diff --git a/docs/components/index.rst b/docs/components/index.rst
index 2f81f23..192773f 100644
--- a/docs/components/index.rst
+++ b/docs/components/index.rst
@@ -24,4 +24,5 @@
    xlat-tables-lib-v2-design
    cot-binding
    realm-management-extension
+   rmm-el3-comms-spec
    granule-protection-tables-design
diff --git a/docs/components/realm-management-extension.rst b/docs/components/realm-management-extension.rst
index 5fa5140..ea921fc 100644
--- a/docs/components/realm-management-extension.rst
+++ b/docs/components/realm-management-extension.rst
@@ -73,6 +73,14 @@
 world. It initializes the RMM and handles Realm Management Interface (RMI)
 SMC calls from Non-secure and Realm worlds.
 
+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
+:ref:`rmm_el3_boot_interface`.
+
+There is also a specification of the runtime services provided by TF-A
+to RMM. This can be found at :ref:`runtime_services_and_interface`.
+
 Test Realm Payload (TRP)
 *************************
 TRP is a small test payload that runs at R-EL2 and implements a subset of
diff --git a/docs/components/rmm-el3-comms-spec.rst b/docs/components/rmm-el3-comms-spec.rst
new file mode 100644
index 0000000..9140d54
--- /dev/null
+++ b/docs/components/rmm-el3-comms-spec.rst
@@ -0,0 +1,413 @@
+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.
+
+.. _rmm_el3_ifc_versioning:
+
+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 :ref:`rmm_el3_boot_manifest`,
+uses a separate version number but with the same scheme.
+
+The version number is a 32-bit type with the following fields:
+
+.. csv-table::
+   :header: "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 0.1 version of Boot Interface ABI and RMM-EL3
+services specification and the 0.1 version of the Boot Manifest.
+
+.. _rmm_el3_boot_interface:
+
+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.
+
+.. _rmm_cold_boot_interface:
+
+Cold Boot Interface
+~~~~~~~~~~~~~~~~~~~
+
+During cold boot RMM expects the following register values:
+
+.. csv-table::
+   :header: "Register", "Value"
+   :widths: 1, 5
+
+   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 :ref:`rmm_el3_ifc_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 4K 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.
+
+.. image:: ../resources/diagrams/rmm_cold_boot_generic.png
+
+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:
+
+.. csv-table::
+   :header: "Register", "Value"
+   :widths: 1, 5
+
+   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
+
+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:
+
+.. csv-table::
+   :header: "Error code", "Description", "ID"
+   :widths: 2 4 1
+
+   ``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.
+
+.. _rmm_el3_boot_manifest:
+
+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 boot manifest independent of the rest of Boot Manifest.
+The current version for the boot manifest is ``v0.1`` and the rules explained
+in :ref:`rmm_el3_ifc_versioning` apply on this version as well.
+
+The boot manifest is divided into two different components:
+
+   - Core Manifest: This is the generic parameters passed to RMM by EL3 common to all platforms.
+   - Platform data: This is defined by the platform owner and contains information specific to that platform.
+
+For the current version of the 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 type specification of the RMM Boot Manifest v0.1, refer to
+:ref:`rmm_el3_manifest_struct`
+
+.. _runtime_services_and_interface:
+
+RMMM-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
+:ref:`rmm_cold_boot_interface` for properties of this shared buffer which both
+EL3 and RMM must adhere to.
+
+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:
+
+.. csv-table::
+   :header: "Error code", "Description", "ID"
+   :widths: 2 4 1
+
+   ``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.
+
+RMM-EL3 runtime services
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following table summarizes the RMM runtime services that need to be
+implemented by EL3 Firmware.
+
+.. csv-table::
+   :header: "FID", "Command"
+   :widths: 2 5
+
+   0xC40001B2,``RMM_ATTEST_GET_REALM_KEY``
+   0xC40001B3,``RMM_ATTEST_GET_PLAT_TOKEN``
+
+RMM_ATTEST_GET_REALM_KEY command
+================================
+
+Retrieve the Realm Attestation Token Signing key from EL3.
+
+FID
+---
+
+``0xC40001B2``
+
+Input values
+------------
+
+.. csv-table::
+   :header: "Name", "Register", "Field", "Type", "Description"
+   :widths: 1 1 1 1 5
+
+   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 :ref:`ecc_curves`
+
+Output values
+-------------
+
+.. csv-table::
+   :header: "Name", "Register", "Field", "Type", "Description"
+   :widths: 1 1 1 1 5
+
+   Result,x0,[63:0],Error Code,Command return status
+   keySize,x1,[63:0],Size,Size of the Realm Attestation Key
+
+Failure conditions
+------------------
+
+The table below shows all the possible error codes returned in ``Result`` upon
+a failure. The errors are ordered by condition check.
+
+.. csv-table::
+   :header: "ID", "Condition"
+   :widths: 1 5
+
+   ``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 :ref:`ecc_curves`
+   ``E_RMM_UNK``,An unknown error occurred whilst processing the command
+   ``E_RMM_OK``,No errors detected
+
+.. _ecc_curves:
+
+Supported ECC Curves
+--------------------
+
+.. csv-table::
+   :header: "ID", "Curve"
+   :widths: 1 5
+
+   0,ECC SECP384R1
+
+RMM_ATTEST_GET_PLAT_TOKEN command
+=================================
+
+Retrieve the Platform Token from EL3.
+
+FID
+---
+
+``0xC40001B3``
+
+Input values
+------------
+
+.. csv-table::
+   :header: "Name", "Register", "Field", "Type", "Description"
+   :widths: 1 1 1 1 5
+
+   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
+
+Output values
+-------------
+
+.. csv-table::
+   :header: "Name", "Register", "Field", "Type", "Description"
+   :widths: 1 1 1 1 5
+
+   Result,x0,[63:0],Error Code,Command return status
+   tokenSize,x1,[63:0],Size,Size of the platform token
+
+Failure conditions
+------------------
+
+The table below shows all the possible error codes returned in ``Result`` upon
+a failure. The errors are ordered by condition check.
+
+.. csv-table::
+   :header: "ID", "Condition"
+   :widths: 1 5
+
+   ``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
+
+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.
+
+It is the responsibility of EL3 that the above registers will not be leaked to
+the NS Host and to maintain the confidentiality of the Realm World.
+
+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
+
+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.
+
+Types
+_____
+
+.. _rmm_el3_manifest_struct:
+
+RMM-EL3 Boot Manifest Version
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The RMM-EL3 Boot Manifest structure contains platform boot information passed
+from EL3 to RMM. The width of the Boot Manifest is 128 bits
+
+.. image:: ../resources/diagrams/rmm_el3_manifest_struct.png
+
+The members of the RMM-EL3 Boot Manifest structure are shown in the following
+table:
+
+.. csv-table::
+   :header: "Name", "Range", "Type", Description
+   :widths: 2 1 1 4
+
+   ``Version Minor``,15:0,uint16_t,Version Minor part of the Boot Manifest Version.
+   ``Version Major``,30:16,uint16_t,Version Major part of the Boot Manifest Version.
+   ``RES0``,31,bit,Reserved. Set to 0.
+   ``Platform Data``,127:64,Address,Pointer to the Platform Data section of the Boot Manifest.