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diff --git a/docs/developer/service-descriptions/index.rst b/docs/developer/service-descriptions/index.rst
index 5962e37..6574a3e 100644
--- a/docs/developer/service-descriptions/index.rst
+++ b/docs/developer/service-descriptions/index.rst
@@ -8,9 +8,10 @@
     attest-service-description
     crypto-service-description
     secure-storage-service-description
+    uefi-smm-services
 
 --------------
 
-*Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2020-2022, Arm Limited and Contributors. All rights reserved.*
 
 SPDX-License-Identifier: BSD-3-Clause
diff --git a/docs/developer/service-descriptions/uefi-smm-services.rst b/docs/developer/service-descriptions/uefi-smm-services.rst
new file mode 100644
index 0000000..3795063
--- /dev/null
+++ b/docs/developer/service-descriptions/uefi-smm-services.rst
@@ -0,0 +1,243 @@
+UEFI SMM Services
+=================
+The Trusted Services project provides support for UEFI System Management Mode (SMM) services via the
+SMM Gateway secure partition. The SMM Gateway adopts the API Gateway design pattern, popular in
+microservices architecture. The pattern decouples clients from backend service providers using an
+API gateway that presents a domain specific interface to clients while delegating operations to a
+set of backend microservices. An API gateway will typically use multiple backend services and may
+perform protocol translation while presenting a single service entry point for clients. The SMM
+Gateway works in a similar manner - clients access SMM services using standard SMM protocol messages,
+carried by an RPC mechanism. Service requests are forwarded by the SMM Gateway to backend service
+providers for operations such as secure persistent storage and signature verification.
+
+SMM Gateway is intended to be used on non-EDK2 platforms as an alternative to the EDK2 StandaloneMM
+(StMM) component. The current SMM Gateway version only supports the SMM Variable service. Additional
+SMM service providers may be added to SMM Gateway if required. By deliberately limiting functionality
+and exploiting backend services, the SMM Gateway SP can be significantly lighter-weight than StMM.
+This option is intended to be used on more resource constrained devices that tend to use u-boot.
+There is of course the possibility that other SMM services will need to be supported in the future.
+In such cases, a judgement should be made as to whether StMM should be used rather than extending the SP.
+
+.. uml:: uml/SmmGatewayOverview.puml
+
+SMM Variable Service
+--------------------
+Overview
+''''''''
+UEFI Variable support is provided by the *smm_variable* service provider component. This service provider
+is structured in the same way as other service providers within the TS project. Features of this
+component are:
+
+  * Source file location:  ``components/service/smm_variable``
+  * Public interface definitions: ``protocols/service/smm_variable``
+  * Can be used with any RPC layer - not tied to MM Communicate RPC.
+  * Volatile and non-volatile storage is accessed via instances of the common *storage_backend* interface.
+
+The *smm-gateway/opteesp* deployment integrates the *smm_variable* service provider with the following:
+
+  * An MM Communicate based RPC endpoint.
+  * A *mock_store* instance for volatile variables.
+  * A *secure_storage_client* for non-volatile variables.
+
+During SP initialization, the *smm-gateway* uses pre-configured information to discover a backend secure
+storage SP for NV storage.
+
+The following diagram illustrates how the *smm_variable* service provider is integrated into the *smm-gateway*.
+
+.. image:: image/smm-gateway-layers.svg
+
+Because the *smm_variable* service provider is independent of any particular environment, alternative deployments
+are possible e.g.
+
+  * *smm_variable* service provider running within a GP TA with storage off-loaded to the GP TEE Internal API.
+  * *smm_variable* service provider running within a secure enclave with its own internal flash storage.
+
+Supported Functions
+'''''''''''''''''''
+The *smm_variable* service provider supports the following functions:
+
+.. list-table::
+  :header-rows: 1
+
+  * - SMM Variable Function
+    - Purpose
+    - Backend service interaction
+  * - SMM_VARIABLE_FUNCTION_GET_VARIABLE
+    - Get variable data identified by GUID/name.
+    - Query index and get object from appropriate storage backend.
+  * - SMM_VARIABLE_FUNCTION_GET_NEXT_VARIABLE_NAME
+    - Called multiple times to enumerate stored variables.
+    - Find variable in index and return next.
+  * - SMM_VARIABLE_FUNCTION_SET_VARIABLE
+    - Adds a new variable or updates an existing one.
+    - | Sets object in storage backend and if necessary, updates index
+      | and syncs to storage.
+  * - SMM_VARIABLE_FUNCTION_QUERY_VARIABLE_INFO
+    - Returns information about the variable store.
+    - Iterates over stored variables to determine space used.
+  * - SMM_VARIABLE_FUNCTION_EXIT_BOOT_SERVICE
+    - Called by OS when boot phase is complete.
+    - | Updates view of runtime state held by smm_variable service provider.
+      | State variable used when implementing state dependent access control.
+  * - SMM_VARIABLE_FUNCTION_VAR_CHECK_VARIABLE_PROPERTY_SET
+    - | Set constraints that are checked on the SetVariable operation.
+      | Allows a platform to set check policy.
+    - | Variable index holds variable check constraints object for each variable.
+      | This is updated by this function.
+  * - SMM_VARIABLE_FUNCTION_VAR_CHECK_VARIABLE_PROPERTY_GET
+    - Get the variable check constraints.
+    - Reads the variable check constraints object.
+  * - SMM_VARIABLE_FUNCTION_GET_PAYLOAD_SIZE
+    - | Returns the maximum variable data size, excluding any
+      | auth header.
+    - | Considers size constraints imposed by backend stores and RPC response
+      | payload constraints.
+
+Supported Variable Attributes
+'''''''''''''''''''''''''''''
+The following variable attributes are supported:
+
+.. list-table::
+  :widths: 3 1 3
+  :header-rows: 1
+
+  * - SMM Variable Attribute
+    - Support
+    - Comment
+  * - EFI_VARIABLE_NON_VOLATILE
+    - yes
+    - Determines which storage backend is used.
+  * - EFI_VARIABLE_BOOTSERVICE_ACCESS
+    - yes
+    - Boot service access controlled by smm_variable service provider.
+  * - EFI_VARIABLE_RUNTIME_ACCESS
+    - yes
+    - Runtime access controlled by smm_variable service provider.
+  * - EFI_VARIABLE_HARDWARE_ERROR_RECORD
+    - no
+    -
+  * - EFI_VARIABLE_AUTHENTICATED_WRITE_ACCESS
+    - no
+    -
+  * - EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS
+    - not yet
+    - Will be needed for secure boot support
+  * - EFI_VARIABLE_APPEND_WRITE
+    - yes
+    - Implemented by overwriting entire variable data.
+
+SMM Variable Tests
+''''''''''''''''''
+The following test components exist for the SMM Variable service:
+
+.. list-table::
+  :header-rows: 1
+
+  * - Test Component
+    - Description
+    - Included in deployments
+  * - ``component/service/smm_variable/backend/test``
+    - | Component tests for the variable_index and variable_store backend
+      | components. Can be run in a native PC environment.
+    - ``deployments/component-test/*``
+  * - ``component/service/smm_variable/test/service``
+    - | End-to-end service level tests that call service operations from
+      | the perspective of a client.  Can be run in a native PC environment
+      | or on the Arm target platform.
+    - | ``deployments/ts-service-test/linux-pc``
+      | ``deployments/uefi-test/arm-linux``
+
+SMM Gateway Build Configuration
+-------------------------------
+The smm-gateway SP image may be built using the default configuration parameters defined
+within relevant source files. In practice, it is likely that at least some configuration
+values will need to be overridden. The following table lists build-time configuration
+parameters that may be overridden by global C pre-processor defines.
+
+.. list-table::
+  :widths: 2 2 2 1
+  :header-rows: 1
+
+  * - Config define
+    - Usage
+    - File
+    - Default value
+  * - SMM_GATEWAY_MAX_UEFI_VARIABLES
+    - Maximum number of variables
+    - ``deployments/smm-gateway/smm_gateway.c``
+    - 40
+  * - SMM_GATEWAY_NV_STORE_SN
+    - The service ID for the backend NV variable store
+    - ``deployments/smm-gateway/smm_gateway.c``
+    - Protected Storage SP
+
+MM Communicate RPC Layer
+------------------------
+To maintain compatibility with existing SMM service clients, an MM Communicate based RPC
+layer has been developed that uses the same 'carveout' buffer scheme as StMM. When SMM
+Gateway is used instead of StMM, existing SMM variable clients should interoperate seamlessly.
+The MM Communicate RPC components implement the standard TS RPC interfaces and can be used as
+a general purpose RPC for calls from normal world to secure world. The following MM Communicate
+RPC components have been added:
+
+  * ``components/rpc/mm_communicate/endpoint/sp`` - an RPC endpoint that handles FFA direct
+    calls with MM Communicate and SMM message carried in a shared 'carveout' buffer. Call requests
+    are demultiplexed to the appropriate service interface based on the service GUID carried in
+    the MM Communicate header.  Suitable for use in SP deployments.
+  * ``components/rpc/mm_communicate/caller/linux`` - an RPC caller that calls service operations
+    associated with the destination service interface from Linux user-space. Uses the MM Communicate
+    protocol, sent over FFA using the Debug FFA kernel driver.  Service level tests that run against
+    the SMM Gateway use this RPC caller for invoking SMM service operations.
+
+The following register mapping is assumed for FFA based direct calls to an SP that handles the MM
+Communicate RPC protocol:
+
+.. list-table::
+  :widths: 1 2 2 2
+  :header-rows: 1
+
+  * - Registers
+    - FF-A layer
+    - MM_COMMUNICATE Request
+    - MM_COMMUNICATE Response
+  * - W0
+    - Function ID
+    - | FFA_MSG_SEND_DIRECT_REQ
+      | (0x8400006F/0xC400006F)
+    - | FFA_MSG_SEND_DIRECT_RESP
+      | (0x84000070/0xC4000070)
+  * - W1
+    - Source/Destination ID
+    - Source/Destination ID
+    - Source/Destination ID
+  * - W2/X2
+    - Reserved
+    - 0x00000000
+    - 0x00000000
+  * - W3/X3
+    - Parameter[0]
+    - Address of the MM communication buffer
+    - | ARM_SVC_ID_SP_EVENT_COMPLETE
+      | (0x84000061/0xC4000061)
+  * - W4/X4
+    - Parameter[1]
+    - Size of the MM communication buffer
+    - SUCCESS/[error code]
+  * - W5/X5
+    - Parameter[2]
+    - 0x00000000
+    - 0x00000000
+  * - W6/X6
+    - Parameter[3]
+    - 0x00000000
+    - 0x00000000
+  * - W7/X7
+    - Parameter[4]
+    - 0x00000000
+    - 0x00000000
+
+--------------
+
+*Copyright (c) 2021-2022, Arm Limited and Contributors. All rights reserved.*
+
+SPDX-License-Identifier: BSD-3-Clause
diff --git a/docs/developer/service-descriptions/uml/SmmGatewayOverview.puml b/docs/developer/service-descriptions/uml/SmmGatewayOverview.puml
new file mode 100644
index 0000000..f53d2a4
--- /dev/null
+++ b/docs/developer/service-descriptions/uml/SmmGatewayOverview.puml
@@ -0,0 +1,14 @@
+'-------------------------------------------------------------------------------
+' Copyright (c) 2021-2022, Arm Limited and Contributors. All rights reserved.
+'
+' SPDX-License-Identifier: BSD-3-Clause
+'
+'-------------------------------------------------------------------------------
+
+@startuml
+
+[u-boot efi services] -down- [smm gateway]
+[smm gateway] -down- [secure storage service]
+[smm gateway] -down- [crypto service]
+
+@enduml