtio file system device Stefan Hajnoczi
@ 2019-03-05 16:12 ` Stefan Hajnoczi
2019-03-05 16:12 ` [virtio-dev] [PATCH v4 2/2] virtio-fs: add DAX window Stefan Hajnoczi
1 sibling, 0 replies; 3+ messages in thread
From: Stefan Hajnoczi @ 2019-03-05 16:12 UTC (permalink / raw)
To: virtio-dev
Cc: Vivek Goyal, Miklos Szeredi, Dr. David Alan Gilbert,
Steven Whitehouse, Sage Weil, Paolo Bonzini, Stefan Hajnoczi
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>
---
content.tex | 3 +
introduction.tex | 3 +
virtio-fs.tex | 201 +++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 207 insertions(+)
create mode 100644 virtio-fs.tex
diff --git a/content.tex b/content.tex
index ef10cc0..ea6f5c9 100644
--- a/content.tex
+++ b/content.tex
@@ -2634,6 +2634,8 @@ Device ID & Virtio Device \\
\hline
24 & Memory device \\
\hline
+26 & file system device \\
+\hline
\end{tabular}
Some of the devices above are unspecified by this document,
@@ -5559,6 +5561,7 @@ 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 c5ccd89..b5b8159 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..984f14f
--- /dev/null
+++ b/virtio-fs.tex
@@ -0,0 +1,201 @@
+\section{File System Device}\label{sec:Device Types / File System Device}
+
+The virtio file system device provides file system access. The device may
+directly manage a file system or act as a gateway to a remote file system. The
+details of how files are accessed 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_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_queues}] is the total number of request virtqueues exposed by
+ the device. The driver MAY use only one request queue,
+ or it can use more to achieve better 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.
+
+\devicenormative{\subsubsection}{Device configuration layout}{Device Types / File System Device / Device configuration layout}
+
+The device MUST set \field{num_queues} to 1 or greater.
+
+\devicenormative{\subsection}{Device Initialization}{Device Types / File System Device / Device Initialization}
+
+On initialization the driver MUST first discover the
+device's 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 submitted by the driver and completed by the device.
+\item Interrupt requests are submitted by the driver to abort requests that the
+ device may have yet to complete.
+\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 and they are
+completed by the device on that same queue. The device processes requests in
+any order. The driver is responsible for ensuring that ordering constraints
+are met by submitting a dependent request only after its prerequisite request
+has completed.
+
+Requests have the following format:
+
+\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
+first request sent by the driver to the device. The first message of any
+session is FUSE_INIT and this allows the device to determine whether the
+session is little-endian or big-endian.
+
+\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 requests queue. This
+queue only contains FUSE_INTERRUPT, FUSE_FORGET, and FUSE_BATCH_FORGET
+requests.
+
+Interrupt and forget requests have a higher priority than normal requests. In
+order to ensure that they can always be delivered, even if all request queues
+are full, a separate queue is used.
+
+\devicenormative{\paragraph}{Device Operation: High Priority Queue}{Device Types / File System Device / Device Operation / Device Operation: High Priority Queue}
+
+The device SHOULD attempt to process the hiprio queue promptly.
+
+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 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 that may contain files owned by
+different 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.
+
+The file system may contain special files including device nodes and setuid
+executable files. These properties are defined by the file type and mode,
+which may be set by the driver when creating new files or changed at a later
+time. These special files present a security risk when the file system is
+shared with another system, such as the host or another guest. 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 the possibility of
+symlink race conditions, exhausting file system capacity, and overwriting or
+deleting files used by others must be taken into account. These issues have a
+long history in multi-user operating systems and should not be overlooked with
+virtio devices.
+
+\subsubsection{Live migration considerations}\label{sec:Device Types / File System Device / Live Migration Considerations}
+
+When a guest is migrated to a new host 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. Therefore it is trivial to migrate before a FUSE session
+has been started with FUSE_INIT.
+
+It is possible to maintain the FUSE session across live migration either by
+transferring the state or by redirecting requests from the new host to the old
+host where the state resides. The details of how to achieve this are
+implementation-dependent and are not visible at the device interface level.
--
2.20.1
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^ permalink raw reply related [flat|nested] 3+ messages in thread* [virtio-dev] [PATCH v4 2/2] virtio-fs: add DAX window
2019-03-05 16:12 [virtio-dev] [PATCH v4 0/2] virtio-fs: add virtio file system device Stefan Hajnoczi
2019-03-05 16:12 ` [virtio-dev] [PATCH v4 1/2] content: " Stefan Hajnoczi
@ 2019-03-05 16:12 ` Stefan Hajnoczi
1 sibling, 0 replies; 3+ messages in thread
From: Stefan Hajnoczi @ 2019-03-05 16:12 UTC (permalink / raw)
To: virtio-dev
Cc: Vivek Goyal, Miklos Szeredi, Dr. David Alan Gilbert,
Steven Whitehouse, Sage Weil, Paolo Bonzini, Stefan Hajnoczi
Describe how shared memory region ID 0 is the DAX window and how
FUSE_SETUPMAPPING maps file ranges into the window.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
Note that this depends on the shared memory resource specification
extension that David Gilbert is working on.
https://lists.oasis-open.org/archives/virtio-comment/201901/msg00000.html
The FUSE_SETUPMAPPING message is part of the virtio-fs Linux patches:
https://gitlab.com/virtio-fs/linux/blob/virtio-fs/include/uapi/linux/fuse.h
---
virtio-fs.tex | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
diff --git a/virtio-fs.tex b/virtio-fs.tex
index 984f14f..010d352 100644
--- a/virtio-fs.tex
+++ b/virtio-fs.tex
@@ -162,6 +162,31 @@ The driver MUST submit FUSE_INTERRUPT, FUSE_FORGET, and FUSE_BATCH_FORGET reques
The driver MUST anticipate that request queues are processed concurrently with the hiprio queue.
+\subsubsection{Device Operation: DAX Window}\label{sec:Device Types / File System Device / Device Operation / Device Operation: DAX Window}
+
+FUSE\_READ and FUSE\_WRITE requests transfer file contents between the
+driver-provided buffer and the device. In cases where data transfer is
+undesirable, the device can map file contents into the DAX window shared memory
+region. The driver then accesses file contents directly in device-owned memory
+without a data transfer.
+
+Shared memory region ID 0 is called the DAX window. The driver maps a file
+range into the DAX window using the FUSE\_SETUPMAPPING request. The mapping is
+removed using the FUSE\_REMOVEMAPPING request.
+
+After FUSE\_SETUPMAPPING has completed successfully the file range is accessible
+from the DAX window at the offset provided by the driver in the request.
+
+\devicenormative{\paragraph}{Device Operation: DAX Window}{Device Types / File System Device / Device Operation / Device Operation: DAX Window}
+
+The device MUST allow mappings that completely or partially overlap existing mappings within the DAX window.
+
+The device MUST reject mappings that would go beyond the end of the DAX window.
+
+\drivernormative{\paragraph}{Device Operation: DAX Window}{Device Types / File System Device / Device Operation / Device Operation: DAX Window}
+
+The driver SHOULD be prepared to find shared memory region ID 0 absent and fall back to FUSE\_READ and FUSE\_WRITE requests.
+
\subsubsection{Security Considerations}\label{sec:Device Types / File System Device / Security Considerations}
The device provides access to a file system that may contain files owned by
--
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