4.20. RMM-EL3 Communication interface

This document defines the communication interface between RMM and EL3. There are two parts in this interface: the boot interface and the runtime interface.

The Boot Interface defines the ABI between EL3 and RMM when the CPU enters R-EL2 for the first time after boot. The cold boot interface defines the ABI for the cold boot path and the warm boot interface defines the same for the warm path.

The RMM-EL3 runtime interface defines the ABI for EL3 services which can be invoked by RMM as well as the register save-restore convention when handling an SMC call from NS.

The below sections discuss these interfaces more in detail.

4.20.1. RMM-EL3 Interface versioning

The RMM Boot and Runtime Interface uses a version number to check compatibility with the register arguments passed as part of Boot Interface and RMM-EL3 runtime interface.

The Boot Manifest, discussed later in section Boot Manifest, uses a separate version number but with the same scheme.

The version number is a 32-bit type with the following fields:

Bits	Value
[0:15]	`VERSION_MINOR`
[16:30]	`VERSION_MAJOR`
[31]	RES0

The version numbers are sequentially increased and the rules for updating them are explained below:

VERSION_MAJOR: This value is increased when changes break compatibility with previous versions. If the changes on the ABI are compatible with the previous one, VERSION_MAJOR remains unchanged.

VERSION_MINOR: This value is increased on any change that is backwards compatible with the previous version. When VERSION_MAJOR is increased, VERSION_MINOR must be set to 0.

RES0: Bit 31 of the version number is reserved 0 as to maintain consistency with the versioning schemes used in other parts of RMM.

This document specifies the 0.2 version of Boot Interface ABI and RMM-EL3 services specification and the 0.2 version of the Boot Manifest.

4.20.2. RMM Boot Interface

This section deals with the Boot Interface part of the specification.

One of the goals of the Boot Interface is to allow EL3 firmware to pass down into RMM certain platform specific information dynamically. This allows RMM to be less platform dependent and be more generic across platform variations. It also allows RMM to be decoupled from the other boot loader images in the boot sequence and remain agnostic of any particular format used for configuration files.

The Boot Interface ABI defines a set of register conventions and also a memory based manifest file to pass information from EL3 to RMM. The Boot Manifest and the associated platform data in it can be dynamically created by EL3 and there is no restriction on how the data can be obtained (e.g by DTB, hoblist or other).

The register convention and the manifest are versioned separately to manage future enhancements and compatibility.

RMM completes the boot by issuing the RMM_BOOT_COMPLETE SMC (0xC40001CF) back to EL3. After the RMM has finished the boot process, it can only be entered from EL3 as part of RMI handling.

If RMM returns an error during boot (in any CPU), then RMM must not be entered from any CPU.

4.20.2.1. Cold Boot Interface

During cold boot RMM expects the following register values:

Register	Value
x0	Linear index of this PE. This index starts from 0 and must be less than the maximum number of CPUs to be supported at runtime (see x2).
x1	Version for this Boot Interface as defined in RMM-EL3 Interface versioning.
x2	Maximum number of CPUs to be supported at runtime. RMM should ensure that it can support this maximum number.
x3	Base address for the shared buffer used for communication between EL3 firmware and RMM. This buffer must be of 4KB size (1 page). The Boot Manifest must be present at the base of this shared buffer during cold boot.

During cold boot, EL3 firmware needs to allocate a 4KB page that will be passed to RMM in x3. This memory will be used as shared buffer for communication between EL3 and RMM. It must be assigned to Realm world and must be mapped with Normal memory attributes (IWB-OWB-ISH) at EL3. At boot, this memory will be used to populate the Boot Manifest. Since the Boot Manifest can be accessed by RMM prior to enabling its MMU, EL3 must ensure that proper cache maintenance operations are performed after the Boot Manifest is populated.

EL3 should also ensure that this shared buffer is always available for use by RMM during the lifetime of the system and that it can be used for runtime communication between RMM and EL3. For example, when RMM invokes attestation service commands in EL3, this buffer can be used to exchange data between RMM and EL3. It is also allowed for RMM to invoke runtime services provided by EL3 utilizing this buffer during the boot phase, prior to return back to EL3 via RMM_BOOT_COMPLETE SMC.

RMM should map this memory page into its Stage 1 page-tables using Normal memory attributes.

During runtime, it is the RMM which initiates any communication with EL3. If that communication requires the use of the shared area, it is expected that RMM needs to do the necessary concurrency protection to prevent the use of the same buffer by other PEs.

The following sequence diagram shows how a generic EL3 Firmware would boot RMM.

4.20.2.2. Warm Boot Interface

At warm boot, RMM is already initialized and only some per-CPU initialization is still pending. The only argument that is required by RMM at this stage is the CPU Id, which will be passed through register x0 whilst x1 to x3 are RES0. This is summarized in the following table:

Register	Value
x0	Linear index of this PE. This index starts from 0 and must be less than the maximum number of CPUs to be supported at runtime (see x2).
x1 - x3	RES0

4.20.2.3. Boot error handling and return values

After boot up and initialization, RMM returns control back to EL3 through a RMM_BOOT_COMPLETE SMC call. The only argument of this SMC call will be returned in x1 and it will encode a signed integer with the error reason as per the following table:

Error code	Description	ID
`E_RMM_BOOT_SUCCESS`	Boot successful	0
`E_RMM_BOOT_ERR_UNKNOWN`	Unknown error	-1
`E_RMM_BOOT_VERSION_NOT_VALID`	Boot Interface version reported by EL3 is not supported by RMM	-2
`E_RMM_BOOT_CPUS_OUT_OF_RAGE`	Number of CPUs reported by EL3 larger than maximum supported by RMM	-3
`E_RMM_BOOT_CPU_ID_OUT_OF_RAGE`	Current CPU Id is higher or equal than the number of CPUs supported by RMM	-4
`E_RMM_BOOT_INVALID_SHARED_BUFFER`	Invalid pointer to shared memory area	-5
`E_RMM_BOOT_MANIFEST_VERSION_NOT_SUPPORTED`	Version reported by the Boot Manifest not supported by RMM	-6
`E_RMM_BOOT_MANIFEST_DATA_ERROR`	Error parsing core Boot Manifest	-7

For any error detected in RMM during cold or warm boot, RMM will return back to EL3 using RMM_BOOT_COMPLETE SMC with an appropriate error code. It is expected that EL3 will take necessary action to disable Realm world for further entry from NS Host on receiving an error. This will be done across all the PEs in the system so as to present a symmetric view to the NS Host. Any further warm boot by any PE should not enter RMM using the warm boot interface.

4.20.2.4. Boot Manifest

During cold boot, EL3 Firmware passes a memory Boot Manifest to RMM containing platform information.

This Boot Manifest is versioned independently of the Boot Interface, to help evolve the former independent of the latter. The current version for the Boot Manifest is v0.2 and the rules explained in RMM-EL3 Interface versioning apply on this version as well.

The Boot Manifest v0.2 has the following fields:

version : Version of the Manifest (v0.2)

plat_data : Pointer to the platform specific data and not specified by this document. These data are optional and can be NULL.

plat_dram : Structure encoding the NS DRAM information on the platform. This field is also optional and platform can choose to zero out this structure if RMM does not need EL3 to send this information during the boot.

For the current version of the Boot Manifest, the core manifest contains a pointer to the platform data. EL3 must ensure that the whole Boot Manifest, including the platform data, if available, fits inside the RMM EL3 shared buffer.

For the data structure specification of Boot Manifest, refer to RMM-EL3 Boot Manifest structure

4.20.3. RMM-EL3 Runtime Interface

This section defines the RMM-EL3 runtime interface which specifies the ABI for EL3 services expected by RMM at runtime as well as the register save and restore convention between EL3 and RMM as part of RMI call handling. It is important to note that RMM is allowed to invoke EL3-RMM runtime interface services during the boot phase as well. The EL3 runtime service handling must not result in a world switch to another world unless specified. Both the RMM and EL3 are allowed to make suitable optimizations based on this assumption.

If the interface requires the use of memory, then the memory references should be within the shared buffer communicated as part of the boot interface. See Cold Boot Interface for properties of this shared buffer which both EL3 and RMM must adhere to.

4.20.3.1. RMM-EL3 runtime service return codes

The return codes from EL3 to RMM is a 32 bit signed integer which encapsulates error condition as described in the following table:

Error code	Description	ID
`E_RMM_OK`	No errors detected	0
`E_RMM_UNK`	Unknown/Generic error	-1
`E_RMM_BAD_ADDR`	The value of an address used as argument was invalid	-2
`E_RMM_BAD_PAS`	Incorrect PAS	-3
`E_RMM_NOMEM`	Not enough memory to perform an operation	-4
`E_RMM_INVAL`	The value of an argument was invalid	-5

If multiple failure conditions are detected in an RMM to EL3 command, then EL3 is allowed to return an error code corresponding to any of the failure conditions.

4.20.3.2. RMM-EL3 runtime services

The following table summarizes the RMM runtime services that need to be implemented by EL3 Firmware.

FID	Command
0xC400018F	`RMM_RMI_REQ_COMPLETE`
0xC40001B0	`RMM_GTSI_DELEGATE`
0xC40001B1	`RMM_GTSI_UNDELEGATE`
0xC40001B2	`RMM_ATTEST_GET_REALM_KEY`
0xC40001B3	`RMM_ATTEST_GET_PLAT_TOKEN`

4.20.3.2.1. RMM_RMI_REQ_COMPLETE command

Notifies the completion of an RMI call to the Non-Secure world.

This call is the only function currently in RMM-EL3 runtime interface which results in a world switch to NS. This call is the reply to the original RMI call and it is forwarded by EL3 to the NS world.

4.20.3.2.1.1. FID

0xC400018F

4.20.3.2.1.2. Input values

Name	Register	Field	Type	Description
fid	x0	[63:0]	UInt64	Command FID
err_code	x1	[63:0]	RmiCommandReturnCode	Error code returned by the RMI service invoked by NS World. See Realm Management Monitor specification for more info

4.20.3.2.1.3. Output values

This call does not return.

4.20.3.2.1.4. Failure conditions

Since this call does not return to RMM, there is no failure condition which can be notified back to RMM.

4.20.3.2.2. RMM_GTSI_DELEGATE command

Delegate a memory granule by changing its PAS from Non-Secure to Realm.

4.20.3.2.2.1. FID

0xC40001B0

4.20.3.2.2.2. Input values

Name	Register	Field	Type	Description
fid	x0	[63:0]	UInt64	Command FID
base_pa	x1	[63:0]	Address	PA of the start of the granule to be delegated

4.20.3.2.2.3. Output values

Name	Register	Field	Type	Description
Result	x0	[63:0]	Error Code	Command return status

4.20.3.2.2.4. Failure conditions

The table below shows all the possible error codes returned in Result upon a failure. The errors are ordered by condition check.

ID	Condition
`E_RMM_BAD_ADDR`	`PA` does not correspond to a valid granule address
`E_RMM_BAD_PAS`	The granule pointed by `PA` does not belong to Non-Secure PAS
`E_RMM_OK`	No errors detected

4.20.3.2.3. RMM_GTSI_UNDELEGATE command

Undelegate a memory granule by changing its PAS from Realm to Non-Secure.

4.20.3.2.3.1. FID

0xC40001B1

4.20.3.2.3.2. Input values

Name	Register	Field	Type	Description
fid	x0	[63:0]	UInt64	Command FID
base_pa	x1	[63:0]	Address	PA of the start of the granule to be undelegated

4.20.3.2.3.3. Output values

Name	Register	Field	Type	Description
Result	x0	[63:0]	Error Code	Command return status

4.20.3.2.3.4. Failure conditions

The table below shows all the possible error codes returned in Result upon a failure. The errors are ordered by condition check.

ID	Condition
`E_RMM_BAD_ADDR`	`PA` does not correspond to a valid granule address
`E_RMM_BAD_PAS`	The granule pointed by `PA` does not belong to Realm PAS
`E_RMM_OK`	No errors detected

4.20.3.2.4. RMM_ATTEST_GET_REALM_KEY command

Retrieve the Realm Attestation Token Signing key from EL3.

4.20.3.2.4.1. FID

0xC40001B2

4.20.3.2.4.2. Input values

Name	Register	Field	Type	Description
fid	x0	[63:0]	UInt64	Command FID
buf_pa	x1	[63:0]	Address	PA where the Realm Attestation Key must be stored by EL3. The PA must belong to the shared buffer
buf_size	x2	[63:0]	Size	Size in bytes of the Realm Attestation Key buffer. `bufPa + bufSize` must lie within the shared buffer
ecc_curve	x3	[63:0]	Enum	Type of the elliptic curve to which the requested attestation key belongs to. See Supported ECC Curves

4.20.3.2.4.3. Output values

Name	Register	Field	Type	Description
Result	x0	[63:0]	Error Code	Command return status
keySize	x1	[63:0]	Size	Size of the Realm Attestation Key

4.20.3.2.4.4. Failure conditions

The table below shows all the possible error codes returned in Result upon a failure. The errors are ordered by condition check.

ID	Condition
`E_RMM_BAD_ADDR`	`PA` is outside the shared buffer
`E_RMM_INVAL`	`PA + BSize` is outside the shared buffer
`E_RMM_INVAL`	`Curve` is not one of the listed in Supported ECC Curves
`E_RMM_UNK`	An unknown error occurred whilst processing the command
`E_RMM_OK`	No errors detected

4.20.3.2.4.5. Supported ECC Curves

ID	Curve
0	ECC SECP384R1

4.20.3.2.5. RMM_ATTEST_GET_PLAT_TOKEN command

Retrieve the Platform Token from EL3.

4.20.3.2.5.1. FID

0xC40001B3

4.20.3.2.5.2. Input values

Name	Register	Field	Type	Description
fid	x0	[63:0]	UInt64	Command FID
buf_pa	x1	[63:0]	Address	PA of the platform attestation token. The challenge object is passed in this buffer. The PA must belong to the shared buffer
buf_size	x2	[63:0]	Size	Size in bytes of the platform attestation token buffer. `bufPa + bufSize` must lie within the shared buffer
c_size	x3	[63:0]	Size	Size in bytes of the challenge object. It corresponds to the size of one of the defined SHA algorithms

4.20.3.2.5.3. Output values

Name	Register	Field	Type	Description
Result	x0	[63:0]	Error Code	Command return status
tokenSize	x1	[63:0]	Size	Size of the platform token

4.20.3.2.5.4. Failure conditions

The table below shows all the possible error codes returned in Result upon a failure. The errors are ordered by condition check.

ID	Condition
`E_RMM_BAD_ADDR`	`PA` is outside the shared buffer
`E_RMM_INVAL`	`PA + BSize` is outside the shared buffer
`E_RMM_INVAL`	`CSize` does not represent the size of a supported SHA algorithm
`E_RMM_UNK`	An unknown error occurred whilst processing the command
`E_RMM_OK`	No errors detected

4.20.4. RMM-EL3 world switch register save restore convention

As part of NS world switch, EL3 is expected to maintain a register context specific to each world and will save and restore the registers appropriately. This section captures the contract between EL3 and RMM on the register set to be saved and restored.

EL3 must maintain a separate register context for the following:

General purpose registers (x0-x30) and sp_el0, sp_el2 stack pointers

EL2 system register context for all enabled features by EL3. These include system registers with the _EL2 prefix. The EL2 physical and virtual timer registers must not be included in this.

As part of SMC forwarding between the NS world and Realm world, EL3 allows x0-x7 to be passed as arguments to Realm and x0-x4 to be used for return arguments back to Non Secure. As per SMCCCv1.2, x4 must be preserved if not being used as return argument by the SMC function and it is the responsibility of RMM to preserve this or use this as a return argument. EL3 will always copy x0-x4 from Realm context to NS Context.

EL3 must save and restore the following as part of world switch:

EL2 system registers with the exception of zcr_el2 register.
PAuth key registers (APIA, APIB, APDA, APDB, APGA).

EL3 will not save some registers as mentioned in the below list. It is the responsibility of RMM to ensure that these are appropriately saved if the Realm World makes use of them:

FP/SIMD registers

SVE registers

SME registers

EL1/0 registers with the exception of PAuth key registers as mentioned above.

zcr_el2 register.

It is essential that EL3 honors this contract to maintain the Confidentiality and integrity of the Realm world.

SMCCC v1.3 allows NS world to specify whether SVE context is in use. In this case, RMM could choose to not save the incoming SVE context but must ensure to clear SVE registers if they have been used in Realm World. The same applies to SME registers.

4.20.5. Types

4.20.5.1. RMM-EL3 Boot Manifest structure

The RMM-EL3 Boot Manifest v0.2 structure contains platform boot information passed from EL3 to RMM. The size of the Boot Manifest is 40 bytes.

The members of the RMM-EL3 Boot Manifest structure are shown in the following table:

Name	Offset	Type	Description
version	0	uint32_t	Boot Manifest version
padding	4	uint32_t	Reserved, set to 0
plat_data	8	uintptr_t	Pointer to Platform Data section
plat_dram	16	ns_dram_info	NS DRAM Layout Info structure

4.20.5.2. NS DRAM Layout Info structure

NS DRAM Layout Info structure contains information about platform Non-secure DRAM layout. The members of this structure are shown in the table below:

Name	Offset	Type	Description
num_banks	0	uint64_t	Number of NS DRAM banks
banks	8	ns_dram_bank *	Pointer to ‘ns_dram_bank’[] array
checksum	16	uint64_t	Checksum

Checksum is calculated as two’s complement sum of ‘num_banks’, ‘banks’ pointer and DRAM banks data array pointed by it.

4.20.5.3. NS DRAM Bank structure

NS DRAM Bank structure contains information about each Non-secure DRAM bank:

Name	Offset	Type	Description
base	0	uintptr_t	Base address
size	8	uint64_t	Size of bank in bytes