PLDM design proposal

[Ben Wei]

Hi Deepak,

Thanks for providing the detailed design and the background info. 
I just have some questions and comments below, 

> Hi All,
>
> I've put down some thoughts below on an initial PLDM design on OpenBMC. 
> The structure of the document is based on the OpenBMC design template. 
> Please review and let me know your feedback. Once we've had a discussion 
> here on the list, I can move this to Gerrit with some more details. I'd 
> say reading the MCTP proposal from Jeremy should be a precursor to 
> reading this.
>
> # PLDM Stack on OpenBMC
>
> Author: Deepak Kodihalli <dkodihal at linux.vnet.ibm.com> <dkodihal>
>
> ## Problem Description
>
> 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. This design doesn't preclude enablement of 
> communication channels not involving the BMC and the host.
>
> ## Background and References
>
> 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.
>
> As stated earlier, PLDM is designed for different flavors of "inside the 
> box" communication. 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://urldefense.proofpoint.com/v2/url?u=https-3A__www.dmtf.org_standards_pmci&d=DwICAg&c=5VD0RTtNlTh3ycd41b3MUw&r=U35IaQ-7Tnwjs7q_Fwf_bQ&m=hfNgxW6BBJnW0LRRa3Hh2xnPH29lrZDGcvqooGTWVjc&s=qOY-dlAn__E9D7LWH6L16US1Sq8lsCZQX1zJv36BLN0&e=
>
> 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 cab 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.
> 
> - PLDM allows bidirectional communication, and sending asynchronous events.
>
> - 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.
>
> ## Requirements
>
> How different BMC/Host/other 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, I believe there would be motivation to build 
> this into a library that could be shared between BMC, host 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 so 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 responder, 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, for e.g, the Host.
>
> - 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, but at the same time it can issue a 
> single message to a specific responder at a time.
>
> - 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 non-existent PLDM functions (these 
> maybe new/existing standard Types, or OEM Types) into the PLDM stack.
>
> ## Proposed Design
>
> The following are high level structural elements of the design:
>
> ### PLDM encode/decode libraries
>
> This library would take a PLDM message, decode it and spit out 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 library such that it can be used by BMC and 
> the host firmware stacks, because it's the encode/decode and protocol 
> piece (and not the handling of a message). I'd like to know if there's 
> enough motivation to have this as a common lib. That would mean 
> additional requirements such as having this as a C lib instead of C++, 
> because of the runtime constraints of host firmware stacks. If there's 
> not enough interest to have this as a common lib, this could just be 
> part of the provider libs (see below), and it could then be written in C++.


Can you elaborate a bit on the pros and cons of having PLDM library as a common C lib vs them
being provider libs only?


>
> There would be one encode/decode lib per PLDM Type. So for e.g. 
> something like /usr/lib/pldm/libbase.so, /usr/lib/pldm/libfru.so, etc.
>
> ### PLDM provider libraries
>
> 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 libs. Like the encode/decode libs, there would be 
> one per PLDM Type (for e.g /usr/lib/pldm/providers/libfru.so).
>
> These libraries would essentially be plug-ins. That lets someone add 
> functionality for new PLDM (standard as well as OEM) Types, and it also 
> lets them replace default handlers. 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.
>
> ### Request/Response Model
>
> There are two approaches that I've described here, and they correlate to 
> the two options in Jeremy's MCTP design for how to notify on incoming 
> PLDM messages: in-process callbacks vs D-Bus signals.
>
> #### With in-process callbacks
>
> In this case, there would be a single PLDM (over MCTP) daemon that 
> implements both the PLDM responder and PLDM requester function. The 
> daemon would link with the encode/decode libs mentioned above, and the 
> MCTP lib.

In the case if we want to run PLDM over NCSI, do you envision having a separate 
NCSI daemon that also link with PLDM decode/encode lib? So in this case there'd
be multiple stream of (separate) PLDM traffic.

>
> The PLDM responder function would involve registering the PLDM provider 
> libs on startup. The PLDM responder implementation would sit in the 
> callback handler from the transport's rx. If it receives PLDM messages 
> of type Request, it will route them to an appropriate handler in a 
> provider lib, get the response back, and send back a PLDM response 
> message via the transport's tx API. If it receives messages of type 
> Response, it will put them on a "Response queue".

Do you see any needs for handler in the provider lib to communicate with
other daemons? For example, PLDM sensor handler may have to query a separate 
sensor daemon (sensord) to get the sensor data before it can respond. 

If the handler needs to communicate with other daemons/applications in the system,
I think this part of the design would be very similar to the "BMC as PLDM requester" design
you've specified below.

e.g. 
The response from sensord may not return right away, and the PLDM handler shouldn't 
block, in this case I think the handler for each PLDM type  would also need a "Request Queue" 
so it may queue up  incoming requests while it processes each request.

Also if each PLDM Type handler needs to communicate with multiple daemons, I'm thinking having
a msg_in  queue (in addition to the Request queue above) so it may receive responses back
from other daemons in the system, and store PLDM IID in meta-data when
communicating with other daemons so the PLDM handler can map each messages in 
msg_in queue to a PLDM request in Request Queue.

In this case each PLDM handler would need multiple threads to handle these separate tasks.

>
> I think designing the BMC as a PLDM requester is interesting. We haven't 
> had this with IPMI, because the BMC was typically an IPMI server. I 
> envision PLDM requester functions to be spread across multiple OpenBMC 
> applications (instead of a single big requester app) - based on the 
> responder they're talking 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.
>
> The PLDM daemon would have to provide a D-Bus interface to send a PLDM 
> request message. This API would be used by apps wanting to send out PLDM 
> requests. If the message payload is too large, the interface could 
> accept an fd (containing the message), instead of an array of bytes. The 
> implementation of this would send the PLDM request message via the 
> transport's tx API, and then conditionally wait on the response queue to 
> have an entry that matches this request (the match is by instance id). 
> The conditional wait (or something equivalent) is required because the 
> app sending the PLDM message must block until getting a response back 
> from the remote PLDM device.
>
> With what's been described above, it's obvious that the responder and 
> requester functions need to be able to run concurrently (this is as per 
> the PLDM spec as well). The BMC can simultaneously act as a responder 
> and requester. Waiting on a rx from the transport layer shouldn't block 
> other BMC apps from sending PLDM messages. So this means the PLDM daemon 
> would have to be multi-threaded, or maybe we can instead achieve this 
> via an event loop.

Do you see both Requester and Responder spawning multiple threads?

I can see them performing similar functionalities, 

e.g. perhaps something like this below:

PLDM Requester 
- listens for other applications/daemons for PLDM requests, and generate and send PLDM 
   Requests to device (1 thread)
- waits for device response,  look up original request sender via response IID and send 
   response back to applications/daemons (1 thread)

PLDM Responder
  - listens for PLDM requests from device, decode request and add it to corresponding handler's Request queue (1 thread)
- each handler:  
	- checks its Request Queue, process request inline (if able to) and adds response to response queue, 
                   If request needs data from other application, send message to other application (1 thread)
            - processes incoming messages from other applications and put them to Response queue (1 thread)
            - process Response queue - send response back to device (1 thread)  

> #### With D-Bus signals
>
> This lets us separate PLDM daemons from the MCTP daemon, and eliminates 
> the need to handle request and response messages concurrently in the 
> same daemon, at the cost of much more D-Bus traffic. The MCTP daemon 
> would emit D-Bus signals describing the type of the PLDM message 
> (request/response) and containing the message payload. Alternatively it 
> could pass the PLDM message over a D-Bus API that the PLDM daemons would 
> implement. The MCTP daemon would also implement a D-Bus API to send PLDM 
> messages, as with the previous approach.
>
> With this approach, I'd recommend two separate PLDM daemons - a 
> responder daemon and a requester daemon. The responder daemon reacts to 
> D-Bus signals corresponding to PLDM Request messages. It handles 
> incoming requests as before. The requester daemon would react to D-Bus 
> signals corresponding to PLDM response messages. It would implement the 
> instance id generation, and would also implement the response queue and 
> the conditional wait on that queue. It would also have to implement a 
> D-Bus API to let other PLDM-enabled OpenBMC apps send PLDM requests. The 
> implementation of that API would send the message to the MCTP daemon, 
> and then block on the response queue to get a response back.

Similar to previous "in-process callback" approach, the  Responder daemon may
have to send D-Bus signals to other applications in order process a PLDM request?

Is there a way for any daemons in the system to register a communication channel
With PLDM handler?

> ### Multiple requesters and responders
>
> The PLDM spec does allow simultaneous connections between multiple 
> responders/requesters. For e.g. the BMC talking to a multi-host system 
> on two different physical channels. Instead of implementing this in one 
> MCTP/PLDM daemon, we could spawn one daemon per physical channel.

OK I see, so in this case a daemon monitor MCTP channel would have its own PLDM
handler, and a daemon monitoring NCSI channel would spawn its PLDM handler,
both streams of PLDM traffic occurs independently of each other and have its own 
series of instance IDs.

> ## Impacts
>
> 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 potential host firmware
> implementations.
>
> ## Testing
>
> 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.

Thanks!
-Ben