diff mbox series

Add virtio SCMI device specification

Message ID 20200220193715.12097-1-peter.hilber@opensynergy.com (mailing list archive)
State New, archived
Headers show
Series Add virtio SCMI device specification | expand

Commit Message

Peter Hilber Feb. 20, 2020, 7:37 p.m. UTC 
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*ontroller *M*anagement *I*nterface protocol allows speaking
to embedded 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
embedded systems.

OpenSynergy has a prototype implementation, and plans to upstream the
Linux kernel driver.

The PDF output (with ugly fonts, apologies) is available at [2].

[1] https://developer.arm.com/docs/den0056/b
[2] https://share.mailbox.org/ajax/share/0d959c190d5a1c47d18eb2fd5a1c40ad81e8d7897ab9ca1e/1/8/Mjk/MjkvOA

Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
---
 conformance.tex  |  27 ++++-
 content.tex      |   1 +
 introduction.tex |   3 +
 virtio-scmi.tex  | 269 +++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 297 insertions(+), 3 deletions(-)
 create mode 100644 virtio-scmi.tex

Comments

Peter Hilber Feb. 21, 2020, 10:53 a.m. UTC | #1 
On 20.02.20 20:37, 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*ontroller *M*anagement *I*nterface protocol allows speaking
> to embedded 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
> embedded systems.
> 
> OpenSynergy has a prototype implementation, and plans to upstream the
> Linux kernel driver.
> 
> The PDF output (with ugly fonts, apologies) is available at [2].
> 
> [1] https://developer.arm.com/docs/den0056/b
> [2] https://share.mailbox.org/ajax/share/0d959c190d5a1c47d18eb2fd5a1c40ad81e8d7897ab9ca1e/1/8/Mjk/MjkvOA

For completeness: Our prototype implementation does not yet
provide the VIRTIO_SCMI_F_P2A_CHANNELS and VIRTIO_SCMI_F_SHARED_MEMORY
features.

Any feedback would be greatly appreciated.
Souvik Chakravarty Feb. 21, 2020, 11:22 a.m. UTC | #2 
Hi Peter,

The overall proposal is mostly in sync with the SCMI specification. A few comments below.

> -----Original Message-----
> From: Peter Hilber <peter.hilber@opensynergy.com>
> Sent: Thursday, February 20, 2020 7:37 PM
>
> 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*ontroller *M*anagement *I*nterface protocol allows speaking to

Its "System Control and Management Interface" (some recurrences are there below which I haven't pointed out).

> embedded system controllers that allow orchestrating things like power

If we are using Virtio, the system controller is probably no longer "embedded".

> 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 embedded
> systems.

Again, what stops this from being deployed beyond embedded?
There is scope for hypervisors which might implement the full SCMI device for non-embedded usages as well.

>
> OpenSynergy has a prototype implementation, and plans to upstream the Linux
> kernel driver.
>
> The PDF output (with ugly fonts, apologies) is available at [2].
>
> [1] https://developer.arm.com/docs/den0056/b
> [2]
> https://share.mailbox.org/ajax/share/0d959c190d5a1c47d18eb2fd5a1c40ad81
> e8d7897ab9ca1e/1/8/Mjk/MjkvOA
>
> Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
> ---
>  conformance.tex  |  27 ++++-
>  content.tex      |   1 +
>  introduction.tex |   3 +
>  virtio-scmi.tex  | 269
> +++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 297 insertions(+), 3 deletions(-)  create mode 100644 virtio-
> scmi.tex
>

<SNIP>

> +
> +\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. Conceptually, each SCMI message transmitted
> +over the cmdq uses its own short-lived SCMI A2P (agent to platform)
> +channel.

Any special significance of the "short-lived" phrase. Does it have any implications on how it will interact with the SCMI driver?

> +
> +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 len;
> +        le32 hdr;
> +        u8 params[<actual parameters size>]; }; \end{lstlisting}
> +
> +The virtio_scmi_request fields are interpreted as follows:
> +
> +\begin{description}
> +\item[\field{len}] (device-readable) size of \field{hdr} and actual
> +\field{params} in bytes \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 len;
> +        le32 hdr;
> +        u8 ret_values[<actual return values size>]; }; \end{lstlisting}
> +
> +The virtio_scmi_response fields are interpreted as follows:
> +

<SNIP>

> +\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.
> +Conceptually, each SCMI message transmitted over the eventq uses its
> +own short-lived SCMI P2A (platform to agent) channel. Buffers in the
> +eventq have the following layout:

Same question. Any special significance of the "short-lived" phrase?

> +
> +\begin{lstlisting}
> +struct virtio_scmi_event_msg {
> +        /* start of device-writable data */
> +        le32 len;
> +        le32 hdr;
> +        u8 payload[<actual payload size>]; }; \end{lstlisting}
> +
> +\begin{description}
> +\item[\field{len}] (device-writable) size of \field{hdr} and actual
> +\field{payload} in bytes \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 SHOULD send a corresponding
> +notification later, once enough buffers become available.

Any reason why this is mandated? It should be possible for the device to drop the notification if there is no buffer available since this provides an implicit flow control as well, since the guest in this case is clearly unable to consume the notifications at a sufficient rate.
Can we make this Recommended instead?

Regards,
Souvik

> +
> +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.
> +
> +\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
> +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}
> --
> 2.20.1

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Peter Hilber Feb. 21, 2020, 7:45 p.m. UTC | #3 
On 21.02.20 12:22, Souvik Chakravarty wrote:
> Hi Peter,
> 
> The overall proposal is mostly in sync with the SCMI specification. A few comments below.

Hi Souvik,

thanks for the review.

> 
>> -----Original Message-----
>> From: Peter Hilber <peter.hilber@opensynergy.com>
>> Sent: Thursday, February 20, 2020 7:37 PM
>>
>> 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*ontroller *M*anagement *I*nterface protocol allows speaking to
> 
> Its "System Control and Management Interface" (some recurrences are there below which I haven't pointed out).
> 
>> embedded system controllers that allow orchestrating things like power
> 
> If we are using Virtio, the system controller is probably no longer "embedded".
> 
>> 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 embedded
>> systems.
> 
> Again, what stops this from being deployed beyond embedded?
> There is scope for hypervisors which might implement the full SCMI device for non-embedded usages as well.

I will update the commit message and device intro text for the next
patch version according to the above comments.

> 
>>
>> OpenSynergy has a prototype implementation, and plans to upstream the Linux
>> kernel driver.
>>
>> The PDF output (with ugly fonts, apologies) is available at [2].
>>
>> [1] https://developer.arm.com/docs/den0056/b
>> [2]
>> https://share.mailbox.org/ajax/share/0d959c190d5a1c47d18eb2fd5a1c40ad81
>> e8d7897ab9ca1e/1/8/Mjk/MjkvOA
>>
>> Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
>> ---
>>  conformance.tex  |  27 ++++-
>>  content.tex      |   1 +
>>  introduction.tex |   3 +
>>  virtio-scmi.tex  | 269
>> +++++++++++++++++++++++++++++++++++++++++++++++
>>  4 files changed, 297 insertions(+), 3 deletions(-)  create mode 100644 virtio-
>> scmi.tex
>>
> 
> <SNIP>
> 
>> +
>> +\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. Conceptually, each SCMI message transmitted
>> +over the cmdq uses its own short-lived SCMI A2P (agent to platform)
>> +channel.
> 
> Any special significance of the "short-lived" phrase. Does it have any implications on how it will interact with the SCMI driver?

"Short-lived" should just illustrate that the conceptual channel is used
for the lifetime of a single message.

The motivation to introduce this concept: It should be clear that both
device and driver may send multiple messages at once (subject to the
other requirements). But the SCMI spec might be read to imply e.g. that
the platform must wait until the P2A "channel is now free and can be
used to send a further message" (DEN0056B p. 12). For the virtio
transport, since each message uses its own (conceptual) channel, it's
clear that the message's channel can never be considered occupied by
another message.

I think the short-lived channels don't match the meaning of channels in
the Linux kernel SCMI driver. From that driver POV, all messages in the
A2P resp. P2A direction would still go over the same channel (i.e. one
struct scmi_chan_info for A2P, one for P2A).

I'd try to reword the paragraph so the intent becomes more clear.

> 
>> +
>> +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 len;
>> +        le32 hdr;
>> +        u8 params[<actual parameters size>]; }; \end{lstlisting}
>> +
>> +The virtio_scmi_request fields are interpreted as follows:
>> +
>> +\begin{description}
>> +\item[\field{len}] (device-readable) size of \field{hdr} and actual
>> +\field{params} in bytes \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 len;
>> +        le32 hdr;
>> +        u8 ret_values[<actual return values size>]; }; \end{lstlisting}
>> +
>> +The virtio_scmi_response fields are interpreted as follows:
>> +
> 
> <SNIP>
> 
>> +\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.
>> +Conceptually, each SCMI message transmitted over the eventq uses its
>> +own short-lived SCMI P2A (platform to agent) channel. Buffers in the
>> +eventq have the following layout:
> 
> Same question. Any special significance of the "short-lived" phrase?

Please see answer above.

> 
>> +
>> +\begin{lstlisting}
>> +struct virtio_scmi_event_msg {
>> +        /* start of device-writable data */
>> +        le32 len;
>> +        le32 hdr;
>> +        u8 payload[<actual payload size>]; }; \end{lstlisting}
>> +
>> +\begin{description}
>> +\item[\field{len}] (device-writable) size of \field{hdr} and actual
>> +\field{payload} in bytes \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 SHOULD send a corresponding
>> +notification later, once enough buffers become available.
> 
> Any reason why this is mandated? It should be possible for the device to drop the notification if there is no buffer available since this provides an implicit flow control as well, since the guest in this case is clearly unable to consume the notifications at a sufficient rate.
> Can we make this Recommended instead?

I was concerned that dropping non-periodic notifications during a
temporary overload could be problematic. If virtio driver
authors/integrators had the same concern, this could result in
unnecessarily large virtqueues to cover infrequent scenarios. By adding
this requirement, I intended to shift the responsibility for the
overload scenario to the virtio device authors.

If this is considered over-engineering, I will demote it to a
recommendation.

Best regards,

Peter
diff mbox series

Patch

diff --git a/conformance.tex b/conformance.tex
index b6fdec0..99f037a 100644
--- a/conformance.tex
+++ b/conformance.tex
@@ -15,7 +15,7 @@  \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
   \begin{itemize}
     \item Clause \ref{sec:Conformance / Driver Conformance}.
     \item One of clauses \ref{sec:Conformance / Driver Conformance / PCI Driver Conformance}, \ref{sec:Conformance / Driver Conformance / MMIO Driver Conformance} or \ref{sec:Conformance / Driver Conformance / Channel I/O Driver Conformance}.
-    \item One of clauses \ref{sec:Conformance / Driver Conformance / Network Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Block Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Console Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Entropy Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Traditional Memory Balloon Driver Conformance}, \ref{sec:Conformance / Driver Conformance / SCSI Host Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Input Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Crypto Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Socket Driver Conformance} or \ref{sec:Conformance / Driver Conformance / IOMMU Driver Conformance}.
+    \item One of clauses \ref{sec:Conformance / Driver Conformance / Network Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Block Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Console Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Entropy Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Traditional Memory Balloon Driver Conformance}, \ref{sec:Conformance / Driver Conformance / SCSI Host Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Input Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Crypto Driver Conformance}, \ref{sec:Conformance / Driver Conformance / Socket Driver Conformance}, \ref{sec:Conformance / Driver Conformance / IOMMU 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}
 \item[Device] A device MUST conform to four conformance clauses:
@@ -32,8 +32,9 @@  \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
 \ref{sec:Conformance / Device Conformance / Input Device Conformance}, 
 \ref{sec:Conformance / Device Conformance / Crypto Device Conformance}, 
 \ref{sec:Conformance / Device Conformance / Socket Device Conformance}, 
-\ref{sec:Conformance / Device Conformance / RPMB Device Conformance} or 
-\ref{sec:Conformance / Device Conformance / IOMMU Device Conformance}.
+\ref{sec:Conformance / Device Conformance / RPMB Device Conformance},
+\ref{sec:Conformance / Device Conformance / IOMMU 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}
 \end{description}
@@ -220,6 +221,15 @@  \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
 \item \ref{drivernormative:Device Types / IOMMU Device / Device operations / Fault reporting}
 \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:
@@ -408,6 +418,17 @@  \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
 \item \ref{devicenormative:Device Types / IOMMU Device / Device operations / Fault reporting}
 \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 b91a132..6c97f04 100644
--- a/content.tex
+++ b/content.tex
@@ -6062,6 +6062,7 @@  \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
 \input{virtio-fs.tex}
 \input{virtio-rpmb.tex}
 \input{virtio-iommu.tex}
+\input{virtio-scmi.tex}
 
 \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
 
diff --git a/introduction.tex b/introduction.tex
index 33da3ec..3a2ee80 100644
--- a/introduction.tex
+++ b/introduction.tex
@@ -66,6 +66,9 @@  \section{Normative References}\label{sec:Normative References}
         \phantomsection\label{intro:eMMC}\textbf{[eMMC]} &
         eMMC Electrical Standard (5.1), JESD84-B51,
         \newline\url{http://www.jedec.org/sites/default/files/docs/JESD84-B51.pdf}\\
+	\phantomsection\label{intro:SCMI}\textbf{[SCMI]} &
+	Arm System Control and Management Interface, DEN0056,
+	\newline\url{https://developer.arm.com/docs/den0056/b}, version B and any future revisions\\
 
 \end{longtable}
 
diff --git a/virtio-scmi.tex b/virtio-scmi.tex
new file mode 100644
index 0000000..576a262
--- /dev/null
+++ b/virtio-scmi.tex
@@ -0,0 +1,269 @@ 
+\section{SCMI Device}\label{sec:Device Types / SCMI Device}
+
+An SCMI device implements the Arm System Controller 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}
+
+XXX
+
+\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. Conceptually, each SCMI message transmitted
+over the cmdq uses its own short-lived 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 len;
+        le32 hdr;
+        u8 params[<actual parameters size>];
+};
+\end{lstlisting}
+
+The virtio_scmi_request fields are interpreted as follows:
+
+\begin{description}
+\item[\field{len}] (device-readable) size of \field{hdr} and actual
+\field{params} in bytes
+\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 len;
+        le32 hdr;
+        u8 ret_values[<actual return values size>];
+};
+\end{lstlisting}
+
+The virtio_scmi_response fields are interpreted as follows:
+
+\begin{description}
+\item[\field{len}] (device-writable) size of \field{hdr} and actual
+\field{ret_values} in bytes
+\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 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.
+
+If the driver populates the eventq, the driver MUST supply only buffers
+which can hold the largest SCMI message (notification or delayed
+response) which will be requested by the driver.
+
+\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.
+Conceptually, each SCMI message transmitted over the eventq uses its own
+short-lived 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 len;
+        le32 hdr;
+        u8 payload[<actual payload size>];
+};
+\end{lstlisting}
+
+\begin{description}
+\item[\field{len}] (device-writable) size of \field{hdr} and actual
+\field{payload} in bytes
+\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 SHOULD 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.
+
+\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
+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}