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Subject: [virtio-dev][RFC v3] virtio-sdm: new device specification

This patch adds the specification of the Signal Dristribution Module virtio
device to the current virtio specification document.

Signed-off-by: Christian Pinto <c.pinto@virtualopensystems.com>
Signed-off-by: Baptiste Reynal <b.reynal@virtualopensystems.com>

v2 -> v3:
- Removed master field from configuration and replaced with device_id
- Added new RESET signal
- Added signals definition into device specs
- Added feature bits associated to signals

v1 -> v2:
- Fixed some typos
- Removed dependencies from QEMU
- Added explanation on how SDM can be used in AMP systems
- Explained semantics of payload field in SDMSignalData struct
 content.tex    |   2 +
 virtio-sdm.tex | 166 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 168 insertions(+)
 create mode 100644 virtio-sdm.tex

diff --git a/content.tex b/content.tex
index 3317916..7fcf779 100644
--- a/content.tex
+++ b/content.tex
@@ -5643,6 +5643,8 @@ descriptor for the \field{sense_len}, \field{residual},
 \field{status_qualifier}, \field{status}, \field{response} and
 \field{sense} fields.
 \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
 Currently there are three device-independent feature bits defined:
diff --git a/virtio-sdm.tex b/virtio-sdm.tex
new file mode 100644
index 0000000..ee43e23
--- /dev/null
+++ b/virtio-sdm.tex
@@ -0,0 +1,166 @@
+\section{Signal Distribution Module}\label{sec:Device Types / SDM Device}
+The virtio Signal Distribution Module is meant to enable Inter Processor signal
+An example are Inter Processor Interrupts used in AMP systems, for the
+processors involved to notify the presence of new data in the communication
+In AMP systems signals are used for various purposes, an example are remoteproc
+or RPMSG. In the former signals are used by a master processor to trigger
+the boot of a slave processor. While the latter, RPMSG, uses virtio queues as a
+message exchange medium between processors. In this case the SDM can be used to
+generate the interrupt associated to the kick of a virtio queue.
+There are three signal types supported by the device, namely the
+\textit{IRQ} signal, \textit{BOOT} signal and \textit{RESET} signal. Such set of
+signals covers most of the needs of an AMP system, where a master processor can
+trigger the boot or reset of slave processors, and processors send IRQs between
+each other.
+\subsection{Device ID}\label{sec:Device Types / SDM Device / Device ID}
+\subsection{Virtqueues}\label{sec:Device Types / SDM Device / Virtqueues}
+\item[0] hg_vq
+\item[1] gh_vq
+Queue 0 is used for device-to-driver communication (i.e., notification of a
+signal), while queue 1 for driver-to-device communication.
+\subsection{Feature bits}\label{sec:Device Types / SDM Device / Feature bits}
+\item[VIRTIO_SDM_F_IRQ_SIG (0)] Device supports the IRQ signal.
+\item[VIRTIO_SDM_F_BOOT_SIG (1)] Device supports the BOOT signal.
+\item[VIRTIO_SDM_F_RESET_SIG (2)] Device supports the RESET signal.
+This set of bits is used by each virtio SDM device to declare which of the
+signals it supports. By specification each device can support all or a subset of
+the defined signals.
+\subsection{Device configuration layout}\label{sec:Device Types / SDM Device /
+Device configuration layout}
+The device configuration is composed by three fields:
+\texttt{max_slaves}, \texttt{current_slaves} and the \texttt{device_id}.
+struct virtio_sdm_config {
+	u16   max_slaves;
+	u16   current_slaves;
+	u32   device_id;
+The SDM device can be instantiated either as a master or as a slave. The master
+slave behavior is meant on purpose to reflect the AMP like type of communication
+where a master processor controls one or more slave processors.
+A master SDM instance can send a signal to any of the slaves instances,
+while slaves can only signal the master.
+The \texttt{master} field of the configuration space is meant to be read by the
+driver to figure out whether a specific instance is a master or a slave.
+The \texttt{max_slaves} field contains the maximum number of slaves supported by
+the SDM device. A configuration change notification is sent to the driver each
+time the value of \texttt{max_slaves} is changed from the device side.
+Finally, the \texttt{current_slaves} field contains the actual number of slaves
+registered to the master SDM. This field is a read only field. Finally the
+\texttt{device_id} field is used by each driver to know the ID of the device it
+is attached to, the field contains 0 in case of a master instance. A driver
+figures out whether it is attached to a master or a slave instance thanks to this
+\subsection{Device Initialization}\label{sec:Device Types / SDM Device /
+evice Initialization}
+During initialization the \texttt{hg_vq} and \texttt{gh_vq} are identified and
+the device is immediately operational. A master driver instance can access the
+number of slaves registered at any time by reading the configuration space of
+the device.
+During the initialization phase the device connects also to the communication
+channel. It has to be noted that the behavior of the device is
+independent from the communication channel used, that is a detail of each
+specific implementation of the SDM device.
+The last phase of the initialization is the negotiation of the feature bits.
+Each device implementation declares the signals supported by offering all or a
+subset of the three feature bits (VIRTIO_SDM_F_IRQ_SIG, VIRTIO_SDM_F_BOOT_SIG,
+VIRTIO_SDM_F_RESET_SIG). The SDM driver will be aware of the set of signals to
+handle thanks to this negotiation phase.
+\subsection{Device Operation}\label{sec:Device Types / SDM Device / Device
+The SDM device handles signals coming from the two following sources:
+\item The local processor to which the device is attached to.
+\item The communication channel connecting to other slaves/master.
+The first case is verified when the processor attached to the SDM device wants
+to send a signal to a second SDM device instance.
+The second case is instead when an SDM device instance receives a signal from
+another SDM device, to be forwarded to the local processor.
+It is important to note that due to the master/slave behavior, slaves cannot
+signal among themselves but only with the master SDM instance.
+In both cases, before sending over the communication channel, the signal is
+packed in the \texttt{SDMSignalData} data structure.
+enum sdm_signal_type {
+    SDM_IRQ,
+    SDM_BOOT,
+struct SDMSignalData {
+    uint32_t type;
+    uint32_t slave;
+    uint32_t payload[2];
+The \texttt{type} field indicates the type of signal to be sent to the
+destination SDM. The current implementation supports three signal types.
+The \texttt{SDM_IRQ} signal is used to send an inter processor interrupt, while
+the \texttt{SDM_BOOT} signal is sent to trigger the boot of the destination
+processor. The boot signal is meant to be used in an AMP like scenario where a
+master processor boots one or more slave processors (e.g., via remoteproc).
+The \texttt{SDM_RESET} is also meant to be used in AMP like scenarios, to
+trigger the reset of the target slave processor. As an assumption a driver
+cannot trigger the reset of the device it is attached to, so each driver
+implementation should ignore reset signals where the source slave corresponds to
+the device ID the driver is attached to.
+This is done by comparing, when a message is recevied, the value of
+the \texttt{slave} field of the \texttt{SDMSignalData} data structure with the
+\texttt{device_id} field of the configuration space.
+The \texttt{slave} field contains the id of the SDM instance destination of the
+signal. Id 0 is reserved for the master, from 1 onwards for the slaves.
+This means that the \texttt{slave} field will always contain 0 when the source
+of the signal is a slave SDM instance, while the actual id of the slave in case
+of a master. When instead a message is recevied, this field contains the ID of
+the slave source of the signal.
+The \texttt{payload} is used to pass extra accompanying information with the
+The semantics of the payload field depends on the signal itself. In case of
+\texttt{SDM_IRQ} signal, the payload field is ignored since interrupts do not
+need any extra information to be handled. In the case of \texttt{SDM_BOOT}
+signal the payload contains the boot address of the slave processor, to be used
+at the destination to initialize the program counter register before the actual
+boot process is started. Finally the payload field is ignored also in case of
+\texttt{SDM_RESET} signal, since no extra information is needed to trigger the
+reset of the target processor.
+The \texttt{SDMSignalData} structure is first filled by the source SDM kernel
+virtio driver and sent over the gh_vq.

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