SPM Backends

This document briefly introduces the backends of Secure Partition Manager (SPM) in TF-M and how to select one for building.

IPC and SFN

The Firmware Framework M (FF-M) [1], [2] provides two different programming models for Secure Partitions.

  • IPC Model

    The Secure Partition processes signals in any order, and can defer responding to a message while continuing to process other signals.

  • SFN Model

    The Secure Partition is made up of a collection of callback functions which implement secure services.

Although the programming model is different, they share the same APIs to interact with the SPM. The behaviours of the APIs share the same implementation with slight differences for the two programming models. This is regarded as the frontend.

The TF-M runtime implementations behind frontend are different. TF-M provides two backends correspondingly.

  • IPC backend

In this backend, the SPM and each Secure Partition have their own execution contexts, which is required to support the IPC model Secure Partitions. This also enables the SPM to provide higher isolation levels. This SPM backend acts like a multiple-process system. It can also adopt SFN model Secure Partitions.

  • SFN backend

The SFN backend provides more efficient executions because it shares a single-thread execution context with all the Secure Partitions. This SPM backend acts like a single library. Therefore, it can only adopt SFN model Secure Partitions. And it does not support higher isolation levels. On the other hand, it consumes less memory compared to the IPC backend.

The following table summaries the relationships between SPM backends, Secure Partition models and isolation levels.

SPM backend

Supported Partition model

Supported Isolation Level

SFN

SFN Partition

1

IPC

IPC and SFN Partition

1, 2 and 3

Implementation Recommendations

If an implementation doesn’t contain any IPC model Secure Partition and only requires isolation level 1, then it is recommended to select the SFN backend to optimize memory consumption and execution performance.

If an implementation contains any IPC model Secure Partition or requires isolation level 2 or 3, then the IPC backend is required.

TF-M Configuration Switches

In the TF-M build system, the CONFIG_TFM_SPM_BACKEND configuration is used to select the backend of SPM. The valid values are SFN and IPC.

-DCONFIG_TFM_SPM_BACKEND=SFN

If CONFIG_TFM_SPM_BACKEND is not set, then IPC is the default value.

References

Copyright (c) 2022, Arm Limited. All rights reserved.