4.19. Realm Management Extension (RME)
FEAT_RME (or RME for short) is an Armv9-A extension and is one component of the Arm Confidential Compute Architecture (Arm CCA). TF-A supports RME starting from version 2.6. This chapter discusses the changes to TF-A to support RME and provides instructions on how to build and run TF-A with RME.
4.19.1. RME support in TF-A
The following diagram shows an Arm CCA software architecture with TF-A as the
EL3 firmware. In the Arm CCA architecture there are two additional security
states and address spaces: Root and Realm. TF-A firmware runs in the
Root world. In the realm world, a Realm Management Monitor firmware (RMM)
manages the execution of Realm VMs and their interaction with the hypervisor.
RME is the hardware extension to support Arm CCA. To support RME, various changes have been introduced to TF-A. We discuss those changes below.
4.19.1.1. Changes to translation tables library
RME adds Root and Realm Physical address spaces. To support this, two new
memory type macros, MT_ROOT and MT_REALM, have been added to the
Translation (XLAT) Tables Library. These macros are used to configure
memory regions as Root or Realm respectively.
Note
Only version 2 of the translation tables library supports the new memory types.
4.19.1.2. Changes to context management
A new CPU context for the Realm world has been added. The existing can be used to manage Realm context.
4.19.1.3. Boot flow changes
In a typical TF-A boot flow, BL2 runs at Secure-EL1. However when RME is enabled, TF-A runs in the Root world at EL3. Therefore, the boot flow is modified to run BL2 at EL3 when RME is enabled. In addition to this, a Realm-world firmware (RMM) is loaded by BL2 in the Realm physical address space.
The boot flow when RME is enabled looks like the following:
BL1 loads and executes BL2 at EL3
BL2 loads images including RMM
BL2 transfers control to BL31
BL31 initializes SPM (if SPM is enabled)
BL31 initializes RMM
BL31 transfers control to Normal-world software
4.19.1.4. Granule Protection Tables (GPT) library
Isolation between the four physical address spaces is enforced by a process called Granule Protection Check (GPC) performed by the MMU downstream any address translation. GPC makes use of Granule Protection Table (GPT) in the Root world that describes the physical address space assignment of every page (granule). A GPT library that provides APIs to initialize GPTs and to transition granules between different physical address spaces has been added. More information about the GPT library can be found in the Granule Protection Tables Library chapter.
4.19.1.5. RMM Dispatcher (RMMD)
RMMD is a new standard runtime service that handles the switch to the Realm world. It initializes the RMM and handles Realm Management Interface (RMI) SMC calls from Non-secure.
There is a contract between RMM and RMMD that defines the arguments that the former needs to take in order to initialize and also the possible return values. This contract is defined in the RMM Boot Interface, which can be found at RMM Boot Interface.
There is also a specification of the runtime services provided by TF-A to RMM. This can be found at RMM-EL3 Runtime Interface.
4.19.1.6. Test Realm Payload (TRP)
TRP is a small test payload that runs at R-EL2 and implements a subset of the Realm Management Interface (RMI) commands to primarily test EL3 firmware and the interface between R-EL2 and EL3. When building TF-A with RME enabled, if the path to an RMM image is not provided, TF-A builds the TRP by default and uses it as the R-EL2 payload.
4.19.2. Building and running TF-A with RME
The recommended procedure for building and running an RME enabled reference platform is by the use of Shrinkwrap.
The tool offers examples of Three-worlds and Four-worlds configurations for running on the FVP_Base_RevC-2xAEMvA model.
Three-world execution: this is the configuration to use if Secure world functionality is not needed.
Four-world execution: this is the configuration to use if both Secure and Realm world functionality is needed.
TF-A is tested with the following software entities in each world as listed below:
Additionally, the TF-RMM project documentation has the specific Shrinkwrap guide.
TF-A CI pipeline includes coverage for building TF-A along with TF-RMM and SPM/Hafnium for running 3 worlds and 4 worlds configurations. In those cases, the normal world payload and test suite originates from TF-a-tests project.