designs: Add MCTP design document
Add initial design document, as posted on the openbmc list, and
incorporating feedback from:
- Emily Shaffer <emilyshaffer@google.com>
- Nilan Naidoo <nilan.naidoo@intel.com>
- Supreeth Venkatesh <supreeth.venkatesh@arm.com>
- Ed Tanous <ed.tanous@intel.com>
Change-Id: I9731b2ca9752bdf826e0e98486cfbc3393336043
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/designs/mctp-standards.svg b/designs/mctp-standards.svg
new file mode 100644
index 0000000..cf5f339
--- /dev/null
+++ b/designs/mctp-standards.svg
@@ -0,0 +1,536 @@
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diff --git a/designs/mctp.md b/designs/mctp.md
new file mode 100644
index 0000000..6985cef
--- /dev/null
+++ b/designs/mctp.md
@@ -0,0 +1,216 @@
+# OpenBMC platform communication channel: MCTP & PLDM
+
+Author: Jeremy Kerr <jk@ozlabs.org> <jk>
+
+## Problem Description
+
+Currently, we have a few different methods of communication between host
+and BMC. This is primarily IPMI-based, but also includes a few
+hardware-specific side-channels, like hiomap. On OpenPOWER hardware at
+least, we've definitely started to hit some of the limitations of IPMI
+(for example, we have need for >255 sensors), as well as the hardware
+channels that IPMI typically uses.
+
+This design aims to use the Management Component Transport Protocol
+(MCTP) to provide a common transport layer over the multiple channels
+that OpenBMC platforms provide. Then, on top of MCTP, we have the
+opportunity to move to newer host/BMC messaging protocols to overcome
+some of the limitations we've encountered with IPMI.
+
+## Background and References
+
+Separating the "transport" and "messaging protocol" parts of the current
+stack allows us to design these parts separately. Currently, IPMI
+defines both of these; we currently have BT and KCS (both defined as
+part of the IPMI 2.0 standard) as the transports, and IPMI itself as the
+messaging protocol.
+
+Some efforts of improving the hardware transport mechanism of IPMI have
+been attempted, but not in a cross-implementation manner so far. This
+does not address some of the limitations of the IPMI data model.
+
+MCTP defines a standard transport protocol, plus a number of separate
+physical layer bindings for the actual transport of MCTP packets. These
+are defined by the DMTF's Platform Management Working group; standards
+are available at:
+
+ https://www.dmtf.org/standards/pmci
+
+The following diagram shows how these standards map to the areas of
+functionality that we may want to implement for OpenBMC. The DSP numbers
+provided are references to DMTF standard documents.
+
+
+
+One of the key concepts here is that separation of transport protocol
+from the physical layer bindings; this means that an MCTP "stack" may be
+using either a I2C, PCI, Serial or custom hardware channel, without the
+higher layers of that stack needing to be aware of the hardware
+implementation. These higher levels only need to be aware that they are
+communicating with a certain entity, defined by an Entity ID (MCTP EID).
+These entities may be any element of the platform that communicates
+over MCTP - for example, the host device, the BMC, or any other
+system peripheral - static or hot-pluggable.
+
+This document is focussed on the "transport" part of the platform design.
+While this does enable new messaging protocols (mainly PLDM), those
+components are not covered in detail much; we will propose those parts
+in separate design efforts. For example, the PLDM design at
+[pldm-stack.md].
+
+As part of the design, the references to MCTP "messages" and "packets"
+are intentional, to match the definitions in the MCTP standard. MCTP
+messages are the higher-level data transferred between MCTP endpoints,
+which packets are typically smaller, and are what is sent over the
+hardware. Messages that are larger than the hardware Maximum Transmit
+Unit (MTU) are split into individual packets by the transmit
+implementation, and reassembled at the receive implementation.
+
+## Requirements
+
+Any channel between host and BMC should:
+
+ - Have a simple serialisation and deserialisation format, to enable
+ implementations in host firmware, which have widely varying runtime
+ capabilities
+
+ - Allow different hardware channels, as we have a wide variety of
+ target platforms for OpenBMC
+
+ - Be usable over simple hardware implementations, but have a facility
+ for higher bandwidth messaging on platforms that require it.
+
+ - Ideally, integrate with newer messaging protocols
+
+## Proposed Design
+
+The MCTP core specification just provides the packetisation, routing and
+addressing mechanisms. The actual transmit/receive of those packets is
+up to the hardware binding of the MCTP transport.
+
+For OpenBMC, we would introduce a "MCTP+applications" daemon, which
+implements the transport over a configurable hardware channel (eg.,
+Serial UART, I2C or PCI), and provides handlers for any incoming MCTP
+application requests. This daemon is responsible for the packetisation
+and routing of MCTP messages from external endpoints, and handling the
+application layer requests.
+
+This daemon has a few components:
+
+ 1) the core MCTP stack
+
+ 2) one or more binding implementations (eg, MCTP-over-serial), which
+ interact with the hardware channel(s).
+
+ 3) one or more MCTP message handlers (eg PLDM or NVME-MI), to handle incoming
+ MCTP messages of specific types
+
+ 4) the core application, consisting of main loop, handler management and
+ MCTP binding management
+
+The proposed implementation here is to produce an MCTP "library" which
+provides the packetisation and routing functions, between:
+
+ - an "upper" messaging transmit/receive interface, for tx/rx of a full
+ message to a specific endpoint (ie, (1) above)
+
+ - a "lower" hardware binding for transmit/receive of individual
+ packets, providing a method for the core to tx/rx each packet to
+ hardware, and defines the parameters of the common packetisation
+ code (ie. (2) above).
+
+The lower interface would be plugged in to one of a number of
+hardware-specific binding implementations. Most of these would be
+included in the library source tree, but others can be plugged-in too,
+perhaps where the physical layer implementation does not make sense to
+include in the platform-agnostic library.
+
+The reason for a library is to allow the same MCTP implementation to be
+used in both OpenBMC and host firmware; the library should be
+bidirectional. To allow this, the library would be written in portable C
+(structured in a way that can be compiled as "extern C" in C++
+codebases), and be able to be configured to suit those runtime
+environments (for example, POSIX IO may not be available on all
+platforms; we should be able to compile the library to suit). The
+licence for the library should also allow this re-use; a dual Apache &
+GPLv2+ licence may be best.
+
+These "lower" binding implementations may have very different methods of
+transferring packets to the physical layer. For example, a serial
+binding implementation for running on a Linux environment may be
+implemented through read()/write() syscalls to a PTY device. An I2C
+binding for use in low-level host firmware environments may interact
+directly with hardware registers to perform packet transfers.
+
+The application-specific handlers (listed as (3) above) implement the
+actual functionality provided over the MCTP channel. Each of these would
+register with the MCTP core library to receive MCTP messages of a
+certain type, and would transmit MCTP messages of that same type. While
+the handlers themselves are out of scope for this design, there are a
+few elements that are important here:
+
+ - Handlers are likely to perform IO to other components of the BMC
+ (such as sending and receiving dbus messages). To allow multiple
+ handlers to co-exist, this IO should be implemented using
+ non-blocking interfaces (eg, using poll()).
+
+ - Handlers should be implemented as separate components from the main
+ daemon, so as not to require completely separate functionality (such
+ as PLDM and NVME-MI) existing in the same codebase. Having the core
+ daemon load handlers as shared objects would allow this.
+
+MCTP is intended to be an optional component of OpenBMC. Platforms using
+OpenBMC are free to adopt it as they see fit.
+
+## Alternatives Considered
+
+There have been two main alternatives to this approach:
+
+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.
+
+Redfish between host and BMC. This would mean that host firmware needs a
+HTTP client, a TCP/IP stack, a JSON (de)serialiser, and support for
+Redfish schema. While this may be present in some environments (for
+example, UEFI-based firmware), this is may not be feasible for all host
+firmware implementations (for example, OpenPOWER). It's possible that we
+could run a simplified Redfish stack - indeed, MCTP has a proposal for a
+Redfish-over-MCTP channel (DSP0218), which uses simplified serialisation
+format and no requirement on HTTP. However, this may involve a large
+amount of complexity in host firmware.
+
+In terms of an MCTP daemon implementation, an alternative is to have the
+core MCTP stack exist in a different process from the application
+handlers. For example, the MCTP core could be only responsible for
+proxying MCTP messages to and from a dbus interface, as is currently
+done for IPMI messages. However, the complexity, messaging overheads and
+state management involved here has indicated that the added separation
+has not been a clear advantage.
+
+## Impacts
+
+Development would be required to implement the MCTP transport, plus any
+new users of the MCTP messaging (eg, a PLDM implementation). These would
+somewhat duplicate the work we have in IPMI handlers.
+
+We'd want to keep IPMI running in parallel, so the "upgrade" path should
+be fairly straightforward.
+
+Design and development needs to involve potential host, management
+controllers and managed device implementations.
+
+## Testing
+
+For the core MCTP library, we are able to run tests there in complete
+isolation (I have already been able to run a prototype MCTP stack
+through the afl fuzzer) to ensure that the core transport protocol
+works.
+
+For MCTP hardware bindings, we would develop channel-specific tests that
+would be run in CI on both host and BMC.
+
+For the OpenBMC MCTP daemon implementation, testing models would depend
+on the structure we adopt in the design section.
