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+SDEI: Software Delegated Exception Interface
+============================================
+
+.. contents::
+    :depth: 2
+
+This document provides an overview of the SDEI dispatcher implementation in
+Trusted Firmware-A (TF-A).
+
+Introduction
+------------
+
+`Software Delegated Exception Interface`_ (SDEI) is an Arm specification for
+Non-secure world to register handlers with firmware to receive notifications
+about system events. Firmware will first receive the system events by way of
+asynchronous exceptions and, in response, arranges for the registered handler to
+execute in the Non-secure EL.
+
+Normal world software that interacts with the SDEI dispatcher (makes SDEI
+requests and receives notifications) is referred to as the *SDEI Client*. A
+client receives the event notification at the registered handler even when it
+was executing with exceptions masked. The list of SDEI events available to the
+client are specific to the platform [#std-event]_. See also `Determining client
+EL`_.
+
+.. _general SDEI dispatch:
+
+The following figure depicts a general sequence involving SDEI client executing
+at EL2 and an event dispatch resulting from the triggering of a bound interrupt.
+A commentary is provided below:
+
+.. image:: ../plantuml/sdei_general.svg
+
+As part of initialisation, the SDEI client binds a Non-secure interrupt [1], and
+the SDEI dispatcher returns a platform dynamic event number [2]. The client then
+registers a handler for that event [3], enables the event [5], and unmasks all
+events on the current PE [7]. This sequence is typical of an SDEI client, but it
+may involve additional SDEI calls.
+
+At a later point in time, when the bound interrupt triggers [9], it's trapped to
+EL3. The interrupt is handed over to the SDEI dispatcher, which then arranges to
+execute the registered handler [10]. The client terminates its execution with
+``SDEI_EVENT_COMPLETE`` [11], following which the dispatcher resumes the
+original EL2 execution [13]. Note that the SDEI interrupt remains active until
+the client handler completes, at which point EL3 does EOI [12].
+
+Other than events bound to interrupts (as depicted in the sequence above, SDEI
+events can be explicitly dispatched in response to other exceptions, for
+example, upon receiving an *SError* or *Synchronous External Abort*. See
+`Explicit dispatch of events`_.
+
+The remainder of this document only discusses the design and implementation of
+SDEI dispatcher in TF-A, and assumes that the reader is familiar with the SDEI
+specification, the interfaces, and their requirements.
+
+.. [#std-event] Except event 0, which is defined by the SDEI specification as a
+                standard event.
+
+Defining events
+---------------
+
+A platform choosing to include the SDEI dispatcher must also define the events
+available on the platform, along with their attributes.
+
+The platform is expected to provide two arrays of event descriptors: one for
+private events, and another for shared events. The SDEI dispatcher provides
+``SDEI_PRIVATE_EVENT()`` and ``SDEI_SHARED_EVENT()`` macros to populate the
+event descriptors. Both macros take 3 arguments:
+
+-  The event number: this must be a positive 32-bit integer.
+
+-  For an event that has a backing interrupt, the interrupt number the event is
+   bound to:
+
+   - If it's not applicable to an event, this shall be left as ``0``.
+
+   - If the event is dynamic, this should be specified as ``SDEI_DYN_IRQ``.
+
+-  A bit map of `Event flags`_.
+
+To define event 0, the macro ``SDEI_DEFINE_EVENT_0()`` should be used. This
+macro takes only one parameter: an SGI number to signal other PEs.
+
+To define an event that's meant to be `explicitly dispatched`__ (i.e., not as a
+result of receiving an SDEI interrupt), the macro ``SDEI_EXPLICIT_EVENT()``
+should be used. It accepts two parameters:
+
+.. __: `Explicit dispatch of events`_
+
+-  The event number (as above);
+
+-  Event priority: ``SDEI_MAPF_CRITICAL`` or ``SDEI_MAPF_NORMAL``, as described
+   below.
+
+Once the event descriptor arrays are defined, they should be exported to the
+SDEI dispatcher using the ``REGISTER_SDEI_MAP()`` macro, passing it the pointers
+to the private and shared event descriptor arrays, respectively. Note that the
+``REGISTER_SDEI_MAP()`` macro must be used in the same file where the arrays are
+defined.
+
+Regarding event descriptors:
+
+-  For Event 0:
+
+   - There must be exactly one descriptor in the private array, and none in the
+     shared array.
+
+   - The event should be defined using ``SDEI_DEFINE_EVENT_0()``.
+
+   - Must be bound to a Secure SGI on the platform.
+
+-  Explicit events should only be used in the private array.
+
+-  Statically bound shared and private interrupts must be bound to shared and
+   private interrupts on the platform, respectively. See the section on
+   `interrupt configuration`__.
+
+   .. __: `Configuration within Exception Handling Framework`_
+
+-  Both arrays should be one-dimensional. The ``REGISTER_SDEI_MAP()`` macro
+   takes care of replicating private events for each PE on the platform.
+
+-  Both arrays must be sorted in the increasing order of event number.
+
+The SDEI specification doesn't have provisions for discovery of available events
+on the platform. The list of events made available to the client, along with
+their semantics, have to be communicated out of band; for example, through
+Device Trees or firmware configuration tables.
+
+See also `Event definition example`_.
+
+Event flags
+~~~~~~~~~~~
+
+Event flags describe the properties of the event. They are bit maps that can be
+``OR``\ ed to form parameters to macros that `define events`__.
+
+.. __: `Defining events`_
+
+-  ``SDEI_MAPF_DYNAMIC``: Marks the event as dynamic. Dynamic events can be
+   bound to (or released from) any Non-secure interrupt at runtime via the
+   ``SDEI_INTERRUPT_BIND`` and ``SDEI_INTERRUPT_RELEASE`` calls.
+
+-  ``SDEI_MAPF_BOUND``: Marks the event as statically bound to an interrupt.
+   These events cannot be re-bound at runtime.
+
+-  ``SDEI_MAPF_NORMAL``: Marks the event as having *Normal* priority. This is
+   the default priority.
+
+-  ``SDEI_MAPF_CRITICAL``: Marks the event as having *Critical* priority.
+
+Event definition example
+------------------------
+
+.. code:: c
+
+   static sdei_ev_map_t plat_private_sdei[] = {
+        /* Event 0 definition */
+        SDEI_DEFINE_EVENT_0(8),
+
+        /* PPI */
+        SDEI_PRIVATE_EVENT(8, 23, SDEI_MAPF_BOUND),
+
+        /* Dynamic private events */
+        SDEI_PRIVATE_EVENT(100, SDEI_DYN_IRQ, SDEI_MAPF_DYNAMIC),
+        SDEI_PRIVATE_EVENT(101, SDEI_DYN_IRQ, SDEI_MAPF_DYNAMIC)
+
+        /* Events for explicit dispatch */
+        SDEI_EXPLICIT_EVENT(2000, SDEI_MAPF_NORMAL);
+        SDEI_EXPLICIT_EVENT(2000, SDEI_MAPF_CRITICAL);
+   };
+
+   /* Shared event mappings */
+   static sdei_ev_map_t plat_shared_sdei[] = {
+        SDEI_SHARED_EVENT(804, 0, SDEI_MAPF_DYNAMIC),
+
+        /* Dynamic shared events */
+        SDEI_SHARED_EVENT(3000, SDEI_DYN_IRQ, SDEI_MAPF_DYNAMIC),
+        SDEI_SHARED_EVENT(3001, SDEI_DYN_IRQ, SDEI_MAPF_DYNAMIC)
+   };
+
+   /* Export SDEI events */
+   REGISTER_SDEI_MAP(plat_private_sdei, plat_shared_sdei);
+
+Configuration within Exception Handling Framework
+-------------------------------------------------
+
+The SDEI dispatcher functions alongside the Exception Handling Framework. This
+means that the platform must assign priorities to both Normal and Critical SDEI
+interrupts for the platform:
+
+-  Install priority descriptors for Normal and Critical SDEI interrupts.
+
+-  For those interrupts that are statically bound (i.e. events defined as having
+   the ``SDEI_MAPF_BOUND`` property), enumerate their properties for the GIC
+   driver to configure interrupts accordingly.
+
+   The interrupts must be configured to target EL3. This means that they should
+   be configured as *Group 0*.  Additionally, on GICv2 systems, the build option
+   ``GICV2_G0_FOR_EL3`` must be set to ``1``.
+
+See also `SDEI porting requirements`_.
+
+Determining client EL
+---------------------
+
+The SDEI specification requires that the *physical* SDEI client executes in the
+highest Non-secure EL implemented on the system. This means that the dispatcher
+will only allow SDEI calls to be made from:
+
+-  EL2, if EL2 is implemented. The Hypervisor is expected to implement a
+   *virtual* SDEI dispatcher to support SDEI clients in Guest Operating Systems
+   executing in Non-secure EL1.
+
+-  Non-secure EL1, if EL2 is not implemented or disabled.
+
+See the function ``sdei_client_el()`` in ``sdei_private.h``.
+
+Explicit dispatch of events
+---------------------------
+
+Typically, an SDEI event dispatch is caused by the PE receiving interrupts that
+are bound to an SDEI event. However, there are cases where the Secure world
+requires dispatch of an SDEI event as a direct or indirect result of a past
+activity, such as receiving a Secure interrupt or an exception.
+
+The SDEI dispatcher implementation provides ``sdei_dispatch_event()`` API for
+this purpose. The API has the following signature:
+
+::
+
+        int sdei_dispatch_event(int ev_num);
+
+The parameter ``ev_num`` is the event number to dispatch. The API returns ``0``
+on success, or ``-1`` on failure.
+
+The following figure depicts a scenario involving explicit dispatch of SDEI
+event. A commentary is provided below:
+
+.. image:: ../plantuml/sdei_explicit_dispatch.svg
+
+As part of initialisation, the SDEI client registers a handler for a platform
+event [1], enables the event [3], and unmasks the current PE [5]. Note that,
+unlike in `general SDEI dispatch`_, this doesn't involve interrupt binding, as
+bound or dynamic events can't be explicitly dispatched (see the section below).
+
+At a later point in time, a critical event [#critical-event]_ is trapped into
+EL3 [7]. EL3 performs a first-level triage of the event, and a RAS component
+assumes further handling [8]. The dispatch completes, but intends to involve
+Non-secure world in further handling, and therefore decides to explicitly
+dispatch an event [10] (which the client had already registered for [1]). The
+rest of the sequence is similar to that in the `general SDEI dispatch`_: the
+requested event is dispatched to the client (assuming all the conditions are
+met), and when the handler completes, the preempted execution resumes.
+
+.. [#critical-event] Examples of critical event are *SError*, *Synchronous
+                     External Abort*, *Fault Handling interrupt*, or *Error
+                     Recovery interrupt* from one of RAS nodes in the system.
+
+Conditions for event dispatch
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+All of the following requirements must be met for the API to return ``0`` and
+event to be dispatched:
+
+-  SDEI events must be unmasked on the PE. I.e. the client must have called
+   ``PE_UNMASK`` beforehand.
+
+-  Event 0 can't be dispatched.
+
+-  The event must be declared using the ``SDEI_EXPLICIT_EVENT()`` macro
+   described above.
+
+-  The event must be private to the PE.
+
+-  The event must have been registered for and enabled.
+
+-  A dispatch for the same event must not be outstanding. I.e. it hasn't already
+   been dispatched and is yet to be completed.
+
+-  The priority of the event (either Critical or Normal, as configured by the
+   platform at build-time) shouldn't cause priority inversion. This means:
+
+   -  If it's of Normal priority, neither Normal nor Critical priority dispatch
+      must be outstanding on the PE.
+
+   -  If it's of a Critical priority, no Critical priority dispatch must be
+      outstanding on the PE.
+
+Further, the caller should be aware of the following assumptions made by the
+dispatcher:
+
+-  The caller of the API is a component running in EL3; for example, a RAS
+   driver.
+
+-  The requested dispatch will be permitted by the Exception Handling Framework.
+   I.e. the caller must make sure that the requested dispatch has sufficient
+   priority so as not to cause priority level inversion within Exception
+   Handling Framework.
+
+-  The caller must be prepared for the SDEI dispatcher to restore the Non-secure
+   context, and mark that the active context.
+
+-  The call will block until the SDEI client completes the event (i.e. when the
+   client calls either ``SDEI_EVENT_COMPLETE`` or ``SDEI_COMPLETE_AND_RESUME``).
+
+-  The caller must be prepared for this API to return failure and handle
+   accordingly.
+
+Porting requirements
+--------------------
+
+The porting requirements of the SDEI dispatcher are outlined in the `porting
+guide`__.
+
+.. __: `SDEI porting requirements`_
+
+Note on writing SDEI event handlers
+-----------------------------------
+
+*This section pertains to SDEI event handlers in general, not just when using
+the TF-A SDEI dispatcher.*
+
+The SDEI specification requires that event handlers preserve the contents of all
+registers except ``x0`` to ``x17``. This has significance if event handler is
+written in C: compilers typically adjust the stack frame at the beginning and
+end of C functions. For example, AArch64 GCC typically produces the following
+function prologue and epilogue:
+
+::
+
+        c_event_handler:
+                stp     x29, x30, [sp,#-32]!
+                mov     x29, sp
+
+                ...
+
+                bl      ...
+
+                ...
+
+                ldp     x29, x30, [sp],#32
+                ret
+
+The register ``x29`` is used as frame pointer in the prologue. Because neither a
+valid ``SDEI_EVENT_COMPLETE`` nor ``SDEI_EVENT_COMPLETE_AND_RESUME`` calls
+return to the handler, the epilogue never gets executed, and registers ``x29``
+and ``x30`` (in the case above) are inadvertently corrupted. This violates the
+SDEI specification, and the normal execution thereafter will result in
+unexpected behaviour.
+
+To work this around, it's advised that the top-level event handlers are
+implemented in assembly, following a similar pattern as below:
+
+::
+
+        asm_event_handler:
+                /* Save link register whilst maintaining stack alignment */
+                stp     xzr, x30, [sp, #-16]!
+                bl      c_event_handler
+
+                /* Restore link register */
+                ldp     xzr, x30, [sp], #16
+
+                /* Complete call */
+                ldr     x0, =SDEI_EVENT_COMPLETE
+                smc     #0
+                b       .
+
+----
+
+*Copyright (c) 2017-2018, Arm Limited and Contributors. All rights reserved.*
+
+.. _SDEI specification: http://infocenter.arm.com/help/topic/com.arm.doc.den0054a/ARM_DEN0054A_Software_Delegated_Exception_Interface.pdf
+.. _SDEI porting requirements: ../getting_started/porting-guide.rst#sdei-porting-requirements