Exception Handling Framework in Trusted Firmware-A
==================================================


.. section-numbering::
    :suffix: .

.. contents::
    :depth: 2

.. |EHF| replace:: Exception Handling Framework
.. |TF-A| replace:: Trusted Firmware-A

This document describes various aspects of handling exceptions by Runtime
Firmware (BL31) that are targeted at EL3, other than SMCs. The |EHF| takes care
of the following exceptions when targeted at EL3:

-  Interrupts
-  Synchronous External Aborts
-  Asynchronous External Aborts

|TF-A|'s handling of synchronous ``SMC`` exceptions raised from lower ELs is
described in the `Firmware Design document`__. However, the |EHF| changes the
semantics of `interrupt handling`__ and `synchronous exceptions`__ other than
SMCs.

.. __: firmware-design.rst#handling-an-smc
.. __: `Interrupt handling`_
.. __: `Effect on SMC calls`_

The |EHF| is selected by setting the build option ``EL3_EXCEPTION_HANDLING`` to
``1``, and is only available for AArch64 systems.

Introduction
------------

Through various control bits in the ``SCR_EL3`` register, the Arm architecture
allows for asynchronous exceptions to be routed to EL3. As described in the
`Interrupt Framework Design`_ document, depending on the chosen interrupt
routing model, TF-A appropriately sets the ``FIQ`` and ``IRQ`` bits of
``SCR_EL3`` register to effect this routing. For most use cases, other than for
the purpose of facilitating context switch between Normal and Secure worlds,
FIQs and IRQs routed to EL3 are not required to be handled in EL3.

However, the evolving system and standards landscape demands that various
exceptions are targeted at and handled in EL3. For instance:

-  Starting with ARMv8.2 architecture extension, many RAS features have been
   introduced to the Arm architecture. With RAS features implemented, various
   components of the system may use one of the asynchronous exceptions to signal
   error conditions to PEs. These error conditions are of critical nature, and
   it's imperative that corrective or remedial actions are taken at the earliest
   opportunity. Therefore, a *Firmware-first Handling* approach is generally
   followed in response to RAS events in the system.

-  The Arm `SDEI specification`_ defines interfaces through which Normal world
   interacts with the Runtime Firmware in order to request notification of
   system events. The SDEI specification requires that these events are notified
   even when the Normal world executes with the exceptions masked. This too
   implies that firmware-first handling is required, where the events are first
   received by the EL3 firmware, and then dispatched to Normal world through
   purely software mechanism.

For |TF-A|, firmware-first handling means that asynchronous exceptions are
suitably routed to EL3, and the Runtime Firmware (BL31) is extended to include
software components that are capable of handling those exceptions that target
EL3. These components—referred to as *dispatchers* [#spd]_ in general—may
choose to:

.. _delegation-use-cases:

-  Receive and handle exceptions entirely in EL3, meaning the exceptions
   handling terminates in EL3.

-  Receive exceptions, but handle part of the exception in EL3, and delegate the
   rest of the handling to a dedicated software stack running at lower Secure
   ELs. In this scheme, the handling spans various secure ELs.

-  Receive exceptions, but handle part of the exception in EL3, and delegate
   processing of the error to dedicated software stack running at lower secure
   ELs (as above); additionally, the Normal world may also be required to
   participate in the handling, or be notified of such events (for example, as
   an SDEI event). In this scheme, exception handling potentially and maximally
   spans all ELs in both Secure and Normal worlds.

On any given system, all of the above handling models may be employed
independently depending on platform choice and the nature of the exception
received.

.. [#spd] Not to be confused with `Secure Payload Dispatcher`__, which is an
   EL3 component that operates in EL3 on behalf of Secure OS.

.. __: firmware-design.rst#secure-el1-payloads-and-dispatchers

The role of Exception Handling Framework
----------------------------------------

Corollary to the use cases cited above, the primary role of the |EHF| is to
facilitate firmware-first handling of exceptions on Arm systems. The |EHF| thus
enables multiple exception dispatchers in runtime firmware to co-exist, register
for, and handle exceptions targeted at EL3. This section outlines the basics,
and the rest of this document expands the various aspects of the |EHF|.

In order to arbitrate exception handling among dispatchers, the |EHF| operation
is based on a priority scheme. This priority scheme is closely tied to how the
Arm GIC architecture defines it, although it's applied to non-interrupt
exceptions too (SErrors, for example).

The platform is required to `partition`__ the Secure priority space into
priority levels as applicable for the Secure software stack. It then assigns the
dispatchers to one or more priority levels. The dispatchers then register
handlers for the priority levels at runtime. A dispatcher can register handlers
for more than one priority level.

.. __: `Partitioning priority levels`_


.. _ehf-figure:

.. image:: draw.io/ehf.svg

A priority level is *active* when a handler at that priority level is currently
executing in EL3, or has delegated the execution to a lower EL. For interrupts,
this is implicit when an interrupt is targeted and acknowledged at EL3, and the
priority of the acknowledged interrupt is used to match its registered handler.
The priority level is likewise implicitly deactivated when the interrupt
handling concludes by EOIing the interrupt.

Non-interrupt exceptions (SErrors, for example) don't have a notion of priority.
In order for the priority arbitration to work, the |EHF| provides APIs in order
for these non-interrupt exceptions to assume a priority, and to interwork with
interrupts. Dispatchers handling such exceptions must therefore explicitly
activate and deactivate the respective priority level as and when they're
handled or delegated.

Because priority activation and deactivation for interrupt handling is implicit
and involves GIC priority masking, it's impossible for a lower priority
interrupt to preempt a higher priority one. By extension, this means that a
lower priority dispatcher cannot preempt a higher-priority one. Priority
activation and deactivation for non-interrupt exceptions, however, has to be
explicit. The |EHF| therefore disallows for lower priority level to be activated
whilst a higher priority level is active, and would result in a panic.
Likewise, a panic would result if it's attempted to deactivate a lower priority
level when a higher priority level is active.

In essence, priority level activation and deactivation conceptually works like a
stack—priority levels stack up in strictly increasing fashion, and need to be
unstacked in strictly the reverse order. For interrupts, the GIC ensures this is
the case; for non-interrupts, the |EHF| monitors and asserts this. See
`Transition of priority levels`_.

Interrupt handling
------------------

The |EHF| is a client of *Interrupt Management Framework*, and registers the
top-level handler for interrupts that target EL3, as described in the `Interrupt
Framework Design`_ document. This has the following implications.

-  On GICv3 systems, when executing in S-EL1, pending Non-secure interrupts of
   sufficient priority are signalled as FIQs, and therefore will be routed to
   EL3. As a result, S-EL1 software cannot expect to handle Non-secure
   interrupts at S-EL1. Essentially, this deprecates the routing mode described
   as `CSS=0, TEL3=0`__.

   .. __: interrupt-framework-design.rst#el3-interrupts

   In order for S-EL1 software to handle Non-secure interrupts while having
   |EHF| enabled, the dispatcher must adopt a model where Non-secure interrupts
   are received at EL3, but are then `synchronously`__ handled over to S-EL1.

   .. __: interrupt-framework-design.rst#secure-payload

-  On GICv2 systems, it's required that the build option ``GICV2_G0_FOR_EL3`` is
   set to ``1`` so that *Group 0* interrupts target EL3.

-  While executing in Secure world, |EHF| sets GIC Priority Mask Register to the
   lowest Secure priority. This means that no Non-secure interrupts can preempt
   Secure execution. See `Effect on SMC calls`_ for more details.

As mentioned above, with |EHF|, the platform is required to partition *Group 0*
interrupts into distinct priority levels. A dispatcher that chooses to receive
interrupts can then *own* one or more priority levels, and register interrupt
handlers for them. A given priority level can be assigned to only one handler. A
dispatcher may register more than one priority level.

Dispatchers are assigned interrupt priority levels in two steps:

Partitioning priority levels
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Interrupts are associated to dispatchers by way of grouping and assigning
interrupts to a priority level. In other words, all interrupts that are to
target a particular dispatcher should fall in a particular priority level. For
priority assignment:

-  Of the 8 bits of priority that Arm GIC architecture permits, bit 7 must be 0
   (secure space).

-  Depending on the number of dispatchers to support, the platform must choose
   to use the top *n* of the 7 remaining bits to identify and assign interrupts
   to individual dispatchers. Choosing *n* bits supports up to 2\ :sup:`n`
   distinct dispatchers. For example, by choosing 2 additional bits (i.e., bits
   6 and 5), the platform can partition into 4 secure priority ranges: ``0x0``,
   ``0x20``, ``0x40``, and ``0x60``. See `Interrupt handling example`_.

Note:

   The Arm GIC architecture requires that a GIC implementation that supports two
   security states must implement at least 32 priority levels; i.e., at least 5
   upper bits of the 8 bits are writeable. In the scheme described above, when
   choosing *n* bits for priority range assignment, the platform must ensure
   that at least ``n+1`` top bits of GIC priority are writeable.

The priority thus assigned to an interrupt is also used to determine the
priority of delegated execution in lower ELs. Delegated execution in lower EL is
associated with a priority level chosen with ``ehf_activate_priority()`` API
(described `later`__). The chosen priority level also determines the interrupts
masked while executing in a lower EL, therefore controls preemption of delegated
execution.

.. __: `ehf-apis`_

The platform expresses the chosen priority levels by declaring an array of
priority level descriptors. Each entry in the array is of type
``ehf_pri_desc_t``, and declares a priority level, and shall be populated by the
``EHF_PRI_DESC()`` macro.

Note:

   The macro ``EHF_PRI_DESC()`` installs the descriptors in the array at a
   computed index, and not necessarily where the macro is placed in the array.
   The size of the array might therefore be larger than what it appears to be.
   The ``ARRAY_SIZE()`` macro therefore should be used to determine the size of
   array.

Finally, this array of descriptors is exposed to |EHF| via. the
``EHF_REGISTER_PRIORITIES()`` macro.

Refer to the `Interrupt handling example`_ for usage. See also: `Interrupt
Prioritisation Considerations`_.

Programming priority
~~~~~~~~~~~~~~~~~~~~

The text in `Partitioning priority levels`_ only describes how the platform
expresses the required levels of priority. It however doesn't choose interrupts
nor program the required priority in GIC.

The `Firmware Design guide`__ explains methods for configuring secure
interrupts. |EHF| requires the platform to enumerate interrupt properties (as
opposed to just numbers) of Secure interrupts. The priority of secure interrupts
must match that as determined in the `Partitioning priority levels`_ section above.

.. __: firmware-design.rst#configuring-secure-interrupts

See `Limitations`_, and also refer to `Interrupt handling example`_ for
illustration.

Registering handler
-------------------

Dispatchers register handlers for their priority levels through the following
API:

.. code:: c

   int ehf_register_priority_handler(int pri, ehf_handler_t handler)

The API takes two arguments:

-  The priority level for which the handler is being registered;

-  The handler to be registered. The handler must be aligned to 4 bytes.

If a dispatcher owns more than one priority levels, it has to call the API for
each of them.

The API will succeed, and return ``0``, only if:

-  There exists a descriptor with the priority level requested.

-  There are no handlers already registered by a previous call to the API.

Otherwise, the API returns ``-1``.

The interrupt handler should have the following signature:

.. code:: c

   typedef int (*ehf_handler_t)(uint32_t intr_raw, uint32_t flags, void *handle,
                   void *cookie);

The parameters are as obtained from the top-level `EL3 interrupt handler`__.

.. __: interrupt-framework-design.rst#el3-runtime-firmware

The `SDEI dispatcher`__, for example, expects the platform to allocate two
different priority levels—``PLAT_SDEI_CRITICAL_PRI``, and
``PLAT_SDEI_NORMAL_PRI``—and registers the same handler to handle both levels.

.. __: sdei.rst

Interrupt handling example
--------------------------

The following annotated snippet demonstrates how a platform might choose to
assign interrupts to fictitious dispatchers:

.. code:: c

   #include <common/interrupt_props.h>
   #include <drivers/arm/gic_common.h>
   #include <exception_mgmt.h>

   ...

   /*
    * This platform uses 2 bits for interrupt association. In total, 3 upper
    * bits are in use.
    *
    *  7 6 5   3      0
    * .-.-.-.----------.
    * |0|b|b|  ..0..   |
    * '-'-'-'----------'
    */
   #define PLAT_PRI_BITS        2

   /* Priorities for individual dispatchers */
   #define DISP0_PRIO           0x00 /* Not used */
   #define DISP1_PRIO           0x20
   #define DISP2_PRIO           0x40
   #define DISP3_PRIO           0x60

   /* Install priority level descriptors for each dispatcher */
   ehf_pri_desc_t plat_exceptions[] = {
        EHF_PRI_DESC(PLAT_PRI_BITS, DISP1_PRIO),
        EHF_PRI_DESC(PLAT_PRI_BITS, DISP2_PRIO),
        EHF_PRI_DESC(PLAT_PRI_BITS, DISP3_PRIO),
   };

   /* Expose priority descriptors to Exception Handling Framework */
   EHF_REGISTER_PRIORITIES(plat_exceptions, ARRAY_SIZE(plat_exceptions),
        PLAT_PRI_BITS);

   ...

   /* List interrupt properties for GIC driver. All interrupts target EL3 */
   const interrupt_prop_t plat_interrupts[] = {
        /* Dispatcher 1 owns interrupts d1_0 and d1_1, so assigns priority DISP1_PRIO */
        INTR_PROP_DESC(d1_0, DISP1_PRIO, INTR_TYPE_EL3, GIC_INTR_CFG_LEVEL),
        INTR_PROP_DESC(d1_1, DISP1_PRIO, INTR_TYPE_EL3, GIC_INTR_CFG_LEVEL),

        /* Dispatcher 2 owns interrupts d2_0 and d2_1, so assigns priority DISP2_PRIO */
        INTR_PROP_DESC(d2_0, DISP2_PRIO, INTR_TYPE_EL3, GIC_INTR_CFG_LEVEL),
        INTR_PROP_DESC(d2_1, DISP2_PRIO, INTR_TYPE_EL3, GIC_INTR_CFG_LEVEL),

        /* Dispatcher 3 owns interrupts d3_0 and d3_1, so assigns priority DISP3_PRIO */
        INTR_PROP_DESC(d3_0, DISP3_PRIO, INTR_TYPE_EL3, GIC_INTR_CFG_LEVEL),
        INTR_PROP_DESC(d3_1, DISP3_PRIO, INTR_TYPE_EL3, GIC_INTR_CFG_LEVEL),
   };

   ...

   /* Dispatcher 1 registers its handler */
   ehf_register_priority_handler(DISP1_PRIO, disp1_handler);

   /* Dispatcher 2 registers its handler */
   ehf_register_priority_handler(DISP2_PRIO, disp2_handler);

   /* Dispatcher 3 registers its handler */
   ehf_register_priority_handler(DISP3_PRIO, disp3_handler);

   ...

See also the `Build-time flow`_ and the `Run-time flow`_.

Activating and Deactivating priorities
--------------------------------------

A priority level is said to be *active* when an exception of that priority is
being handled: for interrupts, this is implied when the interrupt is
acknowledged; for non-interrupt exceptions, viz. SErrors or `SDEI explicit
dispatches`__, this has to be done via. calling ``ehf_activate_priority()``. See
`Run-time flow`_.

.. __: sdei.rst#explicit-dispatch-of-events

Conversely, when the dispatcher has reached a logical resolution for the cause
of the exception, the corresponding priority level ought to be deactivated. As
above, for interrupts, this is implied when the interrupt is EOId in the GIC;
for other exceptions, this has to be done via. calling
``ehf_deactivate_priority()``.

Thanks to `different provisions`__ for exception delegation, there are
potentially more than one work flow for deactivation:

.. __: `delegation-use-cases`_

.. _deactivation workflows:

-  The dispatcher has addressed the cause of the exception, and decided to take
   no further action. In this case, the dispatcher's handler deactivates the
   priority level before returning to the |EHF|. Runtime firmware, upon exit
   through an ``ERET``, resumes execution before the interrupt occurred.

-  The dispatcher has to delegate the execution to lower ELs, and the cause of
   the exception can be considered resolved only when the lower EL returns
   signals complete (via. an ``SMC``) at a future point in time. The following
   sequence ensues:

   #. The dispatcher calls ``setjmp()`` to setup a jump point, and arranges to
      enter a lower EL upon the next ``ERET``.

   #. Through the ensuing ``ERET`` from runtime firmware, execution is delegated
      to a lower EL.

   #. The lower EL completes its execution, and signals completion via. an
      ``SMC``.

   #. The ``SMC`` is handled by the same dispatcher that handled the exception
      previously. Noticing the conclusion of exception handling, the dispatcher
      does ``longjmp()`` to resume beyond the previous jump point.

As mentioned above, the |EHF| provides the following APIs for activating and
deactivating interrupt:

.. _ehf-apis:

-  ``ehf_activate_priority()`` activates the supplied priority level, but only
   if the current active priority is higher than the given one; otherwise
   panics. Also, to prevent interruption by physical interrupts of lower
   priority, the |EHF| programs the *Priority Mask Register* corresponding to
   the PE to the priority being activated.  Dispatchers typically only need to
   call this when handling exceptions other than interrupts, and it needs to
   delegate execution to a lower EL at a desired priority level.

-  ``ehf_deactivate_priority()`` deactivates a given priority, but only if the
   current active priority is equal to the given one; otherwise panics. |EHF|
   also restores the *Priority Mask Register* corresponding to the PE to the
   priority before the call to ``ehf_activate_priority()``. Dispatchers
   typically only need to call this after handling exceptions other than
   interrupts.

The calling of APIs are subject to allowed `transitions`__. See also the
`Run-time flow`_.

.. __: `Transition of priority levels`_

Transition of priority levels
-----------------------------

The |EHF| APIs ``ehf_activate_priority()`` and ``ehf_deactivate_priority()`` can
be called to transition the current priority level on a PE. A given sequence of
calls to these APIs are subject to the following conditions:

-  For activation, the |EHF| only allows for the priority to increase (i.e.
   numeric value decreases);

-  For deactivation, the |EHF| only allows for the priority to decrease (i.e.
   numeric value increases). Additionally, the priority being deactivated is
   required to be the current priority.

If these are violated, a panic will result.

Effect on SMC calls
-------------------

In general, Secure execution is regarded as more important than Non-secure
execution. As discussed elsewhere in this document, EL3 execution, and any
delegated execution thereafter, has the effect of raising GIC's priority
mask—either implicitly by acknowledging Secure interrupts, or when dispatchers
call ``ehf_activate_priority()``. As a result, Non-secure interrupts cannot
preempt any Secure execution.

SMCs from Non-secure world are synchronous exceptions, and are mechanisms for
Non-secure world to request Secure services. They're broadly classified as
*Fast* or *Yielding* (see `SMCCC`__).

.. __: `http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html`

-  *Fast* SMCs are atomic from the caller's point of view. I.e., they return
   to the caller only when the Secure world has finished serving the request.
   Any Non-secure interrupts that become pending meanwhile cannot preempt Secure
   execution.

-  *Yielding* SMCs carry the semantics of a preemptible, lower-priority request.
   A pending Non-secure interrupt can preempt Secure execution handling a
   Yielding SMC. I.e., the caller might observe a Yielding SMC returning when
   either:

   #. Secure world completes the request, and the caller would find ``SMC_OK``
      as the return code.

   #. A Non-secure interrupt preempts Secure execution. Non-secure interrupt is
      handled, and Non-secure execution resumes after ``SMC`` instruction.

   The dispatcher handling a Yielding SMC must provide a different return code
   to the Non-secure caller to distinguish the latter case. This return code,
   however, is not standardised (unlike ``SMC_UNKNOWN`` or ``SMC_OK``, for
   example), so will vary across dispatchers that handle the request.

For the latter case above, dispatchers before |EHF| expect Non-secure interrupts
to be taken to S-EL1 [#irq]_, so would get a chance to populate the designated
preempted error code before yielding to Non-secure world.

The introduction of |EHF| changes the behaviour as described in `Interrupt
handling`_.

When |EHF| is enabled, in order to allow Non-secure interrupts to preempt
Yielding SMC handling, the dispatcher must call ``ehf_allow_ns_preemption()``
API. The API takes one argument, the error code to be returned to the Non-secure
world upon getting preempted.

.. [#irq] In case of GICv2, Non-secure interrupts while in S-EL1 were signalled
          as IRQs, and in case of GICv3, FIQs.

Build-time flow
---------------

Please refer to the `figure`__ above.

.. __: `ehf-figure`_

The build-time flow involves the following steps:

#. Platform assigns priorities by installing priority level descriptors for
   individual dispatchers, as described in `Partitioning priority levels`_.

#. Platform provides interrupt properties to GIC driver, as described in
   `Programming priority`_.

#. Dispatcher calling ``ehf_register_priority_handler()`` to register an
   interrupt handler.

Also refer to the `Interrupt handling example`_.

Run-time flow
-------------

.. _interrupt-flow:

The following is an example flow for interrupts:

#. The GIC driver, during initialization, iterates through the platform-supplied
   interrupt properties (see `Programming priority`_), and configures the
   interrupts. This programs the appropriate priority and group (Group 0) on
   interrupts belonging to different dispatchers.

#. The |EHF|, during its initialisation, registers a top-level interrupt handler
   with the `Interrupt Management Framework`__ for EL3 interrupts. This also
   results in setting the routing bits in ``SCR_EL3``.

   .. __: interrupt-framework-design.rst#el3-runtime-firmware

#. When an interrupt belonging to a dispatcher fires, GIC raises an EL3/Group 0
   interrupt, and is taken to EL3.

#. The top-level EL3 interrupt handler executes. The handler acknowledges the
   interrupt, reads its *Running Priority*, and from that, determines the
   dispatcher handler.

#. The |EHF| programs the *Priority Mask Register* of the PE to the priority of
   the interrupt received.

#. The |EHF| marks that priority level *active*, and jumps to the dispatcher
   handler.

#. Once the dispatcher handler finishes its job, it has to immediately
   *deactivate* the priority level before returning to the |EHF|. See
   `deactivation workflows`_.

.. _non-interrupt-flow:

The following is an example flow for exceptions that targets EL3 other than
interrupt:

#. The platform provides handlers for the specific kind of exception.

#. The exception arrives, and the corresponding handler is executed.

#. The handler calls ``ehf_activate_priority()`` to activate the required
   priority level. This also has the effect of raising GIC priority mask, thus
   preventing interrupts of lower priority from preempting the handling. The
   handler may choose to do the handling entirely in EL3 or delegate to a lower
   EL.

#. Once exception handling concludes, the handler calls
   ``ehf_deactivate_priority()`` to deactivate the priority level activated
   earlier. This also has the effect of lowering GIC priority mask to what it
   was before.

Interrupt Prioritisation Considerations
---------------------------------------

The GIC priority scheme, by design, prioritises Secure interrupts over Normal
world ones. The platform further assigns relative priorities amongst Secure
dispatchers through |EHF|.

As mentioned in `Partitioning priority levels`_, interrupts targeting distinct
dispatchers fall in distinct priority levels. Because they're routed via. the
GIC, interrupt delivery to the PE is subject to GIC prioritisation rules. In
particular, when an interrupt is being handled by the PE (i.e., the interrupt is
in *Active* state), only interrupts of higher priority are signalled to the PE,
even if interrupts of same or lower priority are pending. This has the side
effect of one dispatcher being starved of interrupts by virtue of another
dispatcher handling its (higher priority) interrupts.

The |EHF| doesn't enforce a particular prioritisation policy, but the platform
should carefully consider the assignment of priorities to dispatchers integrated
into runtime firmware. The platform should sensibly delineate priority to
various dispatchers according to their nature. In particular, dispatchers of
critical nature (RAS, for example) should be assigned higher priority than
others (SDEI, for example); and within SDEI, Critical priority SDEI should be
assigned higher priority than Normal ones.

Limitations
-----------

The |EHF| has the following limitations:

-  Although there could be up to 128 Secure dispatchers supported by the GIC
   priority scheme, the size of descriptor array exposed with
   ``EHF_REGISTER_PRIORITIES()`` macro is currently limited to 32. This serves most
   expected use cases. This may be expanded in the future, should use cases
   demand so.

-  The platform must ensure that the priority assigned to the dispatcher in the
   exception descriptor and the programmed priority of interrupts handled by the
   dispatcher match. The |EHF| cannot verify that this has been followed.

----

*Copyright (c) 2018, Arm Limited and Contributors. All rights reserved.*

.. _Interrupt Framework Design: interrupt-framework-design.rst
.. _SDEI specification: http://infocenter.arm.com/help/topic/com.arm.doc.den0054a/ARM_DEN0054A_Software_Delegated_Exception_Interface.pdf