[Virtio-msg] [PATCH v1 2/4] virtio-msg: Add virtio-msg, a message based virtio transport layer

21 Jan 2026

Add a new transport layer that is based on messages.
This transport layer still uses virtqueues as the other transport layers do
but implements transport layer operations by sending and receiving messages
instead of the "MMR" reads and writes used in virtio-mmio and virtio-pci.
This transport is useful when the device and driver are both implemented in
software but the trap and emulate operations of virtio-mmio and virtio-pci
can not be used.
This transport is intended to be used in many situations, including:
* between a host processor and its co-processors
* between two different systems (not SMP) connected via PCIe
* between normal and secure worlds
* host to vm
* vm to vm
Signed-off-by: Bill Mills bill.mills@linaro.org
Signed-off-by: Bertrand Marquis bertrand.marquis@arm.com
Signed-off-by: Edgar E. Iglesias edgar.iglesias@amd.com
Signed-off-by: Arnaud Pouliquen arnaud.pouliquen@foss.st.com
---
 transport-msg.tex | 1640 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 1640 insertions(+)
 create mode 100644 transport-msg.tex

diff --git a/transport-msg.tex b/transport-msg.tex
new file mode 100644
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+\section{Virtio Over Messages}\label{sec:Virtio Transport Options / Virtio Over Messages}
+
+\newcommand{\conceptref}[1]{\hyperref[sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / #1]{#1}}
+\newcommand{\msgref}[1]{\hyperref[sec:Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_#1]{VIRTIO_MSG_#1}}
+\newcommand{\busref}[1]{\hyperref[sec:Virtio Transport Options / Virtio Over Messages / Bus Messages / BUS_MSG_#1]{BUS_MSG_#1}}
+\newcommand{\msgdef}[1]{\subsubsection{VIRTIO_MSG_#1}\label{sec:Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_#1}}
+\newcommand{\busdef}[1]{\subsubsection{BUS_MSG_#1}\label{sec:Virtio Transport Options / Virtio Over Messages / Bus Messages / BUS_MSG_#1}}
+
+This section defines \textbf{virtio-msg}, a transport mechanism that encapsulates
+virtio operations as discrete message exchanges rather than relying on PCI or
+memory-mapped I/O regions. It separates bus-level functionality (e.g., device
+enumeration, hotplug events) from device-specific operations (e.g., feature
+negotiation, virtqueue setup), ensuring that a single, generic transport layer
+can be reused across multiple bus implementations.
+
+virtio-msg addresses several key objectives:
+
+\begin{itemize}
+  \item \textbf{Support multiple bus implementations:}
+  Systems can rely on various communication methods such as hypercalls, local
+  IPC, network channels, or device trees for enumerating devices. virtio-msg
+  defines a common transport interface suitable for any of these mechanisms.
+
+  \item \textbf{Reduce per-bus complexity:}
+  Buses can implement a fully message-based workflow (including optional
+  enumeration via \busref{GET_DEVICES} and hotplug via \busref{EVENT_DEVICE})
+  or they can discover and manage devices through
+  alternative means such as platform firmware data. In either case, they
+  forward transport messages to and from each device.
+
+  \item \textbf{Preserve virtio semantics:}
+  The transport leverages standard virtio concepts (features, configuration
+  space, virtqueues), so existing virtio drivers and device implementations can
+  integrate smoothly once a device is discovered and registered.
+\end{itemize}
+
+\subsection{Basic Concepts}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts}
+
+The virtio-msg transport relies on a set of foundational concepts to ensure
+reusability across different bus implementations and flexibility in device
+capabilities. This section defines those concepts and clarifies how they apply,
+regardless of whether the bus leverages message-based enumeration or platform
+data for device discovery.
+
+\subsubsection{High-Level Architecture}
+
+virtio-msg operates around two layers:
+
+\begin{enumerate}
+  \item \textbf{Bus Layer}: A \emph{bus instance} exposes zero or more virtio
+        devices. It discovers available devices through mechanisms such as:
+    \begin{itemize}
+      \item Optional message-based queries (\busref{GET_DEVICES}),
+      \item External data sources (e.g., device tree, ACPI tables, hypervisor
+        firmware calls),
+      \item Dynamic hotplug notifications (optionally via \busref{EVENT_DEVICE}).
+    \end{itemize}
+    Once a device is identified, regardless of discovery method, the bus
+    uses \msgref{GET_DEVICE_INFO} to read its device ID and vendor ID, then
+    \emph{registers} that device with the host OS so the usual virtio driver
+    probe can occur.
+
+  \item \textbf{Transport Layer}: After the bus knows about a device, the
+        virtio-msg transport handles all device-specific operations:
+    \begin{itemize}
+      \item Retrieving and setting features (\msgref{GET_DEVICE_FEATURES},
+        \msgref{SET_DRIVER_FEATURES}),
+      \item Accessing the device configuration space (\msgref{GET_CONFIG},
+        \msgref{SET_CONFIG}),
+      \item Setting up virtqueues (\msgref{SET_VQUEUE}, \msgref{RESET_VQUEUE}),
+      \item Managing status and runtime notifications (\msgref{SET_DEVICE_STATUS},
+        \msgref{EVENT_USED}, etc.).
+    \end{itemize}
+    These transport messages remain the same across different bus instances,
+    allowing a single virtio-msg driver component to function in multiple
+    environments.
+\end{enumerate}
+
+\subsubsection{Relationship Between Bus and Transport}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / Relationship between bus and transport}
+
+This subsubsection explains the division of responsibilities: the bus layer is
+the mandatory carrier that moves messages between driver and device endpoints
+(e.g., over IPC, shared memory with signalling or hardware messaging), while
+the virtio-msg transport defines the semantics of those messages.
+
+virtio-msg groups messages into two categories:
+
+\begin{description}
+  \item[\textbf{Bus Messages}:]
+    Intended for global bus operations such as enumerating device numbers
+    (\busref{GET_DEVICES}), managing device hotplug events (\busref{EVENT_DEVICE})
+    or assessing bus-wide health (\busref{PING}).
+    These messages are \emph{optional} in environments where
+    device discovery or state changes occur through other means (e.g., device
+    tree). However, if a bus chooses to handle those tasks via messages,
+    it implements the appropriate bus message definitions described in this
+    section.
+
+  \item[\textbf{Transport Messages}:]
+    Used for device-specific operations, such as:
+    \begin{itemize}
+      \item Retrieving or setting features (\msgref{GET_DEVICE_FEATURES},
+        \msgref{SET_DRIVER_FEATURES}),
+      \item Accessing device configuration (\msgref{GET_CONFIG},
+        \msgref{SET_CONFIG}),
+      \item Managing virtqueues (\msgref{SET_VQUEUE}, \msgref{RESET_VQUEUE}),
+      \item Handling device status and notifications (\msgref{SET_DEVICE_STATUS},
+        \msgref{EVENT_USED}, etc.).
+    \end{itemize}
+\end{description}
+
+This separation lets a bus remain minimal if it obtains device information from
+firmware tables, while still supporting fully message-based enumeration and
+hotplug when desired.
+
+\busnormative{\paragraph}{Bus Message Implementation}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Relationship between bus and transport / Bus Messages}
+\begin{itemize}
+  \item A bus implementation that provides enumeration, hotplug, or bus
+        health handling via bus messages SHOULD implement the corresponding Bus
+        Message definitions described in this section.
+\end{itemize}
+
+\busnormative{\paragraph}{Transport Message Forwarding}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Relationship between bus and transport / Transport Message Forwarding}
+\begin{itemize}
+  \item A bus implementation MUST relay each transport message to the device
+        number identified in the message header, regardless of how it
+        discovered or enumerated that device.
+  \item A bus implementation SHOULD treat transport messages as opaque apart
+        from enforcing generic transport limits, such as the advertised maximum
+        message size, and SHOULD NOT modify the transport payload.
+\end{itemize}
+
+\subsubsection{System Topology}
+
+A virtio-msg system contains the following elements:
+
+\begin{itemize}
+  \item \textbf{Bus Instances and Devices}: Each bus instance advertises its
+        capabilities (e.g., transport revision, maximum message size) and
+        discovers devices via message-based queries or external data sources.
+        Every discovered device has a unique \emph{device number}.
+  \item \textbf{Driver}: Communicates with the bus to learn about
+        available devices. Once a device is recognized, the driver uses the
+        common transport messages to perform feature negotiation, configuration,
+        and virtqueue setup.
+  \item \textbf{Device}: Implement virtio device functionality and
+        respond to the transport messages. The bus forwards these messages to
+        the correct device instance based on device number.
+\end{itemize}
+
+\subsubsection{Transport Revisions and Maximum Message Size}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / Revisions}
+
+Each \textbf{virtio-msg bus instance} advertises the following to the transport
+layer:
+\begin{itemize}
+  \item \textbf{Transport revision}: the protocol version supported by the bus
+        instance (independent of the overall Virtio specification version).
+  \item \textbf{Maximum message size}: the largest payload (in bytes) that can
+        be carried per request or response, including the common header.
+  \item \textbf{Transport feature bits}: revision-specific optional features
+        implemented by the bus instance.
+\end{itemize}
+
+The mechanism for obtaining these parameters is implementation-defined and can
+vary between bus instances. Common approaches include:
+\begin{itemize}
+  \item Reading firmware or device-tree data that describes each bus instance,
+  \item Performing a message exchange during bus setup to retrieve the values,
+  \item Relying on per-bus configuration structures or driver-specific defaults.
+\end{itemize}
+
+\busnormative{\paragraph}{Advertising Transport Parameters}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Revisions / Advertising Transport Parameters}
+\begin{itemize}
+  \item Each bus instance MUST make its transport revision, maximum message
+        size, and transport feature bits available to the virtio-msg transport
+        before any transport messages are exchanged for that device.
+  \item A bus instance MUST apply the same limits to both driver-originated and
+        device-originated transport messages; if the values change, the bus
+        MUST inform the transport layer before accepting additional messages.
+\end{itemize}
+
+\drivernormative{\paragraph}{Respecting Bus Limits}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Revisions / Driver Limits}
+\begin{itemize}
+  \item A driver MUST NOT send a transport message whose total size exceeds the
+        maximum message size advertised for the target bus instance.
+  \item A driver MUST NOT rely on transport features or messages that require a
+        higher transport revision than the bus instance reports.
+\end{itemize}
+
+\devicenormative{\paragraph}{Respecting Bus Limits}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Revisions / Device Limits}
+\begin{itemize}
+  \item A device MUST ensure its responses and device-originated messages do
+        not exceed the maximum message size advertised by the bus instance that
+        relays them.
+  \item A device MUST NOT require transport features or messages beyond the
+        transport revision reported by the bus instance, and MUST respond with
+        an error or ignore requests for unsupported features.
+\end{itemize}
+
+\paragraph{virtio-msg revisions}
+
+The following table lists the currently defined virtio-msg revisions:
+
+\begin{tabular}{ |l|l|l|l| }
+\hline
+\field{revision} & \field{maximum size} & \field{features}      & remarks \
+\hline \hline
+1        & 44-65536    & <empty>   & Virtio Message Revision 1 \
+\hline
+\end{tabular}
+
+Note that a change in the virtio standard does not necessarily
+correspond to a change in the virtio-msg revision.
+
+The maximum message size is specified from the transport-layer point of view
+and includes the 8-byte common header plus payload. Any extra encapsulation
+imposed by the underlying bus (for example, a framing header) does not count
+against this limit. Today the largest practical transport payload is 256 bytes:
+messages such as GET_CONFIG and SET_CONFIG can carry up to 256 bytes of device
+configuration in one transfer, and larger configuration regions can be accessed
+through multiple exchanges without requiring a larger per-message limit.
+
+\busnormative{\paragraph}{Message Size Bounds}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Revisions / Message Size}
+\begin{itemize}
+  \item A bus implementation MUST advertise a maximum message size of at least
+        44 bytes.
+  \item A bus implementation SHOULD NOT advertise a maximum message size that
+        exceeds 264 bytes (256-byte payload plus the common header).
+\end{itemize}
+
+\paragraph{Versioning and Forward Compatibility}
+
+A higher transport revision or additional transport feature bits extend the
+protocol with new messages or capabilities. Implementations are expected to
+remain interoperable across revisions: devices and drivers designed for a newer
+revision still implement the mandatory messages and semantics defined in prior
+revisions.
+
+\drivernormative{\paragraph}{Revision Compatibility}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Versioning and Forward Compatibility / Driver}
+\begin{itemize}
+  \item A driver that negotiates transport revision $N$ MUST implement all
+        mandatory driver behavior defined for revisions $1$ through $N$.
+  \item If a driver receives a device-originated message or feature indication
+        that requires an unsupported revision or transport feature, it MUST
+        ignore the message (or treat the request as failed) and MUST NOT act on
+        partially understood data.
+\end{itemize}
+
+\devicenormative{\paragraph}{Revision Compatibility}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Versioning and Forward Compatibility / Device}
+\begin{itemize}
+  \item A device that advertises transport revision $N$ MUST implement all
+        mandatory device behavior defined for revisions $1$ through $N$.
+  \item If a device receives a driver request that relies on an unsupported
+        revision or transport feature, it MUST reject the request using the
+        message-specific error mechanism (if any) or silently ignore it.
+\end{itemize}
+
+\busnormative{\paragraph}{Revision Compatibility}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Versioning and Forward Compatibility / Bus}
+\begin{itemize}
+  \item A bus instance that advertises transport revision $N$ MUST satisfy every
+        bus requirement defined for revisions $1$ through $N$.
+  \item If a bus instance cannot forward a message because it requires an
+        unsupported revision or transport feature, it MUST surface a transport
+        error or drop the message without forwarding it.
+\end{itemize}
+
+\subsubsection{Device Numbers and Enumeration}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts /
+Device Numbers}
+
+Each virtio-msg bus instance contains one or more \emph{devices}, identified
+by a 16-bit \textbf{device number}. Buses discover these device numbers through
+mechanisms such as:
+\begin{itemize}
+  \item \textbf{Message-Based Enumeration}: Using \busref{GET_DEVICES} to query
+        which numbers exist (optional).
+  \item \textbf{Platform Data}: A device tree, ACPI tables, or hypervisor calls
+        might inform the bus of available device numbers and their properties.
+\end{itemize}
+
+Once a bus confirms that a device number is valid—regardless of the discovery
+method—it normally issues \msgref{GET_DEVICE_INFO} to retrieve the device and
+vendor IDs before registering the device with the host OS so the usual Virtio
+driver binding process can begin.
+
+\busnormative{\paragraph}{Device Number Assignment}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Device Numbers / Assignment}
+\begin{itemize}
+  \item A bus implementation MUST assign a unique device number to every
+        device on a given bus instance and MUST NOT forward transport messages
+        for a device number that has not been validated.
+  \item A bus implementation SHOULD provide the driver with sufficient
+        information—either via \busref{GET_DEVICES} or equivalent platform
+        data—to discover each valid device number.
+\end{itemize}
+
+\subsubsection{Configuration Generation Count}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / Configuration Generation Count}
+
+Each device maintains a \textbf{Configuration Generation Count} to prevent
+inconsistent updates. This count is incremented at least once by the device for
+every driver-visible change it makes to its configuration data. The current
+count is exposed in \msgref{EVENT_CONFIG} and \msgref{GET_CONFIG} responses so
+the driver can determine whether its view of the configuration is current. The
+count does not necessarily start at zero and is not automatically reset when
+the device resets.
+
+\devicenormative{\paragraph}{Configuration Generation Count}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Configuration Generation Count / Device}
+\begin{itemize}
+  \item A device MUST increment the generation count before it makes a change
+        that is visible to the driver and MUST ensure that each
+        \msgref{EVENT_CONFIG} carrying configuration data uses a unique
+        generation count.
+  \item If updated configuration data cannot fit in a single
+        \msgref{EVENT_CONFIG}, the device SHOULD send an \msgref{EVENT_CONFIG}
+        with zero data length to advertise the new generation count and MUST
+        make the updated data available via \msgref{GET_CONFIG}.
+  \item If a \msgref{SET_CONFIG} request includes a generation count that does
+        not match the device's current count, the device MUST reject the
+        request.
+\end{itemize}
+
+\drivernormative{\paragraph}{Configuration Generation Count}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Configuration Generation Count / Driver}
+\begin{itemize}
+  \item A driver MUST track the most recent generation count observed (via
+        \msgref{EVENT_CONFIG} or \msgref{GET_CONFIG}) and include it in every
+        \msgref{SET_CONFIG} request.
+  \item If a \msgref{SET_CONFIG} request is rejected due to a mismatched
+        generation count, the driver SHOULD issue \msgref{GET_CONFIG} to obtain
+        the latest configuration data and generation count before retrying.
+\end{itemize}
+
+This mechanism ensures updates are not lost or overwritten due to stale
+information.
+
+\subsubsection{Feature Negotiation Blocks}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / Feature Blocks}
+
+The virtio-msg transport organizes feature bits into 32-bit blocks accessed by
+\msgref{GET_DEVICE_FEATURES} and \msgref{SET_DRIVER_FEATURES}. Each block
+represents up to 32 feature bits:
+
+\begin{itemize}
+  \item \textbf{Block Index}: The starting block (e.g., block 0 for
+        features 0--31, block 1 for features 32--63, etc.).
+  \item \textbf{Number of Blocks}: How many blocks the driver wishes to retrieve
+        or modify in a single message.
+  \item \textbf{Feature Data}: The 32-bit values representing the supported (or
+        driver-requested) feature bits in the selected blocks.
+\end{itemize}
+
+\devicenormative{\paragraph}{Feature Blocks}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Feature Blocks / Device}
+\begin{itemize}
+  \item When \msgref{GET_DEVICE_FEATURES} covers blocks that extend beyond the
+        features a device implements, the device MUST return zero for the
+        feature data in those positions.
+\end{itemize}
+
+\subsubsection{Error Signaling}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / Error Signaling}
+
+Transport errors can arise from malformed messages, routing failures inside a
+bus implementation, or device-side faults while processing a valid request.
+Implementations should handle such faults locally where possible, but a bus may
+surface an error to the virtio-msg transport if it cannot deliver a request or
+obtain a response within its policy.
+
+\busnormative{\paragraph}{Error Handling}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Error Signaling / Bus}
+\begin{itemize}
+  \item A bus implementation MAY report a transport-visible failure (for
+        example, after exhausting a bounded retry policy) when it cannot deliver
+        a request or obtain a response.
+  \item A bus implementation MUST treat malformed headers or unsupported
+        \field{msg_id} values as invalid, MUST discard them without generating
+        additional protocol traffic, and MAY log the condition for diagnostics.
+  \item A bus implementation MUST NOT generate error responses to event
+        (one-way) messages.
+\end{itemize}
+
+\devicenormative{\paragraph}{Error Handling}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Error Signaling / Device}
+\begin{itemize}
+  \item A device receiving a malformed or unsupported transport message MUST
+        discard it without producing further protocol traffic.
+  \item Recovery actions taken in response to an error (such as retries,
+        selective resets, or device removal) MUST follow the normative reset and
+        status semantics defined in
+        \ref{sec:Virtio Transport Options / Virtio Over Messages / Device Operation}.
+\end{itemize}
+
+This specification does not define a dedicated error-reporting message; it only
+permits implementations to surface failures when silent recovery is not
+feasible.
+
+\subsubsection{Endianness}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / Endianness}
+
+Unless otherwise stated, all numeric fields defined for virtio-msg messages use
+little-endian encoding.
+
+\drivernormative{\paragraph}{Endianness}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Endianness / Driver}
+\begin{itemize}
+  \item A driver MUST encode and decode the common header and payload fields
+        defined by this transport using little-endian byte order.
+\end{itemize}
+
+\devicenormative{\paragraph}{Endianness}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Endianness / Device}
+\begin{itemize}
+  \item A device MUST emit and consume the common header and payload fields
+        defined by this transport using little-endian byte order.
+\end{itemize}
+
+\subsubsection{Common Message Format}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / Common Message Format}
+
+All virtio-msg exchanges, whether \emph{bus messages} or \emph{transport
+messages}, begin with an 8-byte header followed by an optional payload. The
+fields below describe the wire format for that header.
+
+The header layout is:
+\begin{lstlisting}
+struct virtio_msg_header {
+    u8       type;      /* request/response + bus/transport */
+    u8       msg_id;    /* message id */
+    le16     dev_num;   /* device number (0 for bus messages) */
+    le16     token;     /* bus-managed correlation identifier */
+    le16     msg_size;  /* total size: header (8) + payload */
+    u8       payload[];
+};
+\end{lstlisting}
+
+Field semantics:
+\begin{itemize}
+  \item \field{type}:
+    \begin{itemize}
+  \item Bit[0]: 0=request, 1=response.
+  \item Bit[1]: 0=transport message, 1=bus message.
+  \item Bits[2..7]: reserved for future use.
+    \end{itemize}
+  \item \field{msg_id}: Message ID identifying the message definition. Ranges
+    are defined in
+    \ref{sec:Virtio Transport Options / Virtio Over Messages / Transport Messages}
+    and
+    \ref{sec:Virtio Transport Options / Virtio Over Messages / Bus Messages}.
+  \item \field{dev_num}: For transport messages, the device number that should
+    receive the message. Bus messages operate on device number 0.
+  \item \field{token}: Correlation identifier managed by the bus. Drivers and
+    devices treat this field as opaque; the bus MAY overwrite it to track
+    outstanding requests or maintain ordering.
+  \item \field{msg_size}: Total size in bytes of the complete message (header
+    plus payload).
+  \item \field{payload}: Operation-specific data. If a bus introduces extra
+    padding bytes, those bytes are not part of the payload semantics.
+\end{itemize}
+
+\drivernormative{\paragraph}{Common Header}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Common Message Format / Driver}
+\begin{itemize}
+  \item A driver MUST set bits 2..7 of \field{type} to zero and MUST treat them
+        as reserved when parsing received headers.
+  \item A driver MUST ensure \field{msg_size} reflects the total message length
+        (header plus payload) and MUST NOT exceed the maximum message size
+        advertised by the bus instance.
+  \item When sending a transport message, a driver MUST set \field{dev_num} to
+        the intended device number.
+  \item If a driver introduces padding bytes that become part of the transport
+        payload, it MUST set those bytes to zero.
+  \item A driver MUST treat the \field{token} field as opaque and MUST NOT rely
+        on its value when processing received messages.
+\end{itemize}
+
+\devicenormative{\paragraph}{Common Header}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Common Message Format / Device}
+\begin{itemize}
+  \item A device MUST set bits 2..7 of \field{type} to zero in transmitted
+        messages and MUST ignore those bits on receive.
+  \item A device MUST ensure \field{msg_size} reflects the total message length
+        (header plus payload) and does not exceed the bus's advertised maximum.
+  \item When sending a transport message, a device MUST set \field{dev_num} to
+        its own device number.
+  \item A device MUST ignore padding bytes that are documented as bus-specific
+        and MUST zero any such bytes it introduces into the transport payload.
+  \item A device MUST treat the \field{token} field as opaque and MUST NOT rely
+        on its value when processing received messages.
+\end{itemize}
+
+\busnormative{\paragraph}{Common Header}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Common Message Format / Bus}
+\begin{itemize}
+  \item A bus implementation MUST deliver bus messages with \field{dev_num}=0
+        and MUST NOT alter \field{dev_num} for transport messages beyond the
+        routing needed to reach the addressed device.
+  \item A bus implementation MUST set bits 2..7 of \field{type} to zero when
+        generating bus messages and MUST ignore those bits when forwarding
+        transport messages.
+  \item If the bus adds framing or padding bytes around the common header or
+        payload, it MUST set those bytes to zero before delivering the message
+        to the opposite side and MUST present the same zero padding when the
+        opposite side reads a message.
+  \item A bus implementation owns the \field{token} field; it MAY insert or
+        overwrite values for correlation purposes and MUST ensure that any such
+        use is transparent to drivers and devices.
+\end{itemize}
+
+Reserved header bits and unspecified header values MUST be transmitted as zero
+and ignored on receive to preserve forward compatibility.
+
+\subsubsection{Message Ordering}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Basic Concepts / Ordering}
+
+Transport messages fall into two classes: requests (which expect responses) and
+events (which are one-way). Drivers and devices rely on the bus to preserve the
+relative ordering of request/response pairs for each device number; they do not
+interpret the \field{token} field directly.
+
+\busnormative{\paragraph}{Message Ordering}{Virtio Transport Options / Virtio Over Messages / Basic Concepts / Ordering / Bus}
+\begin{itemize}
+  \item For each device number, a bus implementation MUST deliver responses to
+        the driver in the same order that it forwarded the corresponding
+        requests to the device.
+  \item A bus implementation MUST ensure that every request forwarded to a
+        device results in exactly one response delivered to the driver (unless
+        the request is defined as an event).
+\end{itemize}
+
+\subsection{Device Discovery}\label{sec:Virtio Transport Options / Virtio Over Messages / Device Discovery}
+
+A virtio-msg implementation can learn about devices via bus-level enumeration
+(e.g., \busref{GET_DEVICES}), platform data (e.g., device tree, ACPI), or
+hypervisor/firmware interfaces. The following informative text describes
+typical flows; the normative obligations follow.
+
+Bus implementations discover their devices through a mix of platform data,
+hypervisor-provided enumeration, or message-based queries such as
+\busref{GET_DEVICES}. This specification defines \busref{GET_DEVICES} for
+environments that prefer message-driven enumeration, but it does not require
+its use when equivalent information is already known out-of-band.
+
+\busnormative{\paragraph}{Device Discovery}{Virtio Transport Options / Virtio Over Messages / Device Discovery / Bus}
+\begin{itemize}
+  \item A bus implementation MUST ensure that every device number it exposes to
+        the driver has been validated via either platform data or a successful
+        query such as \busref{GET_DEVICES}.
+  \item Once a device number is deemed present, the bus implementation SHOULD
+        invoke \msgref{GET_DEVICE_INFO} so it can register the device with the
+        host OS and allow the appropriate virtio driver to bind.
+  \item If a bus implementation provides an alternative enumeration mechanism
+        (e.g., ACPI, device tree), it MAY omit \busref{GET_DEVICES} provided the
+        alternative delivers equivalent information to the driver.
+\end{itemize}
+
+\subsection{Device Initialization}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Initialization}
+
+After a bus has identified a virtio-msg device (whether by message-based
+enumeration or platform-specific discovery), the driver undertakes a series of
+steps to configure and ready the device for normal operation. This section
+details the recommended order of operations and the associated messages.
+
+\subsubsection{Initialization Flow Overview}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Initialization / Overview}
+
+A typical device initialization flow includes:
+\begin{enumerate}
+  \item \textbf{Obtain Device Information:}
+    Query static parameters with \msgref{GET_DEVICE_INFO}.
+  \item \textbf{Negotiate Features:}
+    Read feature blocks via \msgref{GET_DEVICE_FEATURES}, decide which to
+    enable, and write them back with \msgref{SET_DRIVER_FEATURES} before
+    updating the device status.
+  \item \textbf{Initialize Configuration Space:}
+    Read or write configuration data with
+    \msgref{GET_CONFIG}/\msgref{SET_CONFIG}, guarding updates with the
+    configuration generation count.
+  \item \textbf{Set Up Virtqueues:}
+    Configure each virtqueue via \msgref{SET_VQUEUE} and verify the settings
+    with \msgref{GET_VQUEUE}.
+  \item \textbf{Set Device Status:}
+    Use \msgref{SET_DEVICE_STATUS} to drive the device through the standard
+    virtio status states.
+\end{enumerate}
+
+The exact order may vary slightly across implementations, but these steps form
+the baseline for virtio-msg initialization.
+
+\drivernormative{\paragraph}{Initialization Flow}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Overview / Driver}
+\begin{itemize}
+  \item A driver MUST issue \msgref{GET_DEVICE_INFO} before attempting feature
+        negotiation or queue setup.
+  \item A driver MUST complete feature negotiation via
+        \msgref{GET_DEVICE_FEATURES}/\msgref{SET_DRIVER_FEATURES}
+        and confirm the FEATURES_OK state via \msgref{SET_DEVICE_STATUS} before
+        enabling virtqueues.
+  \item A driver MUST configure each virtqueue via \msgref{SET_VQUEUE} and
+        confirm its parameters (e.g., with \msgref{GET_VQUEUE}) before marking
+        the queue ready for I/O.
+  \item A driver MUST drive the device status transitions using
+        \msgref{SET_DEVICE_STATUS}, ensuring the device reaches DRIVER_OK
+        before issuing normal I/O.
+\end{itemize}
+
+\subsubsection{Device Information}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Initialization / Device Information}
+
+Once a device number is confirmed, the driver uses \msgref{GET_DEVICE_INFO} to
+retrieve static identification data (device ID, vendor ID), the number of
+feature bits, configuration space size, maximum virtqueues, shared memory
+segments, and any admin virtqueue details. This information determines which
+virtio driver should bind to the device, how many feature blocks to query, and
+how much configuration space is valid.
+
+\drivernormative{\paragraph}{Device Information}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Device Information / Driver}
+\begin{itemize}
+  \item A driver MUST issue \msgref{GET_DEVICE_INFO} before attempting feature
+        negotiation or queue setup so it can discover the device ID, vendor ID,
+        feature count, configuration size, and virtqueue limits.
+  \item A driver MUST use the configuration size reported via
+        \msgref{GET_DEVICE_INFO} to bound any offsets and lengths supplied in
+        \msgref{GET_CONFIG} and \msgref{SET_CONFIG} requests.
+\end{itemize}
+
+\subsubsection{Feature Negotiation}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Initialization / Device Features}
+
+Drivers read available features in 32-bit blocks using
+\msgref{GET_DEVICE_FEATURES}; the device returns the requested bitfields,
+padding with zeros for any out-of-range blocks. To enable selected features, the
+driver writes them back with \msgref{SET_DRIVER_FEATURES} and then updates the
+device status via \msgref{SET_DEVICE_STATUS}, checking whether the FEATURES_OK
+bit remains set.
+
+\drivernormative{\paragraph}{Feature Negotiation}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Device Features / Driver}
+\begin{itemize}
+  \item A driver MUST use \msgref{GET_DEVICE_FEATURES} to discover the feature
+        bits offered by the device and MUST write back only the features it
+        intends to enable via \msgref{SET_DRIVER_FEATURES}.
+  \item After writing the desired features, the driver MUST attempt to set the
+        FEATURES_OK bit using \msgref{SET_DEVICE_STATUS} and MUST check the
+        returned status to ensure the device accepted the set.
+\end{itemize}
+
+\devicenormative{\paragraph}{Feature Negotiation}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Device Features / Device}
+\begin{itemize}
+  \item When handling \msgref{GET_DEVICE_FEATURES}, a device MUST return zero
+        for any requested bits that fall outside the number of feature bits it
+        implements.
+  \item After receiving \msgref{SET_DRIVER_FEATURES}, a device MUST update its
+        internal feature mask to match the acknowledged set and MUST reflect
+        acceptance or rejection by leaving the FEATURES_OK bit set or clearing
+        it in the status returned by \msgref{SET_DEVICE_STATUS}.
+\end{itemize}
+
+\subsubsection{Device Configuration}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Initialization / Device Configuration}
+
+Drivers use \msgref{GET_CONFIG} to read portions of the configuration space by
+supplying an offset and length; the device returns the requested data plus the
+current configuration generation count. Writing is performed via
+\msgref{SET_CONFIG}, which carries the same offset/length along with the
+driver's notion of the generation count and the new data.
+
+\drivernormative{\paragraph}{Device Configuration}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Device Configuration / Driver}
+\begin{itemize}
+  \item A driver MUST ensure that the offset and length in each
+        \msgref{GET_CONFIG} or \msgref{SET_CONFIG} request stay within the
+        configuration size reported by \msgref{GET_DEVICE_INFO}.
+  \item A driver MUST include its most recently observed configuration
+        generation count in a \msgref{SET_CONFIG} request and SHOULD re-read the
+        configuration (via \msgref{GET_CONFIG}) if the request is rejected for a
+        generation mismatch.
+\end{itemize}
+
+\devicenormative{\paragraph}{Device Configuration}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Device Configuration / Device}
+\begin{itemize}
+  \item A device MUST reject a \msgref{SET_CONFIG} request whose generation
+        count does not match its current value and MUST indicate the rejection
+        in the response.
+  \item A device MUST return the current configuration generation count
+        alongside any data returned via \msgref{GET_CONFIG}.
+\end{itemize}
+
+\subsubsection{Virtqueue Configuration}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Initialization / Virtqueue Configuration}
+
+Drivers query virtqueue parameters with \msgref{GET_VQUEUE}, configure them via
+\msgref{SET_VQUEUE}, and optionally reset them using \msgref{RESET_VQUEUE} (if
+VIRTIO_F_RING_RESET is negotiated). Each queue is typically configured by
+reading its maximum size, provisioning descriptor/available/used buffers, and
+then calling \msgref{SET_VQUEUE} with the chosen size and guest-physical
+addresses.
+
+\drivernormative{\paragraph}{Virtqueue Configuration}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Virtqueue Configuration / Driver}
+\begin{itemize}
+  \item A driver MUST use \msgref{GET_VQUEUE} to determine the maximum queue
+        size and confirm that a queue is inactive before programming it.
+  \item A driver MUST ensure the queue size provided in \msgref{SET_VQUEUE} does
+        not exceed the maximum reported by the device and MUST supply valid
+        descriptor/driver/device addresses before enabling the queue.
+  \item If VIRTIO_F_RING_RESET has been negotiated and a queue requires
+        reinitialization, the driver SHOULD use \msgref{RESET_VQUEUE} before
+        reprogramming it.
+\end{itemize}
+
+\devicenormative{\paragraph}{Virtqueue Configuration}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Virtqueue Configuration / Device}
+\begin{itemize}
+  \item A device MUST report accurate maximum queue sizes in \msgref{GET_VQUEUE}
+        and MUST persist the parameters supplied via \msgref{SET_VQUEUE} (size,
+        descriptor, driver, and device addresses).
+  \item When \msgref{RESET_VQUEUE} is issued (and VIRTIO_F_RING_RESET is
+        negotiated), the device MUST quiesce the queue, release any resources
+        associated with it, and allow the driver to reconfigure it.
+\end{itemize}
+
+\subsubsection{Status Information}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Initialization / Status Information}
+
+Drivers query the device status via \msgref{GET_DEVICE_STATUS} to observe
+progress or detect errors, and they drive the Virtio status transitions via
+\msgref{SET_DEVICE_STATUS}. Writing zero to the status field resets the device,
+invalidating any configuration or virtqueue state.
+
+\drivernormative{\paragraph}{Status Handling}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Status Information / Driver}
+\begin{itemize}
+  \item A driver SHOULD read the device status via \msgref{GET_DEVICE_STATUS}
+        when diagnosing errors or determining whether the device is ready to
+        move to the next initialization phase.
+  \item A driver MUST use \msgref{SET_DEVICE_STATUS} to drive the device through
+        the virtio-defined status states and MUST write 0 to request a device
+        reset when needed.
+\end{itemize}
+
+\devicenormative{\paragraph}{Status Handling}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Status Information / Device}
+\begin{itemize}
+  \item Upon receiving a \msgref{SET_DEVICE_STATUS} write of 0, a device MUST
+        reset its internal state, invalidate existing configuration and
+        virtqueue settings, and present the status field as 0.
+  \item A device MUST report its current status accurately via
+        \msgref{GET_DEVICE_STATUS}, including whether the FEATURES_OK bit has
+        been accepted or cleared.
+\end{itemize}
+
+\subsubsection{Finalizing Initialization}
+
+After configuring virtqueues and agreeing on features, the driver transitions
+the device to DRIVER_OK (and any other required status bits) via
+\msgref{SET_DEVICE_STATUS}. At that point, the device is considered ready for
+normal virtqueue I/O.
+
+\drivernormative{\paragraph}{Final Status}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Finalizing Initialization / Driver}
+\begin{itemize}
+  \item A driver MUST set DRIVER_OK via \msgref{SET_DEVICE_STATUS} only after
+        it has completed feature negotiation and initialized all required
+        virtqueues.
+  \item A driver MUST NOT queue normal I/O until the device reports a status
+        that includes DRIVER_OK.
+  \item A driver MUST NOT supply buffers or send driver notifications for a
+        virtqueue until that queue has been configured via \msgref{SET_VQUEUE}.
+\end{itemize}
+
+\devicenormative{\paragraph}{Final Status}{Virtio Transport Options / Virtio Over Messages / Device Initialization / Finalizing Initialization / Device}
+\begin{itemize}
+  \item A device MUST NOT process normal virtqueue I/O until the driver has set
+        DRIVER_OK.
+  \item Once DRIVER_OK is set, the device MUST begin processing virtqueue
+        requests subject to the negotiated features and queue configurations.
+\end{itemize}
+
+\subsection{Device Operation}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Operation}
+
+Once a virtio-msg device is fully initialized (see
+\ref{sec:Virtio Transport Options / Virtio Over Messages / Device Initialization}),
+the driver and device exchange messages to perform I/O and respond to
+configuration changes. This section details the primary messaging paths for
+\emph{driver notifications}, \emph{device notifications}, and the handling of
+runtime resets or shutdown sequences.
+
+\subsubsection{Driver Notifications}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Operation / Driver Notifications}
+
+After buffers are made available to a virtqueue, the driver issues
+\msgref{EVENT_AVAIL} to notify the device. A bus implementation may translate
+these notifications into an out-of-band mechanism or deliver them in-band.
+
+\drivernormative{\paragraph}{Driver Notifications}{Virtio Transport Options / Virtio Over Messages / Device Operation / Driver Notifications / Driver}
+\begin{itemize}
+  \item A driver MUST send \msgref{EVENT_AVAIL} when it places new buffers on a
+        virtqueue.
+  \item Each \msgref{EVENT_AVAIL} MUST identify the target virtqueue index.
+  \item If VIRTIO_F_NOTIFICATION_DATA has been negotiated, the driver MUST
+        populate the “next offset and wrap” fields so the device can locate the
+        head of the updated ring; otherwise those fields MUST be zero.
+\end{itemize}
+
+\busnormative{\paragraph}{Driver Notifications}{Virtio Transport Options / Virtio Over Messages / Device Operation / Driver Notifications / Bus}
+\begin{itemize}
+  \item A bus implementation MAY convert \msgref{EVENT_AVAIL} into an
+        out-of-band notification but, if it does not, it MUST relay the message
+        over the virtio-msg channel.
+  \item A bus implementation MUST NOT drop \msgref{EVENT_AVAIL} unless it has
+        arranged for the device to detect new buffers through polling or an
+        equivalent mechanism.
+\end{itemize}
+
+\subsubsection{Device Notifications}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Operation / Device Notifications}
+
+\msgref{EVENT_CONFIG} and \msgref{EVENT_USED} provide asynchronous notifications
+from the device (or device-side bus) to the driver. The bus may forward these
+messages in-band or synthesize them based on other signals such as interrupts
+or polling.
+
+\devicenormative{\paragraph}{Device Notifications}{Virtio Transport Options / Virtio Over Messages / Device Operation / Device Notifications / Device}
+\begin{itemize}
+  \item A device (or device-side bus) MUST send \msgref{EVENT_CONFIG} whenever
+        it makes a configuration change or status update that becomes visible to
+        the driver. The message MUST include the new configuration generation
+        count and MAY include the updated configuration data.
+  \item If the configuration data is omitted from \msgref{EVENT_CONFIG}, the
+        device SHOULD include the relevant offsets/lengths so the driver can
+        re-fetch the data via \msgref{GET_CONFIG}.
+  \item A device SHOULD send \msgref{EVENT_USED} to inform the driver when
+        buffers on a virtqueue have been consumed, unless the device relies on
+        an alternative, agreed-upon completion mechanism.
+\end{itemize}
+
+\drivernormative{\paragraph}{Device Notifications}{Virtio Transport Options / Virtio Over Messages / Device Operation / Device Notifications / Driver}
+\begin{itemize}
+  \item Upon receiving \msgref{EVENT_CONFIG}, a driver SHOULD update its view of
+        the configuration using the provided data (or by issuing
+        \msgref{GET_CONFIG} if necessary) before resuming normal I/O.
+  \item Upon receiving \msgref{EVENT_USED}, a driver MUST examine the indicated
+        virtqueue (for example, by reading the used ring) to reclaim completed
+        buffers.
+\end{itemize}
+
+\busnormative{\paragraph}{Device Notifications}{Virtio Transport Options / Virtio Over Messages / Device Operation / Device Notifications / Bus}
+\begin{itemize}
+  \item A bus implementation MUST forward \msgref{EVENT_CONFIG} and
+        \msgref{EVENT_USED} notifications (or their equivalents) to the driver,
+        either in-band or by synthesizing the appropriate message.
+  \item If a bus implementation relies on polling or other mechanisms instead of
+        direct notifications, it SHOULD limit that mode to scenarios where no
+        other notification method is available.
+\end{itemize}
+
+\subsubsection{Configuration Changes During Operation}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Operation / Configuration Changes During Operation}
+
+Drivers may update configuration fields at runtime using \msgref{SET_CONFIG}
+when features such as device modes or limits need to change. Devices can also
+update configuration data autonomously but must signal those changes via
+\msgref{EVENT_CONFIG}.
+
+\drivernormative{\paragraph}{Runtime Configuration}{Virtio Transport Options / Virtio Over Messages / Device Operation / Configuration Changes During Operation / Driver}
+\begin{itemize}
+  \item A driver MAY issue \msgref{SET_CONFIG} after initialization, provided it
+        includes the current configuration generation count and follows the same
+        validation rules as during setup.
+  \item Upon receiving an \msgref{EVENT_CONFIG}, the driver SHOULD refresh its
+        view of the configuration (via the data provided or by reissuing
+        \msgref{GET_CONFIG}) before relying on the updated values.
+\end{itemize}
+
+\devicenormative{\paragraph}{Runtime Configuration}{Virtio Transport Options / Virtio Over Messages / Device Operation / Configuration Changes During Operation / Device}
+\begin{itemize}
+  \item If a device updates its configuration after initialization, it MUST send
+        \msgref{EVENT_CONFIG} to inform the driver of the change and provide the
+        new configuration generation count.
+\end{itemize}
+
+\subsubsection{Virtqueue Changes During Operation}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Operation / Virtqueue Changes During Operation}
+
+Drivers may provision unused virtqueues later in the device lifetime by issuing
+\msgref{SET_VQUEUE}, and they may reconfigure existing queues if the
+VIRTIO_F_RING_RESET feature has been negotiated.
+
+\drivernormative{\paragraph}{Runtime Virtqueue Changes}{Virtio Transport Options / Virtio Over Messages / Device Operation / Virtqueue Changes During Operation / Driver}
+\begin{itemize}
+  \item A driver MAY configure additional virtqueues after initialization using
+        \msgref{SET_VQUEUE}, provided it follows the same validation steps
+        (e.g., checking the maximum queue size).
+  \item If VIRTIO_F_RING_RESET is negotiated, the driver SHOULD use
+        \msgref{RESET_VQUEUE} before reprogramming a queue to avoid races with
+        the device.
+\end{itemize}
+
+\devicenormative{\paragraph}{Runtime Virtqueue Changes}{Virtio Transport Options / Virtio Over Messages / Device Operation / Virtqueue Changes During Operation / Device}
+\begin{itemize}
+  \item A device MUST honor \msgref{SET_VQUEUE} requests issued after
+        initialization and update the queue parameters accordingly.
+  \item When \msgref{RESET_VQUEUE} is received (and VIRTIO_F_RING_RESET is
+        negotiated), the device MUST quiesce the queue and allow the driver to
+        reconfigure it without processing stale data.
+\end{itemize}
+
+\subsubsection{Device Reset and Shutdown}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Operation / Reset}
+
+Drivers reset a device by writing 0 to the status field via \msgref{SET_DEVICE_STATUS},
+which forces the device to discard pending operations and return to the initial state.
+A device may also signal that a reset is required by sending \msgref{EVENT_CONFIG}
+with the DEVICE_NEEDS_RESET bit set.
+
+\drivernormative{\paragraph}{Reset and Shutdown}{Virtio Transport Options / Virtio Over Messages / Device Operation / Reset / Driver}
+\begin{itemize}
+  \item A driver MAY request a device reset at any time by writing 0 through
+        \msgref{SET_DEVICE_STATUS} and MAY reinitialize the device afterwards if
+        it intends to resume operation.
+  \item If a device signals DEVICE_NEEDS_RESET (for example, via
+        \msgref{EVENT_CONFIG}), the driver SHOULD write 0 to
+        \msgref{SET_DEVICE_STATUS} and reinitialize the device before resuming
+        I/O.
+\end{itemize}
+
+\devicenormative{\paragraph}{Reset and Shutdown}{Virtio Transport Options / Virtio Over Messages / Device Operation / Reset / Device}
+\begin{itemize}
+  \item Upon receiving a \msgref{SET_DEVICE_STATUS} write of 0, a device MUST
+        reset its internal state, discard pending virtqueue operations, and
+        present the status field as 0.
+  \item If the device encounters an unrecoverable error that requires driver
+        intervention, it SHOULD signal DEVICE_NEEDS_RESET (such as via
+        \msgref{EVENT_CONFIG}) so the driver can initiate a reset.
+\end{itemize}
+
+\subsubsection{Hotplug and Removal}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Device Operation / Hotplug and Removal}
+
+If the bus supports dynamic addition or removal of devices, it can advertise
+those changes with \busref{EVENT_DEVICE} (READY or REMOVED). Some platforms may
+instead rely on external signals such as ACPI, device tree updates, or
+hypervisor events; virtio-msg does not mandate a specific mechanism.
+
+\busnormative{\paragraph}{Hotplug and Removal}{Virtio Transport Options / Virtio Over Messages / Device Operation / Hotplug and Removal / Bus}
+\begin{itemize}
+  \item A bus implementation that uses \busref{EVENT_DEVICE} MUST send the READY
+        event when a device becomes accessible and the REMOVED event when it is
+        no longer available.
+  \item Regardless of how hotplug information is delivered, once a new device is
+        reported the bus SHOULD query it (e.g., via \msgref{GET_DEVICE_INFO})
+        and register it with the host OS. When a device is removed, the bus
+        SHOULD prompt the OS to quiesce and release the associated driver.
+\end{itemize}
+
+\subsection{Transport Messages}\label{sec:Virtio Transport Options / Virtio Over Messages / Transport Messages}
+
+Transport messages configure and operate virtio devices once they have been
+discovered. Unlike bus messages (which cover enumeration or hotplug), transport
+messages focus on feature negotiation, configuration access, virtqueue setup,
+and runtime notifications. The subsections below describe each message and how
+it is used in the virtio-msg transport.
+
+\subsubsection{Overview}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Transport Messages / Overview}
+
+Drivers send transport messages to a specific device number, and the virtio-msg
+device replies or emits events accordingly. The bus simply forwards these
+messages after enforcing generic checks such as maximum size or verifying that
+the target device exists. Most transport messages are request/response pairs,
+with events reserved for asynchronous notifications.
+
+\paragraph{Messages IDs and issuers}
+
+\begin{tabular}{|l|l|l|}
+\hline
+Name & ID & Sender \
+\hline
+\hline
+Reserved                   & 0x0 & \
+\hline
+Reserved                   & 0x1 & \
+\hline
+\msgref{GET_DEVICE_INFO}   & 0x2  & Driver \
+\hline
+\msgref{GET_DEVICE_FEATURES} & 0x3  & Driver \
+\hline
+\msgref{SET_DRIVER_FEATURES} & 0x4  & Driver \
+\hline
+\msgref{GET_CONFIG}        & 0x5  & Driver \
+\hline
+\msgref{SET_CONFIG}        & 0x6  & Driver \
+\hline
+\msgref{GET_DEVICE_STATUS} & 0x7  & Driver \
+\hline
+\msgref{SET_DEVICE_STATUS} & 0x8  & Driver \
+\hline
+\msgref{GET_VQUEUE}        & 0x9  & Driver \
+\hline
+\msgref{SET_VQUEUE}        & 0xA  & Driver \
+\hline
+\msgref{RESET_VQUEUE}      & 0xB  & Driver \
+\hline
+\msgref{GET_SHM}           & 0xC  & Driver \
+\hline
+\msgref{EVENT_CONFIG}      & 0x40 & Device \
+\hline
+\msgref{EVENT_AVAIL}       & 0x41 & Driver \
+\hline
+\msgref{EVENT_USED}        & 0x42 & Device \
+\hline
+\end{tabular}
+
+IDs are grouped as follows; bit 6 distinguishes requests (0) from events (1):
+
+\begin{tabular}{|l|l|p{7cm}|}
+\hline
+Range & Usage & Notes \
+\hline
+0x00-0x3F & Request/Response & Standard transport requests that expect responses. \
+0x40-0x7F & Events & Standard transport events (no response). \
+0x80-0xBF & Implementation-defined Requests & Optional, transport-specific request extensions. \
+0xC0-0xFF & Implementation-defined Events & Optional, transport-specific event extensions. \
+\hline
+\end{tabular}
+
+\devicenormative{\paragraph}{Core Messages}{Virtio Transport Options / Virtio Over Messages / Transport Messages / Mandatory / Device}
+\begin{itemize}
+  \item A device MUST implement \msgref{GET_DEVICE_INFO},
+        \msgref{GET_DEVICE_FEATURES}, \msgref{SET_DRIVER_FEATURES},
+        \msgref{GET_CONFIG}, \msgref{SET_CONFIG}, \msgref{GET_DEVICE_STATUS},
+        \msgref{SET_DEVICE_STATUS}, \msgref{GET_VQUEUE}, \msgref{SET_VQUEUE},
+        and \msgref{RESET_VQUEUE}.
+\end{itemize}
+
+\drivernormative{\paragraph}{Core Messages}{Virtio Transport Options / Virtio Over Messages / Transport Messages / Mandatory / Driver}
+\begin{itemize}
+  \item A driver MUST support sending and interpreting the messages listed
+        above in order to initialize and operate a virtio-msg device.
+\end{itemize}
+
+As described in Sections
+\ref{sec:Virtio Transport Options / Virtio Over Messages / Device Operation / Driver Notifications}
+and
+\ref{sec:Virtio Transport Options / Virtio Over Messages / Device Operation / Device Notifications},
+runtime notifications (\msgref{EVENT_AVAIL}, \msgref{EVENT_USED}, and
+\msgref{EVENT_CONFIG}) may be delivered in-band or via equivalent out-of-band
+mechanisms.
+
+\msgdef{GET_DEVICE_INFO}
+
+The driver issues \msgref{GET_DEVICE_INFO} to discover static identification
+data and limits for a device. It is the only transport message permitted before
+the device transitions out of the inactive state.
+
+\begin{lstlisting}
+struct virtio_msg_get_device_info_req {
+        /* no payload */
+};
+
+struct virtio_msg_get_device_info_resp {
+        le32 device_id;        /* virtio device type */
+        le32 vendor_id;        /* implementation-defined vendor ID */
+        le32 num_feature_bits; /* total feature bits (multiples of 32) */
+        le32 config_size;      /* bytes in the device configuration space */
+        le32 max_virtqueues;   /* maximum virtqueues supported */
+        le16 admin_vq_start;   /* index of the first admin virtqueue */
+        le16 admin_vq_count;   /* number of admin virtqueues */
+};
+\end{lstlisting}
+
+\devicenormative{\paragraph}{GET_DEVICE_INFO}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_GET_DEVICE_INFO / Device}
+\begin{itemize}
+  \item A device MUST respond to \msgref{GET_DEVICE_INFO} while inactive and
+        MUST supply consistent values for device ID, vendor ID, feature count (a
+        multiple of 32 bits), configuration size, and virtqueue limits.
+\end{itemize}
+
+\drivernormative{\paragraph}{GET_DEVICE_INFO}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_GET_DEVICE_INFO / Driver}
+\begin{itemize}
+  \item A driver SHOULD treat the response as the authoritative source for
+        feature count, configuration size, and virtqueue limits before
+        proceeding with initialization.
+\end{itemize}
+
+\msgdef{GET_DEVICE_FEATURES}
+
+Drivers retrieve device feature bits in 32-bit blocks via
+\msgref{GET_DEVICE_FEATURES}; the response echoes the requested block index and
+returns one or more 32-bit values with the feature bits in that range.
+
+\begin{lstlisting}
+struct virtio_msg_get_device_features_req {
+        le32 block_index; /* starting block (0 == bits 0-31) */
+        le32 num_blocks;  /* number of 32-bit blocks requested */
+};
+
+struct virtio_msg_get_device_features_resp {
+        le32 block_index; /* echoed starting block */
+        le32 num_blocks;  /* echoed number of blocks */
+        le32 features[];  /* num_blocks entries, zero-padded if needed */
+};
+\end{lstlisting}
+
+\devicenormative{\paragraph}{GET_DEVICE_FEATURES}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_GET_DEVICE_FEATURES / Device}
+\begin{itemize}
+  \item A device MUST return zero for any feature bits beyond those it
+        implements when responding to \msgref{GET_DEVICE_FEATURES}.
+\end{itemize}
+
+\drivernormative{\paragraph}{GET_DEVICE_FEATURES}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_GET_DEVICE_FEATURES / Driver}
+\begin{itemize}
+  \item A driver MUST treat the returned data as 32-bit feature blocks starting
+        at the requested index, padding with zeros as needed, before deciding
+        which features to enable.
+\end{itemize}
+
+\msgdef{SET_DRIVER_FEATURES}
+
+Drivers use \msgref{SET_DRIVER_FEATURES} to acknowledge the subset of feature
+bits they wish to enable. The payload mirrors \msgref{GET_DEVICE_FEATURES},
+supplying an index, count, and 32-bit feature data for the selected blocks.
+
+\begin{lstlisting}
+struct virtio_msg_set_driver_features_req {
+        le32 block_index; /* starting block being acknowledged */
+        le32 num_blocks;  /* number of blocks provided */
+        le32 features[];  /* num_blocks entries with desired bits */
+};
+
+struct virtio_msg_set_driver_features_resp {
+        /* no payload */
+};
+\end{lstlisting}
+
+\drivernormative{\paragraph}{SET_DRIVER_FEATURES}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_SET_DRIVER_FEATURES / Driver}
+\begin{itemize}
+  \item A driver MUST set only the feature bits it intends to enable in the
+        blocks supplied to \msgref{SET_DRIVER_FEATURES}.
+\end{itemize}
+
+\devicenormative{\paragraph}{SET_DRIVER_FEATURES}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_SET_DRIVER_FEATURES / Device}
+\begin{itemize}
+  \item After processing \msgref{SET_DRIVER_FEATURES}, a device MUST update its
+        acknowledged feature set to match the data supplied and MUST report that
+        set consistently in subsequent \msgref{GET_DEVICE_FEATURES} responses.
+        If it cannot support the requested set, it SHOULD clear the FEATURES_OK
+        bit in the device status.
+\end{itemize}
+
+\msgdef{GET_CONFIG}
+
+\msgref{GET_CONFIG} reads a portion of the configuration space. The request
+specifies the byte offset and length; the response echoes those values, returns
+the data, and includes the current configuration generation count.
+
+\begin{lstlisting}
+struct virtio_msg_get_config_req {
+        le32 offset; /* byte offset into configuration space */
+        le32 length; /* number of bytes requested */
+};
+
+struct virtio_msg_get_config_resp {
+        le32 generation; /* current configuration generation count */
+        le32 offset;     /* echoed offset */
+        le32 length;     /* echoed length */
+        u8   data[];     /* configuration payload */
+};
+\end{lstlisting}
+
+\drivernormative{\paragraph}{GET_CONFIG}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_GET_CONFIG / Driver}
+\begin{itemize}
+  \item A driver MUST ensure the requested offset and length in \msgref{GET_CONFIG}
+        stay within the configuration size reported by \msgref{GET_DEVICE_INFO}.
+\end{itemize}
+
+\devicenormative{\paragraph}{GET_CONFIG}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_GET_CONFIG / Device}
+\begin{itemize}
+  \item A device MUST return the current configuration generation count with
+        every \msgref{GET_CONFIG} response.
+\end{itemize}
+
+\msgdef{SET_CONFIG}
+
+\msgref{SET_CONFIG} writes a portion of configuration space, supplying the
+offset, length, the driver's view of the generation count, and the new data.
+The device echoes the offset/length, returns the resulting generation count,
+and may mirror the applied data or set the length to zero if the write was
+rejected.
+
+\begin{lstlisting}
+struct virtio_msg_set_config_req {
+        le32 generation; /* driver's view of generation count */
+        le32 offset;     /* byte offset being written */
+        le32 length;     /* number of bytes written */
+        u8   data[];     /* configuration payload */
+};
+
+struct virtio_msg_set_config_resp {
+        le32 generation; /* resulting generation count */
+        le32 offset;     /* echoed offset */
+        le32 length;     /* bytes applied (0 if rejected) */
+        u8   data[];     /* optional echoed data */
+};
+\end{lstlisting}
+
+\drivernormative{\paragraph}{SET_CONFIG}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_SET_CONFIG / Driver}
+\begin{itemize}
+  \item A driver MUST include its most recent configuration generation count in
+        each \msgref{SET_CONFIG} request and MUST keep the offset and length
+        within the reported configuration size.
+\end{itemize}
+
+\devicenormative{\paragraph}{SET_CONFIG}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_SET_CONFIG / Device}
+\begin{itemize}
+  \item A device MUST reject a \msgref{SET_CONFIG} request if the supplied
+        generation count does not match its current value and MUST report that
+        rejection in the response.
+\end{itemize}
+
+\msgdef{GET_DEVICE_STATUS}
+
+\msgref{GET_DEVICE_STATUS} reads the current device status (e.g., ACKNOWLEDGE,
+DRIVER, DRIVER_OK, FEATURES_OK, or DEVICE_NEEDS_RESET bits). The request has
+no payload; the response returns the 32-bit status value.
+
+\begin{lstlisting}
+struct virtio_msg_get_device_status_req {
+        /* no payload */
+};
+
+struct virtio_msg_get_device_status_resp {
+        le32 status; /* current device status bits */
+};
+\end{lstlisting}
+
+\devicenormative{\paragraph}{GET_DEVICE_STATUS}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_GET_DEVICE_STATUS / Device}
+\begin{itemize}
+  \item A device MUST report its current status accurately via
+        \msgref{GET_DEVICE_STATUS}, including whether FEATURES_OK or
+        DEVICE_NEEDS_RESET are set.
+\end{itemize}
+
+\msgdef{SET_DEVICE_STATUS}
+
+\msgref{SET_DEVICE_STATUS} writes a new device status value. Drivers use it to
+progress through the virtio-defined states or to request a reset by writing 0.
+The device responds with its resulting status, which may differ (for example,
+if it refuses FEATURES_OK or sets DEVICE_NEEDS_RESET).
+
+\begin{lstlisting}
+struct virtio_msg_set_device_status_req {
+        le32 status; /* desired device status value */
+};
+
+struct virtio_msg_set_device_status_resp {
+        le32 status; /* resulting device status */
+};
+\end{lstlisting}
+
+\drivernormative{\paragraph}{SET_DEVICE_STATUS}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_SET_DEVICE_STATUS / Driver}
+\begin{itemize}
+  \item A driver MUST write 0 via \msgref{SET_DEVICE_STATUS} to request a device
+        reset and MUST re-read the status (e.g., via \msgref{GET_DEVICE_STATUS})
+        if it needs to confirm acceptance.
+\end{itemize}
+
+\devicenormative{\paragraph}{SET_DEVICE_STATUS}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_SET_DEVICE_STATUS / Device}
+\begin{itemize}
+  \item A device MAY clear FEATURES_OK or set DEVICE_NEEDS_RESET in its
+        response if it cannot accept the requested status, but it MUST report
+        the resulting status accurately.
+\end{itemize}
+
+\msgdef{GET_VQUEUE}
+
+\msgref{GET_VQUEUE} returns information about a specific virtqueue, including
+its maximum size, current size, and, if already configured, the descriptor,
+driver, and device area addresses.
+
+\begin{lstlisting}
+struct virtio_msg_get_vqueue_req {
+        le32 index; /* virtqueue index */
+};
+
+struct virtio_msg_get_vqueue_resp {
+        le32 index;        /* echoed virtqueue index */
+        le32 max_size;     /* maximum queue size */
+        le32 cur_size;     /* current size (0 if unconfigured) */
+        le32 reserved;     /* must be zero */
+        le64 desc_addr;    /* descriptor area address */
+        le64 driver_addr;  /* driver area address */
+        le64 device_addr;  /* device area address */
+};
+\end{lstlisting}
+
+\devicenormative{\paragraph}{GET_VQUEUE}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_GET_VQUEUE / Device}
+\begin{itemize}
+  \item A device MUST report accurate maxima and current queue sizes for each
+        virtqueue and MUST return zero as the current size if the queue has not
+        yet been configured.
+\end{itemize}
+
+\msgdef{SET_VQUEUE}
+
+\msgref{SET_VQUEUE} programs a virtqueue's size and buffer addresses. The driver
+selects a queue index, supplies the desired size (not exceeding the maximum
+reported via \msgref{GET_VQUEUE}), and provides guest-physical addresses for the
+descriptor, driver, and device areas.
+
+\begin{lstlisting}
+struct virtio_msg_set_vqueue_req {
+        le32 index;       /* virtqueue index */
+        le32 reserved0;   /* must be zero */
+        le32 size;        /* number of descriptors */
+        le32 reserved1;   /* must be zero */
+        le64 desc_addr;   /* descriptor area address */
+        le64 driver_addr; /* driver area address */
+        le64 device_addr; /* device area address */
+};
+
+struct virtio_msg_set_vqueue_resp {
+        /* no payload */
+};
+\end{lstlisting}
+
+\drivernormative{\paragraph}{SET_VQUEUE}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_SET_VQUEUE / Driver}
+\begin{itemize}
+  \item A driver MUST set the virtqueue size field to a value not exceeding the
+        maximum reported by \msgref{GET_VQUEUE} and MUST ensure the supplied
+        addresses point to properly aligned memory.
+\end{itemize}
+
+\devicenormative{\paragraph}{SET_VQUEUE}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_SET_VQUEUE / Device}
+\begin{itemize}
+  \item A device MUST configure the queue using the parameters provided via
+        \msgref{SET_VQUEUE} and treat the queue as inactive until
+        \msgref{SET_VQUEUE} has successfully completed.
+\end{itemize}
+
+\msgdef{RESET_VQUEUE}
+
+\msgref{RESET_VQUEUE} resets an individual virtqueue (if VIRTIO_F_RING_RESET
+has been negotiated). It requires only the queue index; the device responds
+after the queue has been quiesced.
+
+\begin{lstlisting}
+struct virtio_msg_reset_vqueue_req {
+        le32 index; /* virtqueue index */
+};
+
+struct virtio_msg_reset_vqueue_resp {
+        /* no payload */
+};
+\end{lstlisting}
+
+\drivernormative{\paragraph}{RESET_VQUEUE}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_RESET_VQUEUE / Driver}
+\begin{itemize}
+  \item A driver SHOULD issue \msgref{RESET_VQUEUE} only if
+        VIRTIO_F_RING_RESET has been negotiated and it needs to reconfigure
+        or recover a specific queue.
+\end{itemize}
+
+\devicenormative{\paragraph}{RESET_VQUEUE}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_RESET_VQUEUE / Device}
+\begin{itemize}
+  \item Upon receiving \msgref{RESET_VQUEUE}, a device MUST stop processing the
+        indicated queue, reset its internal state for that queue, and allow the
+        driver to reconfigure it via \msgref{SET_VQUEUE}.
+\end{itemize}
+
+\msgdef{GET_SHM}
+
+\msgref{GET_SHM} returns the location and size of a device-owned shared memory
+region. The request carries the region index; the response echoes the index and
+provides the base address and length. A length of zero indicates that the
+specified region does not exist.
+
+\begin{lstlisting}
+struct virtio_msg_get_shm_req {
+        le32 index; /* shared memory region index */
+};
+
+struct virtio_msg_get_shm_resp {
+        le32 index;  /* echoed index */
+        le32 length; /* region length (0 if nonexistent) */
+        le32 address;/* region address */
+};
+\end{lstlisting}
+
+\devicenormative{\paragraph}{GET_SHM}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_GET_SHM / Device}
+\begin{itemize}
+  \item A device MUST return zero length if the requested shared memory region
+        does not exist and MUST report accurate address/length information for
+        regions it supports.
+\end{itemize}
+
+\msgdef{EVENT_CONFIG}
+
+\msgref{EVENT_CONFIG} notifies the driver about configuration or status changes.
+The payload includes the current device status, configuration generation count,
+and optionally the updated configuration data (offset plus length). A length of
+zero indicates that the driver should re-fetch the affected range via
+\msgref{GET_CONFIG}.
+
+\begin{lstlisting}
+struct virtio_msg_event_config {
+        le32 device_status; /* new device status value */
+        le32 generation;    /* configuration generation count */
+        le32 offset;        /* configuration offset */
+        le32 length;        /* number of bytes (may be zero) */
+        u8   data[];        /* optional configuration data */
+};
+\end{lstlisting}
+
+\drivernormative{\paragraph}{EVENT_CONFIG}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_EVENT_CONFIG / Driver}
+\begin{itemize}
+  \item When \msgref{EVENT_CONFIG} provides configuration data, the driver
+        SHOULD apply it; otherwise it SHOULD re-read the affected range via
+        \msgref{GET_CONFIG} before proceeding.
+\end{itemize}
+
+\devicenormative{\paragraph}{EVENT_CONFIG}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_EVENT_CONFIG / Device}
+\begin{itemize}
+  \item A device MUST send \msgref{EVENT_CONFIG} whenever it changes
+        configuration data or status in a way that is visible to the driver,
+        including the new configuration generation count.
+\end{itemize}
+
+\msgdef{EVENT_AVAIL}
+
+\msgref{EVENT_AVAIL} notifies the device that a virtqueue has new buffers. The
+message includes the queue index and, if VIRTIO_F_NOTIFICATION_DATA is
+negotiated, a combined next-offset/next-wrap field so the device can jump
+directly to the updated position. Otherwise that field is zero.
+
+\begin{lstlisting}
+struct virtio_msg_event_avail {
+        le32 vq_index;    /* virtqueue index */
+        le32 next_offset; /* bits[30:0] offset, bit[31] wrap; zero if unused */
+};
+\end{lstlisting}
+
+\drivernormative{\paragraph}{EVENT_AVAIL}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_EVENT_AVAIL / Driver}
+\begin{itemize}
+  \item If VIRTIO_F_NOTIFICATION_DATA has not been negotiated, a driver MUST
+        set the next offset/wrap field to zero in \msgref{EVENT_AVAIL} messages.
+        If it has been negotiated, the driver MUST fill in the field with the
+        current available ring position.
+\end{itemize}
+
+\busnormative{\paragraph}{EVENT_AVAIL}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_EVENT_AVAIL / Bus}
+\begin{itemize}
+  \item A bus implementation that uses out-of-band notifications SHOULD prevent
+        negotiation of VIRTIO_F_NOTIFICATION_DATA so that devices do not
+        expect in-band offset/wrap values.
+\end{itemize}
+
+\msgdef{EVENT_USED}
+
+\msgref{EVENT_USED} notifies the driver that buffers on a particular virtqueue
+have been consumed. The payload carries only the queue index; the driver uses
+its used ring to determine which descriptors completed.
+
+\begin{lstlisting}
+struct virtio_msg_event_used {
+        le32 vq_index; /* virtqueue index */
+};
+\end{lstlisting}
+
+\devicenormative{\paragraph}{EVENT_USED}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_EVENT_USED / Device}
+\begin{itemize}
+  \item A device SHOULD send \msgref{EVENT_USED} (or an equivalent notification)
+        when it processes buffers on a virtqueue unless the driver has
+        explicitly disabled such notifications.
+\end{itemize}
+
+\drivernormative{\paragraph}{EVENT_USED}{Virtio Transport Options / Virtio Over Messages / Transport Messages / VIRTIO_MSG_EVENT_USED / Driver}
+\begin{itemize}
+  \item Upon receiving \msgref{EVENT_USED}, a driver MUST examine the indicated
+        virtqueue (for example, by reading the used ring) to reclaim completed
+        buffers.
+\end{itemize}
+
+\subsection{Bus Messages}\label{sec:Virtio Transport Options / Virtio Over Messages / Bus Messages}
+
+Bus messages cover device discovery, hotplug notifications, and bus-level
+liveness checks. Implementations that already provide equivalent functionality
+through platform mechanisms (e.g., ACPI, device tree, hypervisor events) may
+reuse that infrastructure instead of the messages defined here. The following
+subsections describe standardized bus messages that virtio-msg supports today.
+
+\subsubsection{Overview}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Bus Messages / Overview}
+
+Each bus implementation chooses which of these messages to support. Some may
+implement all of them for a fully message-based workflow, while others may rely
+on out-of-band enumeration and use only the subset that fits their needs.
+
+\paragraph{Messages IDs and issuers}
+
+\begin{tabular}{|l|l|l|}
+\hline
+Name & ID & Sender \
+\hline
+\hline
+Reserved                    & 0x0  &        \
+\hline
+Reserved                    & 0x1  &        \
+\hline
+\busref{GET_DEVICES}        & 0x2  & Driver \
+\hline
+\busref{PING}               & 0x3  & Any    \
+\hline
+\busref{EVENT_DEVICE}       & 0x40 & Device \
+\hline
+\end{tabular}
+
+Bus message IDs are grouped as follows:
+
+\begin{tabular}{|l|l|p{7cm}|}
+\hline
+Range & Usage & Notes \
+\hline
+0x00-0x3F & Request/Response & Standardized bus request messages. \
+0x40-0x7F & Events & Standardized bus event messages. \
+0x80-0xBF & Implementation-defined Requests & Optional, bus-specific request messages. \
+0xC0-0xFF & Implementation-defined Events & Optional, bus-specific event messages. \
+\hline
+\end{tabular}
+
+\paragraph{Bus Specific Messages}
+\label{sec:Virtio Transport Options / Virtio Over Messages / Bus Operation / Bus Specific Messages}
+
+A range of message IDs are reserved for use by the specific bus
+implementation. These messages can be used for any implementation specific
+usage. Example usage could include:
+
+\begin{itemize}
+  \item Configuration of out-of-band notification methods
+  \item Setup shared memory regions to be used for buffers or virtqueues
+  \item Declare bus specific error conditions
+  \item Provide extra debug or logging information
+\end{itemize}
+
+\busdef{GET_DEVICES}
+
+The driver-side bus uses this request to enumerate device numbers on the
+device-side bus. The response describes a “window” of device numbers and signals
+which entries are present.
+
+\begin{lstlisting}
+struct virtio_bus_msg_get_devices_req {
+        le16 offset; /* starting device number */
+        le16 count;  /* number of device-number slots (multiple of 8) */
+};
+
+struct virtio_bus_msg_get_devices_resp {
+        le16 offset;      /* echoed starting device number */
+        le16 count;       /* window length actually returned */
+        le16 next_offset; /* 0 or suggested start for next query */
+        u8   bitmap[];    /* count/8 bytes, LSB-first packing */
+};
+\end{lstlisting}
+
+The \textbf{(offset, count)} tuple defines a window of \textbf{count}
+consecutive device numbers beginning at \textbf{offset}. The number of present
+devices equals the number of 1-bits in the bitmap. Responders SHOULD return the
+requested \textbf{count} unless constrained (e.g., by maximum message size);
+otherwise they MAY return a smaller value, in which case the bitmap covers the
+reduced window.
+
+Example: a request with \textbf{offset}=0, \textbf{count}=16 might produce
+\textbf{bitmap}=0b00100101 00000000 and \textbf{next_offset}=16. That indicates
+devices 0, 2, and 5 are present within the 0–15 window and suggests continuing
+at device number 16.
+
+\paragraph{Intended usage}
+Drivers MAY use this message to enumerate device numbers. Treat each window as
+a snapshot, advance using \textbf{next_offset}, and confirm candidates via
+\msgref{GET_DEVICE_INFO} before issuing other transport messages.
+
+\busnormative{\paragraph}{GET_DEVICES}{Virtio Transport Options / Virtio Over Messages / Bus Messages / GET_DEVICES / Bus}
+\begin{itemize}
+  \item The bus-side responder MUST enforce that \textbf{offset} and
+        \textbf{count} are multiples of 8, and it MUST return a bitmap of length
+        \textbf{count/8} packed least-significant-bit first.
+  \item \textbf{next_offset} MUST be 0 (indicating no further windows) or an
+        aligned value $\ge \textbf{offset} + \textbf{count}$ recommending where
+        the driver should query next.
+  \item Responders SHOULD return the requested \textbf{count} unless constrained
+        (e.g., by maximum message size); if a smaller count is returned, the
+        bitmap MUST still represent the window defined by the echoed
+        \textbf{offset} and \textbf{count}.
+\end{itemize}
+
+\busdef{PING}
+
+\busref{PING} is a simple liveness check that can be sent by either side of the
+bus; the response echoes the 32-bit data value from the request.
+
+\begin{lstlisting}
+struct virtio_bus_msg_ping_req {
+        le32 data; /* arbitrary value chosen by the sender */
+};
+
+struct virtio_bus_msg_ping_resp {
+        le32 data; /* echoed request data */
+};
+\end{lstlisting}
+
+\paragraph{Intended usage}
+Drivers or bus implementations MAY use \busref{PING} as a keepalive. If a reply
+is missing or delayed beyond policy, the initiator SHOULD verify device status
+(e.g., via \msgref{GET_DEVICE_STATUS}) and take recovery actions as needed.
+
+\busnormative{\paragraph}{PING}{Virtio Transport Options / Virtio Over Messages / Bus Messages / PING / Bus}
+\begin{itemize}
+  \item The bus-side responder MUST echo the 32-bit data field from the request
+        exactly in the \busref{PING} response.
+\end{itemize}
+
+\busdef{EVENT_DEVICE}
+
+\busref{EVENT_DEVICE} signals device-level hotplug changes. The driver-side bus
+receives these indications from the device-side bus; the table below lists the
+defined device bus states.
+
+\begin{lstlisting}
+struct virtio_bus_msg_event_device {
+        le16 device_number;
+        le16 device_bus_state; /* see table below */
+};
+\end{lstlisting}
+
+\begin{tabular}{|l|l|p{7cm}|}
+\hline
+Constant & Value & Meaning \
+\hline
+DEVICE_BUS_STATE_READY    & 0x0001 & Device is present and ready. \
+DEVICE_BUS_STATE_REMOVED  & 0x0002 & Device is no longer present. \
+Reserved                    & 0x0003–0x7FFF & Reserved for standard use. \
+Implementation-defined      & 0x8000–0xFFFF & MAY be used by the bus implementation. \
+\hline
+\end{tabular}
+
+\paragraph{Intended usage}
+Upon receiving DEVICE_BUS_STATE_READY, drivers SHOULD probe the device via
+\msgref{GET_DEVICE_INFO} and proceed with initialization if successful. Upon
+DEVICE_BUS_STATE_REMOVED, drivers MUST stop queueing new work, quiesce and
+reset as applicable, and release resources. Drivers SHOULD tolerate duplicate or
+out-of-order events and MAY rely on additional bus-level monitoring (e.g.,
+\busref{PING}) if needed.
-- 
2.34.1


    

2026

2025

2024

[Virtio-msg] [PATCH v1 2/4] virtio-msg: Add virtio-msg, a message based virtio transport layer