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Subject: Re: [PATCH v6] Add virtio SCMI device specification
On 12.02.21 10:59, Peter Hilber wrote:
> This patch proposes a new virtio device for the Arm SCMI protocol.
>
> The device provides a simple transport for the Arm SCMI protocol[1]. The
> *S*ystem *C*ontrol and *M*anagement *I*nterface protocol allows speaking
> to system controllers that allow orchestrating things like power
> management, system state management and sensor access. The SCMI protocol
> is used on SoCs where multiple cores and co-processors need access to
> these resources.
>
> The virtio transport allows making use of this protocol in virtualized
> systems.
>
> [1] https://developer.arm.com/docs/den0056/c
>
> Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
> ---
>
> Notes:
>
> Since sending out v5 in May 2020, there has been no additional
> feedback w.r.t. the spec. An RfC patch series for the driver is now
> available at [1]. OpenSynergy also has a proprietary implementation
> of the device (without the VIRTIO_SCMI_F_SHARED_MEMORY feature so
> far).
>
> So I would like to request a vote on adding the device in a few weeks.
>
I would like to request a vote on adding the virtio SCMI device to the spec.
Fixes: https://github.com/oasis-tcs/virtio-spec/issues/100
Best regards,
Peter
> The PDF output is available at [2].
>
> [1] https://lore.kernel.org/linux-arm-kernel/20201105212116.411422-1-peter.hilber@opensynergy.com/
> [2] https://share.mailbox.org/ajax/share/056076e70571144f50c4ca7571144b319a1d7236dda1cd3b/1/8/MzQ/MzQvMQ
>
> Changes for v6:
>
> - Refer to new SCMI spec version 3.0 (DEN0056C), which has a
> provision for SHM race conditions.
>
> - Resolve conflicts with other added device documentation.
>
> Changes for v5:
>
> - Remove requirements on shared memory added in v4, since an upcoming
> new version of the Arm SCMI spec[1] will provide a generic, more
> powerful way to handle concurrent access to shared memory.
>
> Changes for v4:
>
> - Add more requirements on shared memory regions after feedback from
> Alex BennÃe.
>
> Changes for v3:
>
> - Add tentative device ID 32 in device section.
>
> - Remove redundant 'len' fields. The length of the payload fields can
> already be deduced from the generic virtqueue 'len' fields. Therefore,
> remove the redundant device-specific 'len' fields.
>
> - Reword requirement that driver must not put too small buffers into
> eventq.
>
> Changes for v2:
>
> - CC virtio-dev list.
> - Define size of erroneous delayed/not delayed responses.
> - Use correct long name for SCMI.
> - Remove restriction to `embedded' in commit message.
> - Add motivation for conceptual per-message-channels.
> - Device may now just drop notification if no buffers are available.
>
>
> conformance.tex | 34 ++++--
> content.tex | 1 +
> introduction.tex | 3 +
> virtio-scmi.tex | 271 +++++++++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 303 insertions(+), 6 deletions(-)
> create mode 100644 virtio-scmi.tex
>
> diff --git a/conformance.tex b/conformance.tex
> index 17c390d..a164cbb 100644
> --- a/conformance.tex
> +++ b/conformance.tex
> @@ -27,9 +27,10 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
> \ref{sec:Conformance / Driver Conformance / Socket Driver Conformance},
> \ref{sec:Conformance / Driver Conformance / RPMB Driver Conformance},
> \ref{sec:Conformance / Driver Conformance / IOMMU Driver Conformance},
> -\ref{sec:Conformance / Driver Conformance / Sound Driver Conformance}
> -\ref{sec:Conformance / Driver Conformance / Memory Driver Conformance} or
> -\ref{sec:Conformance / Driver Conformance / I2C Adapter Driver Conformance}.
> +\ref{sec:Conformance / Driver Conformance / Sound Driver Conformance},
> +\ref{sec:Conformance / Driver Conformance / Memory Driver Conformance},
> +\ref{sec:Conformance / Driver Conformance / I2C Adapter Driver Conformance} or
> +\ref{sec:Conformance / Driver Conformance / SCMI Driver Conformance}.
>
> \item Clause \ref{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}.
> \end{itemize}
> @@ -49,9 +50,10 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
> \ref{sec:Conformance / Device Conformance / Socket Device Conformance},
> \ref{sec:Conformance / Device Conformance / RPMB Device Conformance},
> \ref{sec:Conformance / Device Conformance / IOMMU Device Conformance},
> -\ref{sec:Conformance / Device Conformance / Sound Device Conformance}
> -\ref{sec:Conformance / Device Conformance / Memory Device Conformance} or
> -\ref{sec:Conformance / Device Conformance / I2C Adapter Device Conformance}.
> +\ref{sec:Conformance / Device Conformance / Sound Device Conformance},
> +\ref{sec:Conformance / Device Conformance / Memory Device Conformance},
> +\ref{sec:Conformance / Device Conformance / I2C Adapter Device Conformance} or
> +\ref{sec:Conformance / Device Conformance / SCMI Device Conformance}.
>
> \item Clause \ref{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}.
> \end{itemize}
> @@ -277,6 +279,15 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
> \item \ref{drivernormative:Device Types / I2C Adapter Device / Device Operation}
> \end{itemize}
>
> +\conformance{\subsection}{SCMI Driver Conformance}\label{sec:Conformance / Driver Conformance / SCMI Driver Conformance}
> +
> +An SCMI driver MUST conform to the following normative statements:
> +
> +\begin{itemize}
> +\item \ref{drivernormative:Device Types / SCMI Device / Device Operation / cmdq Operation}
> +\item \ref{drivernormative:Device Types / SCMI Device / Device Operation / Setting Up eventq Buffers}
> +\end{itemize}
> +
> \conformance{\section}{Device Conformance}\label{sec:Conformance / Device Conformance}
>
> A device MUST conform to the following normative statements:
> @@ -505,6 +516,17 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
> \item \ref{devicenormative:Device Types / I2C Adapter Device / Device Operation}
> \end{itemize}
>
> +\conformance{\subsection}{SCMI Device Conformance}\label{sec:Conformance / Device Conformance / SCMI Device Conformance}
> +
> +An SCMI device MUST conform to the following normative statements:
> +
> +\begin{itemize}
> +\item \ref{devicenormative:Device Types / SCMI Device / Feature bits}
> +\item \ref{devicenormative:Device Types / SCMI Device / Device Operation / cmdq Operation}
> +\item \ref{devicenormative:Device Types / SCMI Device / Device Operation / eventq Operation}
> +\item \ref{devicenormative:Device Types / SCMI Device / Device Operation / Shared Memory Operation}
> +\end{itemize}
> +
> \conformance{\section}{Legacy Interface: Transitional Device and Transitional Driver Conformance}\label{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}
> A conformant implementation MUST be either transitional or
> non-transitional, see \ref{intro:Legacy
> diff --git a/content.tex b/content.tex
> index 835f1ea..40ab30a 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -6532,6 +6532,7 @@ \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
> \input{virtio-sound.tex}
> \input{virtio-mem.tex}
> \input{virtio-i2c.tex}
> +\input{virtio-scmi.tex}
>
> \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
>
> diff --git a/introduction.tex b/introduction.tex
> index bc0217f..7204b24 100644
> --- a/introduction.tex
> +++ b/introduction.tex
> @@ -76,6 +76,9 @@ \section{Normative References}\label{sec:Normative References}
> \phantomsection\label{intro:I2C}\textbf{[I2C]} &
> I2C-bus specification and user manual,
> \newline\url{https://www.nxp.com/docs/en/user-guide/UM10204.pdf}\\
> + \phantomsection\label{intro:SCMI}\textbf{[SCMI]} &
> + Arm System Control and Management Interface, DEN0056,
> + \newline\url{https://developer.arm.com/docs/den0056/c}, version C and any future revisions\\
>
> \end{longtable}
>
> diff --git a/virtio-scmi.tex b/virtio-scmi.tex
> new file mode 100644
> index 0000000..c4b8dd0
> --- /dev/null
> +++ b/virtio-scmi.tex
> @@ -0,0 +1,271 @@
> +\section{SCMI Device}\label{sec:Device Types / SCMI Device}
> +
> +An SCMI device implements the Arm System Control and Management
> +Interface (SCMI). SCMI can be used for sensors, power state management,
> +clock management and performance management among other things.
> +
> +This section relies on definitions from the \hyperref[intro:SCMI]{SCMI
> +specification}.
> +
> +Virtio SCMI device and driver are mapped to SCMI platform and agent
> +respectively. The device is visible to a particular SCMI agent. The
> +device allows a guest to communicate as an SCMI agent using one or more
> +SCMI protocols. The default SCMI protocols are defined in the
> +\hyperref[intro:SCMI]{SCMI specification}. Virtio provides a transport
> +medium for exchanging SCMI messages between the SCMI agent and platform.
> +The virtio SCMI transport allows the queueing of multiple messages and
> +responses.
> +
> +SCMI FastChannels are not supported.
> +
> +\subsection{Device ID}\label{sec:Device Types / SCMI Device / Device ID}
> +
> +32
> +
> +\subsection{Virtqueues}\label{sec:Device Types / SCMI Device / Virtqueues}
> +
> +\begin{description}
> +\item[0] cmdq
> +\item[1] eventq
> +\end{description}
> +
> +The cmdq is used by the driver to send commands to the device. The
> +device replies with responses (not delayed responses) over the cmdq.
> +
> +The eventq is used by the device to send notifications and delayed
> +responses. The eventq only exists if VIRTIO_SCMI_F_P2A_CHANNELS was
> +negotiated.
> +
> +\subsection{Feature bits}\label{sec:Device Types / SCMI Device / Feature bits}
> +
> +\begin{description}
> +\item[VIRTIO_SCMI_F_P2A_CHANNELS (0)] Device implements some SCMI
> +notifications, or delayed responses.
> +\item[VIRTIO_SCMI_F_SHARED_MEMORY (1)] Device implements any SCMI
> +statistics shared memory region.
> +\end{description}
> +
> +VIRTIO_SCMI_F_P2A_CHANNELS is used to determine the existence of the
> +eventq. The eventq is required for SCMI notifications and delayed
> +responses.
> +
> +VIRTIO_SCMI_F_SHARED_MEMORY is used to determine whether the device
> +provides any SCMI statistics shared memory region. SCMI statistics
> +shared memory regions are defined by some SCMI protocols.
> +
> +The SCMI protocols provide the PROTOCOL_MESSAGE_ATTRIBUTES commands to
> +inquire about the particular SCMI notifications and delayed responses
> +implemented by the device. The SCMI protocols provide additional
> +commands to detect other features implemented by the device.
> +
> +\devicenormative{\subsubsection}{Feature bits}{Device Types / SCMI Device / Feature bits}
> +
> +The device MUST offer VIRTIO_SCMI_F_P2A_CHANNELS if the device can
> +implement at least one SCMI notification, or delayed response.
> +
> +The device MUST offer VIRTIO_SCMI_F_SHARED_MEMORY if the device can
> +implement at least one SCMI statistics shared memory region.
> +
> +\subsection{Device configuration layout}\label{sec:Device Types / SCMI Device / Device configuration layout}
> +
> +There is no configuration data for the device.
> +
> +\subsection{Device Initialization}\label{sec:Device Types / SCMI Device / Device Initialization}
> +
> +The
> +\hyperref[sec:General Initialization And Device Operation / Device Initialization]{general
> +requirements on device initialization} apply.
> +
> +\subsection{Device Operation}\label{sec:Device Types / SCMI Device / Device Operation}
> +
> +The SCMI transport used for the device puts each SCMI message into a
> +dedicated virtio buffer. The driver uses the cmdq for transmitting SCMI
> +commands and receiving the corresponding SCMI responses. The device uses
> +the eventq for transmitting SCMI notifications and delayed responses.
> +Each message includes an SCMI protocol header and payload, as defined by
> +the \hyperref[intro:SCMI]{SCMI specification}.
> +
> +\subsubsection{cmdq Operation}\label{sec:Device Types / SCMI Device / Device Operation / cmdq Operation}
> +
> +Each buffer in the cmdq holds a single SCMI command once the buffer has
> +been made available. When the buffer has been marked as used, it
> +contains the SCMI response. An arbitrary number of such SCMI messages
> +can be in transit at the same time. Conceptually, each SCMI message in
> +the cmdq uses its own SCMI A2P (agent to platform) channel.
> +
> +The SCMI response is in the same virtio buffer as the corresponding SCMI
> +command. The response contains the return values which SCMI specifies
> +for each command, whether synchronous or asynchronous. Delayed responses
> +are distinct SCMI messages transmitted over the eventq.
> +
> +Buffers in the cmdq contain both the request and the response. A request
> +has the following layout:
> +
> +\begin{lstlisting}
> +struct virtio_scmi_request {
> + le32 hdr;
> + u8 params[<actual parameters size>];
> +};
> +\end{lstlisting}
> +
> +The virtio_scmi_request fields are interpreted as follows:
> +
> +\begin{description}
> +\item[\field{hdr}] (device-readable) contains the SCMI message header
> +\item[\field{params}] (device-readable) comprises the SCMI message
> +parameters
> +\end{description}
> +
> +A cmdq response has the following layout:
> +
> +\begin{lstlisting}
> +struct virtio_scmi_response {
> + le32 hdr;
> + u8 ret_values[<actual return values size>];
> +};
> +\end{lstlisting}
> +
> +The virtio_scmi_response fields are interpreted as follows:
> +
> +\begin{description}
> +\item[\field{hdr}] (device-writable) contains the SCMI message header
> +\item[\field{ret_values}] (device-writable) comprises the SCMI message
> +return values
> +\end{description}
> +
> +If VIRTIO_SCMI_F_P2A_CHANNELS was not negotiated, the device responds to
> +SCMI commands as if no SCMI notifications or delayed responses were
> +implemented.
> +
> +\devicenormative{\paragraph}{cmdq Operation}{Device Types / SCMI Device / Device Operation / cmdq Operation}
> +
> +The device MAY process available commands out of order and in parallel.
> +
> +The device MUST process all available commands eventually, even in the
> +case of bursts of multiple command messages.
> +
> +If the \field{status} field in the \field{virtio_scmi_response}
> +\field{ret_values} has a value other than SUCCESS, the device MUST set
> +the size of \field{ret_values} to the size of the \field{status} field.
> +
> +If the driver requests an SCMI notification or a delayed response and
> +there are currently NOT enough available buffers in the eventq, the
> +device SHOULD still return SCMI status code SUCCESS.
> +
> +If VIRTIO_SCMI_F_P2A_CHANNELS was not negotiated, the device MUST deny
> +any request for an SCMI notification or a delayed response by returning
> +SCMI status code NOT_SUPPORTED.
> +
> +If VIRTIO_SCMI_F_P2A_CHANNELS was not negotiated, the device MUST NOT
> +indicate in the PROTOCOL_MESSAGE_ATTRIBUTES return values that any SCMI
> +notification, or delayed response, is implemented.
> +
> +\drivernormative{\paragraph}{cmdq Operation}{Device Types / SCMI Device / Device Operation / cmdq Operation}
> +
> +Before sending a command, the driver MUST wait for responses to all
> +commands whose completion the driver considers prerequisites to
> +executing the command.
> +
> +With every command message, the driver MUST provide enough
> +device-writable memory to enable the device to return corresponding
> +return values.
> +
> +If VIRTIO_SCMI_F_P2A_CHANNELS was not negotiated, the driver MUST NOT
> +request any SCMI notification, nor any delayed response.
> +
> +\subsubsection{Setting Up eventq Buffers}
> +
> +The driver has to populate the eventq before the device can use it.
> +
> +\drivernormative{\paragraph}{Setting Up eventq Buffers}{Device Types / SCMI Device / Device Operation / Setting Up eventq Buffers}
> +
> +If VIRTIO_SCMI_F_P2A_CHANNELS was negotiated, the driver SHOULD populate
> +the eventq with buffers.
> +
> +The driver MUST NOT put device-readable descriptors into the eventq.
> +
> +The driver MUST NOT put into the eventq any buffer which is smaller than
> +the largest SCMI P2A (platform to agent) message which the driver will
> +request.
> +
> +\subsubsection{eventq Operation}
> +
> +Each buffer in the eventq holds (once the buffer is marked as used)
> +either a single SCMI notification, or a single SCMI delayed response. An
> +arbitrary number of such SCMI messages can be in transit at the same
> +time. Conceptually, each SCMI message transmitted over the eventq uses
> +its own SCMI P2A (platform to agent) channel. Buffers in the eventq have
> +the following layout:
> +
> +\begin{lstlisting}
> +struct virtio_scmi_event_msg {
> + /* start of device-writable data */
> + le32 hdr;
> + u8 payload[<actual payload size>];
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{hdr}] (device-writable) contains the SCMI message header
> +\item[\field{payload}] (device-writable) comprises the SCMI message
> +payload
> +\end{description}
> +
> +\devicenormative{\paragraph}{eventq Operation}{Device Types / SCMI Device / Device Operation / eventq Operation}
> +
> +If the device intends to send a notification and there are no available
> +buffers in the eventq, the device MAY drop the notification, or send a
> +corresponding notification later, once enough buffers become available.
> +
> +The device MAY send the notification later if the events which cause the
> +notification take place in quick succession.
> +
> +If the device sends the notification later, the device MAY send the
> +notification with updated data, unless the specific SCMI protocol
> +disallows this.
> +
> +If the device intends to send a notification and there are available
> +buffers, but one of the buffers is too small to fit the notification,
> +the device MAY omit the notification.
> +
> +If the device intends to send a delayed response and there are no
> +available buffers in the eventq, the device MUST send the corresponding
> +delayed response once enough buffers become available.
> +
> +If the \field{status} field in a delayed response \field{payload} has a
> +value other than SUCCESS, the device MUST set the size of
> +\field{payload} to the size of the \field{status} field.
> +
> +\subsubsection{Shared Memory Operation}
> +
> +Various SCMI protocols define statistics shared memory regions (for
> +statistics and sensor values).
> +
> +\devicenormative{\paragraph}{Shared Memory Operation}{Device Types / SCMI Device / Device Operation / Shared Memory Operation}
> +
> +If VIRTIO_SCMI_F_SHARED_MEMORY was negotiated, the device MAY implement
> +an SCMI statistics shared memory region using a virtio shared memory
> +region.
> +
> +If the device implements a shared memory region, the device MUST assign
> +the corresponding shmid as per the following table:
> +
> +\begin{tabular}{|l|l|}
> +\hline
> +SCMI statistics shared memory region & Virtio shmid \\
> +\hline \hline
> +Reserved (invalid) & 0 \\
> +\hline
> +Power state statistics shared memory region & 1 \\
> +\hline
> +Performance domain statistics shared memory region & 2 \\
> +\hline
> +Sensor Values Shared Memory & 3 \\
> +\hline
> +Reserved for future use & 4 to 0x7F \\
> +\hline
> +Vendor-specific statistics shared memory regions & 0x80 to 0xFF \\
> +\hline
> +Reserved for future use & 0x100 and greater \\
> +\hline
> +\end{tabular}
>
> base-commit: f5fd3fca7e4006108b3f9ba91c0b76c5eb6c0726
>
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