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+*******************************
+Secure Partition Manager Design
+*******************************
+
+
+
+.. contents::
+
+Background
+==========
+
+In some market segments that primarily deal with client-side devices like mobile
+phones, tablets, STBs and embedded devices, a Trusted OS instantiates trusted
+applications to provide security services like DRM, secure payment and
+authentication. The Global Platform TEE Client API specification defines the API
+used by Non-secure world applications to access these services. A Trusted OS
+fulfils the requirements of a security service as described above.
+
+Management services are typically implemented at the highest level of privilege
+in the system, i.e. EL3 in Trusted Firmware-A (TF-A). The service requirements are
+fulfilled by the execution environment provided by TF-A.
+
+The following diagram illustrates the corresponding software stack:
+
+|Image 1|
+
+In other market segments that primarily deal with server-side devices (e.g. data
+centres and enterprise servers) the secure software stack typically does not
+include a Global Platform Trusted OS. Security functions are accessed through
+other interfaces (e.g. ACPI TCG TPM interface, UEFI runtime variable service).
+
+Placement of management and security functions with diverse requirements in a
+privileged Exception Level (i.e. EL3 or S-EL1) makes security auditing of
+firmware more difficult and does not allow isolation of unrelated services from
+each other either.
+
+Introduction
+============
+
+A **Secure Partition** is a software execution environment instantiated in
+S-EL0 that can be used to implement simple management and security services.
+Since S-EL0 is an unprivileged Exception Level, a Secure Partition relies on
+privileged firmware (i.e. TF-A) to be granted access to system and processor
+resources. Essentially, it is a software sandbox in the Secure world that runs
+under the control of privileged software, provides one or more services and
+accesses the following system resources:
+
+- Memory and device regions in the system address map.
+
+- PE system registers.
+
+- A range of synchronous exceptions (e.g. SMC function identifiers).
+
+Note that currently TF-A only supports handling one Secure Partition.
+
+A Secure Partition enables TF-A to implement only the essential secure
+services in EL3 and instantiate the rest in a partition in S-EL0.
+Furthermore, multiple Secure Partitions can be used to isolate unrelated
+services from each other.
+
+The following diagram illustrates the place of a Secure Partition in a typical
+Armv8-A software stack. A single or multiple Secure Partitions provide secure
+services to software components in the Non-secure world and other Secure
+Partitions.
+
+|Image 2|
+
+The TF-A build system is responsible for including the Secure Partition image
+in the FIP. During boot, BL2 includes support to authenticate and load the
+Secure Partition image. A BL31 component called **Secure Partition Manager
+(SPM)** is responsible for managing the partition. This is semantically
+similar to a hypervisor managing a virtual machine.
+
+The SPM is responsible for the following actions during boot:
+
+- Allocate resources requested by the Secure Partition.
+
+- Perform architectural and system setup required by the Secure Partition to
+  fulfil a service request.
+
+- Implement a standard interface that is used for initialising a Secure
+  Partition.
+
+The SPM is responsible for the following actions during runtime:
+
+- Implement a standard interface that is used by a Secure Partition to fulfil
+  service requests.
+
+- Implement a standard interface that is used by the Non-secure world for
+  accessing the services exported by a Secure Partition. A service can be
+  invoked through a SMC.
+
+Alternatively, a partition can be viewed as a thread of execution running under
+the control of the SPM. Hence common programming concepts described below are
+applicable to a partition.
+
+Description
+===========
+
+The previous section introduced some general aspects of the software
+architecture of a Secure Partition. This section describes the specific choices
+made in the current implementation of this software architecture. Subsequent
+revisions of the implementation will include a richer set of features that
+enable a more flexible architecture.
+
+Building TF-A with Secure Partition support
+-------------------------------------------
+
+SPM is supported on the Arm FVP exclusively at the moment. The current
+implementation supports inclusion of only a single Secure Partition in which a
+service always runs to completion (e.g. the requested services cannot be
+preempted to give control back to the Normal world).
+
+It is not currently possible for BL31 to integrate SPM support and a Secure
+Payload Dispatcher (SPD) at the same time; they are mutually exclusive. In the
+SPM bootflow, a Secure Partition image executing at S-EL0 replaces the Secure
+Payload image executing at S-EL1 (e.g. a Trusted OS). Both are referred to as
+BL32.
+
+A working prototype of a SP has been implemented by re-purposing the EDK2 code
+and tools, leveraging the concept of the *Standalone Management Mode (MM)* in
+the UEFI specification (see the PI v1.6 Volume 4: Management Mode Core
+Interface). This will be referred to as the *Standalone MM Secure Partition* in
+the rest of this document.
+
+To enable SPM support in TF-A, the source code must be compiled with the build
+flag ``ENABLE_SPM=1``, along with ``EL3_EXCEPTION_HANDLING=1``. On Arm
+platforms the build option ``ARM_BL31_IN_DRAM`` must be set to 1. Also, the
+location of the binary that contains the BL32 image
+(``BL32=path/to/image.bin``) must be specified.
+
+First, build the Standalone MM Secure Partition. To build it, refer to the
+`instructions in the EDK2 repository`_.
+
+Then build TF-A with SPM support and include the Standalone MM Secure Partition
+image in the FIP:
+
+::
+
+    BL32=path/to/standalone/mm/sp BL33=path/to/bl33.bin \
+    make PLAT=fvp ENABLE_SPM=1 ARM_BL31_IN_DRAM=1 fip all
+
+Describing Secure Partition resources
+-------------------------------------
+
+TF-A exports a porting interface that enables a platform to specify the system
+resources required by the Secure Partition. Some instructions are given below.
+However, this interface is under development and it may change as new features
+are implemented.
+
+- A Secure Partition is considered a BL32 image, so the same defines that apply
+  to BL32 images apply to a Secure Partition: ``BL32_BASE`` and ``BL32_LIMIT``.
+
+- The following defines are needed to allocate space for the translation tables
+  used by the Secure Partition: ``PLAT_SP_IMAGE_MMAP_REGIONS`` and
+  ``PLAT_SP_IMAGE_MAX_XLAT_TABLES``.
+
+- The functions ``plat_get_secure_partition_mmap()`` and
+  ``plat_get_secure_partition_boot_info()`` have to be implemented. The file
+  ``plat/arm/board/fvp/fvp_common.c`` can be used as an example. It uses the
+  defines in ``include/plat/arm/common/arm_spm_def.h``.
+
+  - ``plat_get_secure_partition_mmap()`` returns an array of mmap regions that
+    describe the memory regions that the SPM needs to allocate for a Secure
+    Partition.
+
+  - ``plat_get_secure_partition_boot_info()`` returns a
+    ``secure_partition_boot_info_t`` struct that is populated by the platform
+    with information about the memory map of the Secure Partition.
+
+For an example of all the changes in context, you may refer to commit
+``e29efeb1b4``, in which the port for FVP was introduced.
+
+Accessing Secure Partition services
+-----------------------------------
+
+The `SMC Calling Convention`_ (*Arm DEN 0028B*) describes SMCs as a conduit for
+accessing services implemented in the Secure world. The ``MM_COMMUNICATE``
+interface defined in the `Management Mode Interface Specification`_ (*Arm DEN
+0060A*) is used to invoke a Secure Partition service as a Fast Call.
+
+The mechanism used to identify a service within the partition depends on the
+service implementation. It is assumed that the caller of the service will be
+able to discover this mechanism through standard platform discovery mechanisms
+like ACPI and Device Trees. For example, *Volume 4: Platform Initialisation
+Specification v1.6. Management Mode Core Interface* specifies that a GUID is
+used to identify a management mode service. A client populates the GUID in the
+``EFI_MM_COMMUNICATE_HEADER``. The header is populated in the communication
+buffer shared with the Secure Partition.
+
+A Fast Call appears to be atomic from the perspective of the caller and returns
+when the requested operation has completed. A service invoked through the
+``MM_COMMUNICATE`` SMC will run to completion in the partition on a given CPU.
+The SPM is responsible for guaranteeing this behaviour. This means that there
+can only be a single outstanding Fast Call in a partition on a given CPU.
+
+Exchanging data with the Secure Partition
+-----------------------------------------
+
+The exchange of data between the Non-secure world and the partition takes place
+through a shared memory region. The location of data in the shared memory area
+is passed as a parameter to the ``MM_COMMUNICATE`` SMC. The shared memory area
+is statically allocated by the SPM and is expected to be either implicitly known
+to the Non-secure world or discovered through a platform discovery mechanism
+e.g. ACPI table or device tree. It is possible for the Non-secure world to
+exchange data with a partition only if it has been populated in this shared
+memory area. The shared memory area is implemented as per the guidelines
+specified in Section 3.2.3 of the `Management Mode Interface Specification`_
+(*Arm DEN 0060A*).
+
+The format of data structures used to encapsulate data in the shared memory is
+agreed between the Non-secure world and the Secure Partition. For example, in
+the `Management Mode Interface specification`_ (*Arm DEN 0060A*), Section 4
+describes that the communication buffer shared between the Non-secure world and
+the Management Mode (MM) in the Secure world must be of the type
+``EFI_MM_COMMUNICATE_HEADER``. This data structure is defined in *Volume 4:
+Platform Initialisation Specification v1.6. Management Mode Core Interface*.
+Any caller of a MM service will have to use the ``EFI_MM_COMMUNICATE_HEADER``
+data structure.
+
+Runtime model of the Secure Partition
+=====================================
+
+This section describes how the Secure Partition interfaces with the SPM.
+
+Interface with SPM
+------------------
+
+In order to instantiate one or more secure services in the Secure Partition in
+S-EL0, the SPM should define the following types of interfaces:
+
+- Interfaces that enable access to privileged operations from S-EL0. These
+  operations typically require access to system resources that are either shared
+  amongst multiple software components in the Secure world or cannot be directly
+  accessed from an unprivileged Exception Level.
+
+- Interfaces that establish the control path between the SPM and the Secure
+  Partition.
+
+This section describes the APIs currently exported by the SPM that enable a
+Secure Partition to initialise itself and export its services in S-EL0. These
+interfaces are not accessible from the Non-secure world.
+
+Conduit
+^^^^^^^
+
+The `SMC Calling Convention`_ (*Arm DEN 0028B*) specification describes the SMC
+and HVC conduits for accessing firmware services and their availability
+depending on the implemented Exception levels. In S-EL0, the Supervisor Call
+exception (SVC) is the only architectural mechanism available for unprivileged
+software to make a request for an operation implemented in privileged software.
+Hence, the SVC conduit must be used by the Secure Partition to access interfaces
+implemented by the SPM.
+
+A SVC causes an exception to be taken to S-EL1. TF-A assumes ownership of S-EL1
+and installs a simple exception vector table in S-EL1 that relays a SVC request
+from a Secure Partition as a SMC request to the SPM in EL3. Upon servicing the
+SMC request, Arm Trusted Firmware returns control directly to S-EL0 through an
+ERET instruction.
+
+Calling conventions
+^^^^^^^^^^^^^^^^^^^
+
+The `SMC Calling Convention`_ (*Arm DEN 0028B*) specification describes the
+32-bit and 64-bit calling conventions for the SMC and HVC conduits. The SVC
+conduit introduces the concept of SVC32 and SVC64 calling conventions. The SVC32
+and SVC64 calling conventions are equivalent to the 32-bit (SMC32) and the
+64-bit (SMC64) calling conventions respectively.
+
+Communication initiated by SPM
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A service request is initiated from the SPM through an exception return
+instruction (ERET) to S-EL0. Later, the Secure Partition issues an SVC
+instruction to signal completion of the request. Some example use cases are
+given below:
+
+- A request to initialise the Secure Partition during system boot.
+
+- A request to handle a runtime service request.
+
+Communication initiated by Secure Partition
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A request is initiated from the Secure Partition by executing a SVC instruction.
+An ERET instruction is used by TF-A to return to S-EL0 with the result of the
+request.
+
+For instance, a request to perform privileged operations on behalf of a
+partition (e.g.  management of memory attributes in the translation tables for
+the Secure EL1&0 translation regime).
+
+Interfaces
+^^^^^^^^^^
+
+The current implementation reserves function IDs for Fast Calls in the Standard
+Secure Service calls range (see `SMC Calling Convention`_ (*Arm DEN 0028B*)
+specification) for each API exported by the SPM. This section defines the
+function prototypes for each function ID. The function IDs specify whether one
+or both of the SVC32 and SVC64 calling conventions can be used to invoke the
+corresponding interface.
+
+Secure Partition Event Management
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The Secure Partition provides an Event Management interface that is used by the
+SPM to delegate service requests to the Secure Partition. The interface also
+allows the Secure Partition to:
+
+- Register with the SPM a service that it provides.
+- Indicate completion of a service request delegated by the SPM
+
+Miscellaneous interfaces
+------------------------
+
+``SPM_VERSION_AARCH32``
+^^^^^^^^^^^^^^^^^^^^^^^
+
+- Description
+
+  Returns the version of the interface exported by SPM.
+
+- Parameters
+
+  - **uint32** - Function ID
+
+    - SVC32 Version: **0x84000060**
+
+- Return parameters
+
+  - **int32** - Status
+
+    On success, the format of the value is as follows:
+
+    - Bit [31]: Must be 0
+    - Bits [30:16]: Major Version. Must be 0 for this revision of the SPM
+      interface.
+    - Bits [15:0]: Minor Version. Must be 1 for this revision of the SPM
+      interface.
+
+    On error, the format of the value is as follows:
+
+    - ``NOT_SUPPORTED``: SPM interface is not supported or not available for the
+      client.
+
+- Usage
+
+  This function returns the version of the Secure Partition Manager
+  implementation. The major version is 0 and the minor version is 1. The version
+  number is a 31-bit unsigned integer, with the upper 15 bits denoting the major
+  revision, and the lower 16 bits denoting the minor revision. The following
+  rules apply to the version numbering:
+
+  - Different major revision values indicate possibly incompatible functions.
+
+  - For two revisions, A and B, for which the major revision values are
+    identical, if the minor revision value of revision B is greater than the
+    minor revision value of revision A, then every function in revision A must
+    work in a compatible way with revision B. However, it is possible for
+    revision B to have a higher function count than revision A.
+
+- Implementation responsibilities
+
+  If this function returns a valid version number, all the functions that are
+  described subsequently must be implemented, unless it is explicitly stated
+  that a function is optional.
+
+See `Error Codes`_ for integer values that are associated with each return
+code.
+
+Secure Partition Initialisation
+-------------------------------
+
+The SPM is responsible for initialising the architectural execution context to
+enable initialisation of a service in S-EL0. The responsibilities of the SPM are
+listed below. At the end of initialisation, the partition issues a
+``SP_EVENT_COMPLETE_AARCH64`` call (described later) to signal readiness for
+handling requests for services implemented by the Secure Partition. The
+initialisation event is executed as a Fast Call.
+
+Entry point invocation
+^^^^^^^^^^^^^^^^^^^^^^
+
+The entry point for service requests that should be handled as Fast Calls is
+used as the target of the ERET instruction to start initialisation of the Secure
+Partition.
+
+Architectural Setup
+^^^^^^^^^^^^^^^^^^^
+
+At cold boot, system registers accessible from S-EL0 will be in their reset
+state unless otherwise specified. The SPM will perform the following
+architectural setup to enable execution in S-EL0
+
+MMU setup
+^^^^^^^^^
+
+The platform port of a Secure Partition specifies to the SPM a list of regions
+that it needs access to and their attributes. The SPM validates this resource
+description and initialises the Secure EL1&0 translation regime as follows.
+
+1. Device regions are mapped with nGnRE attributes and Execute Never
+   instruction access permissions.
+
+2. Code memory regions are mapped with RO data and Executable instruction access
+   permissions.
+
+3. Read Only data memory regions are mapped with RO data and Execute Never
+   instruction access permissions.
+
+4. Read Write data memory regions are mapped with RW data and Execute Never
+   instruction access permissions.
+
+5. If the resource description does not explicitly describe the type of memory
+   regions then all memory regions will be marked with Code memory region
+   attributes.
+
+6. The ``UXN`` and ``PXN`` bits are set for regions that are not executable by
+   S-EL0 or S-EL1.
+
+System Register Setup
+^^^^^^^^^^^^^^^^^^^^^
+
+System registers that influence software execution in S-EL0 are setup by the SPM
+as follows:
+
+1. ``SCTLR_EL1``
+
+   - ``UCI=1``
+   - ``EOE=0``
+   - ``WXN=1``
+   - ``nTWE=1``
+   - ``nTWI=1``
+   - ``UCT=1``
+   - ``DZE=1``
+   - ``I=1``
+   - ``UMA=0``
+   - ``SA0=1``
+   - ``C=1``
+   - ``A=1``
+   - ``M=1``
+
+2. ``CPACR_EL1``
+
+   - ``FPEN=b'11``
+
+3. ``PSTATE``
+
+   - ``D,A,I,F=1``
+   - ``CurrentEL=0`` (EL0)
+   - ``SpSel=0`` (Thread mode)
+   - ``NRW=0`` (AArch64)
+
+General Purpose Register Setup
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+SPM will invoke the entry point of a service by executing an ERET instruction.
+This transition into S-EL0 is special since it is not in response to a previous
+request through a SVC instruction. This is the first entry into S-EL0. The
+general purpose register usage at the time of entry will be as specified in the
+"Return State" column of Table 3-1 in Section 3.1 "Register use in AArch64 SMC
+calls" of the `SMC Calling Convention`_ (*Arm DEN 0028B*) specification. In
+addition, certain other restrictions will be applied as described below.
+
+1. ``SP_EL0``
+
+   A non-zero value will indicate that the SPM has initialised the stack pointer
+   for the current CPU.
+
+   The value will be 0 otherwise.
+
+2. ``X4-X30``
+
+   The values of these registers will be 0.
+
+3. ``X0-X3``
+
+   Parameters passed by the SPM.
+
+   - ``X0``: Virtual address of a buffer shared between EL3 and S-EL0. The
+     buffer will be mapped in the Secure EL1&0 translation regime with read-only
+     memory attributes described earlier.
+
+   - ``X1``: Size of the buffer in bytes.
+
+   - ``X2``: Cookie value (*IMPLEMENTATION DEFINED*).
+
+   - ``X3``: Cookie value (*IMPLEMENTATION DEFINED*).
+
+Runtime Event Delegation
+------------------------
+
+The SPM receives requests for Secure Partition services through a synchronous
+invocation (i.e. a SMC from the Non-secure world). These requests are delegated
+to the partition by programming a return from the last
+``SP_EVENT_COMPLETE_AARCH64`` call received from the partition. The last call
+was made to signal either completion of Secure Partition initialisation or
+completion of a partition service request.
+
+``SP_EVENT_COMPLETE_AARCH64``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+- Description
+
+  Signal completion of the last SP service request.
+
+- Parameters
+
+  - **uint32** - Function ID
+
+    - SVC64 Version: **0xC4000061**
+
+  - **int32** - Event Status Code
+
+    Zero or a positive value indicates that the event was handled successfully.
+    The values depend upon the original event that was delegated to the Secure
+    partition. They are described as follows.
+
+    - ``SUCCESS`` : Used to indicate that the Secure Partition was initialised
+      or a runtime request was handled successfully.
+
+    - Any other value greater than 0 is used to pass a specific Event Status
+      code in response to a runtime event.
+
+    A negative value indicates an error. The values of Event Status code depend
+    on the original event.
+
+- Return parameters
+
+  - **int32** - Event ID/Return Code
+
+    Zero or a positive value specifies the unique ID of the event being
+    delegated to the partition by the SPM.
+
+    In the current implementation, this parameter contains the function ID of
+    the ``MM_COMMUNICATE`` SMC. This value indicates to the partition that an
+    event has been delegated to it in response to an ``MM_COMMUNICATE`` request
+    from the Non-secure world.
+
+    A negative value indicates an error. The format of the value is as follows:
+
+    - ``NOT_SUPPORTED``: Function was called from the Non-secure world.
+
+    See `Error Codes`_ for integer values that are associated with each return
+    code.
+
+  - **uint32** - Event Context Address
+
+    Address of a buffer shared between the SPM and Secure Partition to pass
+    event specific information. The format of the data populated in the buffer
+    is implementation defined.
+
+    The buffer is mapped in the Secure EL1&0 translation regime with read-only
+    memory attributes described earlier.
+
+    For the SVC64 version, this parameter is a 64-bit Virtual Address (VA).
+
+    For the SVC32 version, this parameter is a 32-bit Virtual Address (VA).
+
+  - **uint32** - Event context size
+
+    Size of the memory starting at Event Address.
+
+  - **uint32/uint64** - Event Cookie
+
+    This is an optional parameter. If unused its value is SBZ.
+
+- Usage
+
+  This function signals to the SPM that the handling of the last event delegated
+  to a partition has completed. The partition is ready to handle its next event.
+  A return from this function is in response to the next event that will be
+  delegated to the partition. The return parameters describe the next event.
+
+- Caller responsibilities
+
+  A Secure Partition must only call ``SP_EVENT_COMPLETE_AARCH64`` to signal
+  completion of a request that was delegated to it by the SPM.
+
+- Callee responsibilities
+
+  When the SPM receives this call from a Secure Partition, the corresponding
+  syndrome information can be used to return control through an ERET
+  instruction, to the instruction immediately after the call in the Secure
+  Partition context. This syndrome information comprises of general purpose and
+  system register values when the call was made.
+
+  The SPM must save this syndrome information and use it to delegate the next
+  event to the Secure Partition. The return parameters of this interface must
+  specify the properties of the event and be populated in ``X0-X3/W0-W3``
+  registers.
+
+Secure Partition Memory Management
+----------------------------------
+
+A Secure Partition executes at S-EL0, which is an unprivileged Exception Level.
+The SPM is responsible for enabling access to regions of memory in the system
+address map from a Secure Partition. This is done by mapping these regions in
+the Secure EL1&0 Translation regime with appropriate memory attributes.
+Attributes refer to memory type, permission, cacheability and shareability
+attributes used in the Translation tables. The definitions of these attributes
+and their usage can be found in the `Armv8-A ARM`_ (*Arm DDI 0487*).
+
+All memory required by the Secure Partition is allocated upfront in the SPM,
+even before handing over to the Secure Partition for the first time. The initial
+access permissions of the memory regions are statically provided by the platform
+port and should allow the Secure Partition to run its initialisation code.
+
+However, they might not suit the final needs of the Secure Partition because its
+final memory layout might not be known until the Secure Partition initialises
+itself. As the Secure Partition initialises its runtime environment it might,
+for example, load dynamically some modules. For instance, a Secure Partition
+could implement a loader for a standard executable file format (e.g. an PE-COFF
+loader for loading executable files at runtime). These executable files will be
+a part of the Secure Partition image. The location of various sections in an
+executable file and their permission attributes (e.g. read-write data, read-only
+data and code) will be known only when the file is loaded into memory.
+
+In this case, the Secure Partition needs a way to change the access permissions
+of its memory regions. The SPM provides this feature through the
+``SP_MEMORY_ATTRIBUTES_SET_AARCH64`` SVC interface. This interface is available
+to the Secure Partition during a specific time window: from the first entry into
+the Secure Partition up to the first ``SP_EVENT_COMPLETE`` call that signals the
+Secure Partition has finished its initialisation. Once the initialisation is
+complete, the SPM does not allow changes to the memory attributes.
+
+This section describes the standard SVC interface that is implemented by the SPM
+to determine and change permission attributes of memory regions that belong to a
+Secure Partition.
+
+``SP_MEMORY_ATTRIBUTES_GET_AARCH64``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+- Description
+
+  Request the permission attributes of a memory region from S-EL0.
+
+- Parameters
+
+  - **uint32** Function ID
+
+    - SVC64 Version: **0xC4000064**
+
+  - **uint64** Base Address
+
+    This parameter is a 64-bit Virtual Address (VA).
+
+    There are no alignment restrictions on the Base Address. The permission
+    attributes of the translation granule it lies in are returned.
+
+- Return parameters
+
+  - **int32** - Memory Attributes/Return Code
+
+    On success the format of the Return Code is as follows:
+
+    - Bits[1:0] : Data access permission
+
+      - b'00 : No access
+      - b'01 : Read-Write access
+      - b'10 : Reserved
+      - b'11 : Read-only access
+
+    - Bit[2]: Instruction access permission
+
+      - b'0 : Executable
+      - b'1 : Non-executable
+
+    - Bit[30:3] : Reserved. SBZ.
+
+    - Bit[31]   : Must be 0
+
+    On failure the following error codes are returned:
+
+    - ``INVALID_PARAMETERS``: The Secure Partition is not allowed to access the
+      memory region the Base Address lies in.
+
+    - ``NOT_SUPPORTED`` : The SPM does not support retrieval of attributes of
+      any memory page that is accessible by the Secure Partition, or the
+      function was called from the Non-secure world. Also returned if it is
+      used after ``SP_EVENT_COMPLETE_AARCH64``.
+
+    See `Error Codes`_ for integer values that are associated with each return
+    code.
+
+- Usage
+
+  This function is used to request the permission attributes for S-EL0 on a
+  memory region accessible from a Secure Partition. The size of the memory
+  region is equal to the Translation Granule size used in the Secure EL1&0
+  translation regime. Requests to retrieve other memory region attributes are
+  not currently supported.
+
+- Caller responsibilities
+
+  The caller must obtain the Translation Granule Size of the Secure EL1&0
+  translation regime from the SPM through an implementation defined method.
+
+- Callee responsibilities
+
+  The SPM must not return the memory access controls for a page of memory that
+  is not accessible from a Secure Partition.
+
+``SP_MEMORY_ATTRIBUTES_SET_AARCH64``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+- Description
+
+  Set the permission attributes of a memory region from S-EL0.
+
+- Parameters
+
+  - **uint32** - Function ID
+
+    - SVC64 Version: **0xC4000065**
+
+  - **uint64** - Base Address
+
+    This parameter is a 64-bit Virtual Address (VA).
+
+    The alignment of the Base Address must be greater than or equal to the size
+    of the Translation Granule Size used in the Secure EL1&0 translation
+    regime.
+
+  - **uint32** - Page count
+
+    Number of pages starting from the Base Address whose memory attributes
+    should be changed. The page size is equal to the Translation Granule Size.
+
+  - **uint32** - Memory Access Controls
+
+    - Bits[1:0] : Data access permission
+
+      - b'00 : No access
+      - b'01 : Read-Write access
+      - b'10 : Reserved
+      - b'11 : Read-only access
+
+    - Bit[2] : Instruction access permission
+
+      - b'0 : Executable
+      - b'1 : Non-executable
+
+    - Bits[31:3] : Reserved. SBZ.
+
+    A combination of attributes that mark the region with RW and Executable
+    permissions is prohibited. A request to mark a device memory region with
+    Executable permissions is prohibited.
+
+- Return parameters
+
+  - **int32** - Return Code
+
+    - ``SUCCESS``: The Memory Access Controls were changed successfully.
+
+    - ``DENIED``: The SPM is servicing a request to change the attributes of a
+      memory region that overlaps with the region specified in this request.
+
+    - ``INVALID_PARAMETER``: An invalid combination of Memory Access Controls
+      has been specified. The Base Address is not correctly aligned. The Secure
+      Partition is not allowed to access part or all of the memory region
+      specified in the call.
+
+    - ``NO_MEMORY``: The SPM does not have memory resources to change the
+      attributes of the memory region in the translation tables.
+
+    - ``NOT_SUPPORTED``: The SPM does not permit change of attributes of any
+      memory region that is accessible by the Secure Partition. Function was
+      called from the Non-secure world. Also returned if it is used after
+      ``SP_EVENT_COMPLETE_AARCH64``.
+
+    See `Error Codes`_ for integer values that are associated with each return
+    code.
+
+- Usage
+
+  This function is used to change the permission attributes for S-EL0 on a
+  memory region accessible from a Secure Partition. The size of the memory
+  region is equal to the Translation Granule size used in the Secure EL1&0
+  translation regime. Requests to change other memory region attributes are not
+  currently supported.
+
+  This function is only available at boot time. This interface is revoked after
+  the Secure Partition sends the first ``SP_EVENT_COMPLETE_AARCH64`` to signal
+  that it is initialised and ready to receive run-time requests.
+
+- Caller responsibilities
+
+  The caller must obtain the Translation Granule Size of the Secure EL1&0
+  translation regime from the SPM through an implementation defined method.
+
+- Callee responsibilities
+
+  The SPM must preserve the original memory access controls of the region of
+  memory in case of an unsuccessful call.  The SPM must preserve the consistency
+  of the S-EL1 translation regime if this function is called on different PEs
+  concurrently and the memory regions specified overlap.
+
+Error Codes
+-----------
+
+.. csv-table::
+   :header: "Name", "Value"
+
+   ``SUCCESS``,0
+   ``NOT_SUPPORTED``,-1
+   ``INVALID_PARAMETER``,-2
+   ``DENIED``,-3
+   ``NO_MEMORY``,-5
+   ``NOT_PRESENT``,-7
+
+--------------
+
+*Copyright (c) 2017-2018, Arm Limited and Contributors. All rights reserved.*
+
+.. _Armv8-A ARM: https://developer.arm.com/docs/ddi0487/latest/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile
+.. _instructions in the EDK2 repository: https://github.com/tianocore/edk2-staging/blob/AArch64StandaloneMm/HowtoBuild.MD
+.. _Management Mode Interface Specification: http://infocenter.arm.com/help/topic/com.arm.doc.den0060a/DEN0060A_ARM_MM_Interface_Specification.pdf
+.. _SDEI Specification: http://infocenter.arm.com/help/topic/com.arm.doc.den0054a/ARM_DEN0054A_Software_Delegated_Exception_Interface.pdf
+.. _SMC Calling Convention: http://infocenter.arm.com/help/topic/com.arm.doc.den0028b/ARM_DEN0028B_SMC_Calling_Convention.pdf
+
+.. |Image 1| image:: ../diagrams/secure_sw_stack_tos.png
+.. |Image 2| image:: ../diagrams/secure_sw_stack_sp.png