docs/components/sdei.rst - filogic/atf - Gitiles

 SDEI: Software Delegated Exception Interface
 ============================================

 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:: ../resources/diagrams/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:

 .. code:: c

         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:: ../resources/diagrams/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
 .. _Software Delegated Exception Interface: `SDEI specification`_
	SDEI: Software Delegated Exception Interface
	============================================

	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:: ../resources/diagrams/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:

	.. code:: c

	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:: ../resources/diagrams/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
	.. _Software Delegated Exception Interface: `SDEI specification`_