Author: Deepak Kodihalli dkodihal@linux.vnet.ibm.com
Primary assignee: Deepak Kodihalli
Created: 2019-01-22
On OpenBMC, in-band IPMI is currently the primary industry-standard means of communication between the BMC and the Host firmware. We've started hitting some inherent limitations of IPMI on OpenPOWER servers: a limited number of sensors, and a lack of a generic control mechanism (sensors are a generic monitoring mechanism) are the major ones. There is a need to improve upon the communication protocol, but at the same time inventing a custom protocol is undesirable.
This design aims to employ Platform Level Data Model (PLDM), a standard application layer communication protocol defined by the DMTF. PLDM draws inputs from IPMI, but it overcomes most of the latter's limitations. PLDM is also designed to run on standard transport protocols, for e.g. MCTP (also designed by the DMTF). MCTP provides for a common transport layer over several physical channels, by defining hardware bindings. The solution of PLDM over MCTP also helps overcome some of the limitations of the hardware channels that IPMI uses.
PLDM's purpose is to enable all sorts of "inside the box communication": BMC - Host, BMC - BMC, BMC - Network Controller and BMC - Other (for e.g. sensor) devices.
PLDM is designed to be an effective interface and data model that provides efficient access to low-level platform inventory, monitoring, control, event, and data/parameters transfer functions. For example, temperature, voltage, or fan sensors can have a PLDM representation that can be used to monitor and control the platform using a set of PLDM messages. PLDM defines data representations and commands that abstract the platform management hardware.
PLDM groups commands under broader functions, and defines separate specifications for each of these functions (also called PLDM "Types"). The currently defined Types (and corresponding specs) are : PLDM base (with associated IDs and states specs), BIOS, FRU, Platform monitoring and control, Firmware Update and SMBIOS. All these specifications are available at:
https://www.dmtf.org/standards/pmci
Some of the reasons PLDM sounds promising (some of these are advantages over IPMI):
Common in-band communication protocol.
Already existing PLDM Type specifications that cover the most common communication requirements. Up to 64 PLDM Types can be defined (the last one is OEM). At the moment, 6 are defined. Each PLDM type can house up to 256 PLDM commands.
PLDM sensors are 2 bytes in length.
PLDM introduces the concept of effecters - a control mechanism. Both sensors and effecters are associated to entities (similar to IPMI, entities can be physical or logical), where sensors are a mechanism for monitoring and effecters are a mechanism for control. Effecters can be numeric or state based. PLDM defines commonly used entities and their IDs, but there 8K slots available to define OEM entities.
A very active PLDM related working group in the DMTF.
The plan is to run PLDM over MCTP. MCTP is defined in a spec of its own, and a proposal on the MCTP design is in discussion already. There's going to be an intermediate PLDM over MCTP binding layer, which lets us send PLDM messages over MCTP. This is defined in a spec of its own, and the design for this binding will be proposed separately.
How different BMC applications make use of PLDM messages is outside the scope of this requirements doc. The requirements listed here are related to the PLDM protocol stack and the request/response model:
Marshalling and unmarshalling of PLDM messages, defined in various PLDM Type specs, must be implemented. This can of course be staged based on the need of specific Types and functions. Since this is just encoding and decoding PLDM messages, this can be a library that could shared between the BMC, and other firmware stacks. The specifics of each PLDM Type (such as FRU table structures, sensor PDR structures, etc) are implemented by this lib.
Mapping PLDM concepts to native OpenBMC concepts must be implemented. For e.g.: mapping PLDM sensors to phosphor-hwmon hosted D-Bus objects, mapping PLDM FRU data to D-Bus objects hosted by phosphor-inventory-manager, etc. The mapping shouldn't be restrictive to D-Bus alone (meaning it shouldn't be necessary to put objects on the Bus just to serve PLDM requests, a problem that exists with phosphor-host-ipmid today). Essentially these are platform specific PLDM message handlers.
The BMC should be able to act as a PLDM responder as well as a PLDM requester. As a PLDM requester, the BMC can monitor/control other devices. As a PLDM responder, the BMC can react to PLDM messages directed to it via requesters in the platform.
As a PLDM requester, the BMC must be able to discover other PLDM enabled components in the platform.
As a PLDM requester, the BMC must be able to send simultaneous messages to different responders.
As a PLDM requester, the BMC must be able to handle out of order responses.
As a PLDM responder, the BMC may simultaneously respond to messages from different requesters, but the spec doesn't mandate this. In other words the responder could be single-threaded.
It should be possible to plug-in OEM PLDM types/functions into the PLDM stack.
This document covers the architectural, interface, and design details. It provides recommendations for implementations, but implementation details are outside the scope of this document.
The design aims at having a single PLDM daemon serve both the requester and responder functions, and having transport specific endpoints to communicate on different channels.
The design enables concurrency aspects of the requester and responder functions, but the goal is to employ asynchronous IO and event loops, instead of multiple threads, wherever possible.
The following are high level structural elements of the design:
This library would take a PLDM message, decode it and extract the different fields of the message. Conversely, given a PLDM Type, command code, and the command's data fields, it would make a PLDM message. The thought is to design this as a common library, that can be used by the BMC and other firmware stacks, because it's the encode/decode and protocol piece (and not the handling of a message).
These libraries would implement the platform specific handling of incoming PLDM requests (basically helping with the PLDM responder implementation, see next bullet point), so for instance they would query D-Bus objects (or even something like a JSON file) to fetch platform specific information to respond to the PLDM message. They would link with the encode/decode lib.
It should be possible to plug-in a provider library, that lets someone add functionality for new PLDM (standard as well as OEM) Types. The libraries would implement a "register" API to plug-in handlers for specific PLDM messages. Something like:
template <typename Handler, typename... args> auto register(uint8_t type, uint8_t command, Handler handler);
This allows for providing a strongly-typed C++ handler registration scheme. It would also be possible to validate the parameters passed to the handler at compile time.
The PLDM daemon links with the encode/decode and provider libs. The daemon would have to implement the following functions:
The receiver wakes up on getting notified of incoming PLDM messages (via D-Bus signal or callback from the transport layer) from a remote PLDM device. If the message type is "Request" it would route them to a PLDM provider library. Via the library, asynchronous D-Bus calls (using sdbusplus-asio) would be made, so that the receiver can register a handler for the D-Bus response, instead of having to wait for the D-Bus response. This way it can go back to listening for incoming PLDM messages.
In the D-Bus response handler, the receiver will send out the PLDM response message via the transport's send message API. If the transport's send message API blocks for a considerably long duration, then it would have to be run in a thread of it's own.
If the incoming PLDM message is of type "Response", then the receiver emits a D-Bus signal pointing to the response message. Any time the message is too large to fit in a D-Bus payload, the message is written to a file, and a read-only file descriptor pointing to that file is contained in the D-Bus signal.
Designing the BMC as a PLDM requester is interesting. We haven't had this with IPMI, because the BMC was typically an IPMI server. PLDM requester functions will be spread across multiple OpenBMC applications (instead of a single big requester app) - based on the responder they're talking to and the high level function they implement. For example, there could be an app that lets the BMC upgrade firmware for other devices using PLDM - this would be a generic app in the sense that the same set of commands might have to be run irrespective of the device on the other side. There could also be an app that does fan control on a remote device, based on sensors from that device and algorithms specific to that device.
A requester app/flow comprises of the following :
Linkage with a PLDM encode/decode library, to be able to pack PLDM requests and unpack PLDM responses.
A D-Bus API to generate a unique PLDM instance id. The id needs to be unique across all outgoing PLDM messages (from potentially different processes). This needs to be on D-Bus because the id needs to be unique across PLDM requester app processes.
A requester client API that provides blocking and non-blocking functions to transfer a PLDM request message and to receive the corresponding response message, over MCTP (the blocking send() will return a PLDM response). This will be a thin wrapper over the socket API provided by the mctp demux daemon. This will provide APIs for common tasks so that the same may not be re-implemented in each PLDM requester app. This set of API will be built into the encode/decode library (so libpldm would house encode/decode APIs, and based on a compile time flag, the requester APIs as well). A PLDM requester app can choose to not use the client requester APIs, and instead can directly talk to the MCTP demux daemon.
a) With blocking API
+---------------+ +----------------+ +----------------+ +-----------------+
|BMC requester/ | |PLDM requester | |PLDM responder | |PLDM Daemon |
|client app | |lib (part of | | | | |
| | |libpldm) | | | | |
+-------+-------+ +-------+--------+ +--------+-------+ +---------+-------+
| | | |
|App starts | | |
| | | |
+------------------------------->setup connection with | |
|init(non_block=false) |MCTP daemon | |
| | | |
+<-------+return_code+----------+ | |
| | | |
| | | |
| | | |
+------------------------------>+ | |
|encode_pldm_cmd(cmd code, args)| | |
| | | |
+<----+returns pldm_msg+--------+ | |
| | | |
| | | |
|----------------------------------------------------------------------------------------------->|
|DBus.getPLDMInstanceId() | | |
| | | |
|<-------------------------returns PLDM instance id----------------------------------------------|
| | | |
+------------------------------>+ | |
|send_msg(mctp_eids, pldm_msg) +----------------------------->+ |
| |write msg to MCTP socket | |
| +----------------------------->+ |
| |call blocking recv() on socket| |
| | | |
| +<-+returns pldm_response+-----+ |
| | | |
| +----+ | |
| | | verify eids, instance id| |
| +<---+ | |
| | | |
+<--+returns pldm_response+-----+ | |
| | | |
| | | |
| | | |
+------------------------------>+ | |
|decode_pldm_cmd(pldm_resp, | | |
| output args) | | |
| | | |
+------------------------------>+ | |
|close_connection() | | |
+ + + +
b) With non-blocking API
+---------------+ +----------------+ +----------------+ +---------------+
|BMC requester/ | |PLDM requester | |PLDM responder | |PLDM daemon |
|client app | |lib (part of | | | | |
| | |libpldm) | | | | |
+-------+-------+ +-------+--------+ +--------+-------+ +--------+------+
| | | |
|App starts | | |
| | | |
+------------------------------->setup connection with | |
|init(non_block=true |MCTP daemon | |
| int* o_mctp_fd) | | |
| | | |
+<-------+return_code+----------+ | |
| | | |
| | | |
| | | |
+------------------------------>+ | |
|encode_pldm_cmd(cmd code, args)| | |
| | | |
+<----+returns pldm_msg+--------+ | |
| | | |
|-------------------------------------------------------------------------------------------->|
|DBus.getPLDMInstanceId() | | |
| | | |
|<-------------------------returns PLDM instance id-------------------------------------------|
| | | |
| | | |
+------------------------------>+ | |
|send_msg(eids, pldm_msg, +----------------------------->+ |
| non_block=true) |write msg to MCTP socket | |
| +<---+return_code+-------------+ |
+<-+returns rc, doesn't block+--+ | |
| | | |
+------+ | | |
| |Add EPOLLIN on mctp_fd | | |
| |to self.event_loop | | |
+<-----+ | | |
| + | |
+<----------------------+PLDM response msg written to mctp_fd+-+ |
| + | |
+------+EPOLLIN on mctp_fd | | |
| |received | | |
| | | | |
+<-----+ | | |
| | | |
+------------------------------>+ | |
|decode_pldm_cmd(pldm_response) | | |
| | | |
+------------------------------>+ | |
|close_connection() | | |
+ + + +
a) Define D-Bus interfaces to send and receive PLDM messages :
method sendPLDM(uint8 mctp_eid, uint8 msg[]) signal recvPLDM(uint8 mctp_eid, uint8 pldm_instance_id, uint8 msg[])
PLDM requester apps can then invoke the above applications. While this simplifies things for the user, it has two disadvantages :
The PLDM daemon might have to talk to remote PLDM devices via different channels. While a level of abstraction might be provided by MCTP, the PLDM daemon would have to implement a D-Bus interface to target a specific transport channel, so that requester apps on the BMC can send messages over that transport. Also, it should be possible to plug-in platform specific D-Bus objects that implement an interface to target a platform specific transport.
Continue using IPMI, but start making more use of OEM extensions to suit the requirements of new platforms. However, given that the IPMI standard is no longer under active development, we would likely end up with a large amount of platform-specific customisations. This also does not solve the hardware channel issues in a standard manner. On OpenPOWER hardware at least, we've started to hit some of the limitations of IPMI (for example, we have need for >255 sensors).
Development would be required to implement the PLDM protocol, the request/response model, and platform specific handling. Low level design is required to implement the protocol specifics of each of the PLDM Types. Such low level design is not included in this proposal.
Design and development needs to involve the firmware stacks of management controllers and management devices of a platform management subsystem.
Testing can be done without having to depend on the underlying transport layer.
The responder function can be tested by mocking a requester and the transport layer: this would essentially test the protocol handling and platform specific handling. The requester function can be tested by mocking a responder: this would test the instance id handling and the send/receive functions.
APIs from the shared libraries can be tested via fuzzing.