Unverified Commit 9345d9a0 authored by Antonio Niño Díaz's avatar Antonio Niño Díaz Committed by GitHub
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Merge pull request #1693 from jeenu-arm/ehf-doc

EHF and RAS documentation
parents 48e32a13 63eb2410
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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 <exception_mgmt.h>
#include <gic_common.h>
#include <interrupt_props.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
......@@ -981,6 +981,11 @@ services ``handle()`` function.
On return from the handler the result registers are populated in X0-X3 before
restoring the stack and CPU state and returning from the original SMC.
Exception Handling Framework
----------------------------
Please refer to the `Exception Handling Framework`_ document.
Power State Coordination Interface
----------------------------------
......@@ -2641,5 +2646,6 @@ References
.. _SMC Calling Convention PDD: http://infocenter.arm.com/help/topic/com.arm.doc.den0028b/ARM_DEN0028B_SMC_Calling_Convention.pdf
.. _TF-A Interrupt Management Design guide: ./interrupt-framework-design.rst
.. _Xlat_tables design: xlat-tables-lib-v2-design.rst
.. _Exception Handling Framework: exception-handling.rst
.. |Image 1| image:: diagrams/rt-svc-descs-layout.png?raw=true
......@@ -153,7 +153,10 @@ EL3 interrupts
However, when ``EL3_EXCEPTION_HANDLING`` is ``1``, this routing model is
invalid as EL3 interrupts are unconditionally routed to EL3, and EL3
interrupts will always preempt Secure EL1/EL0 execution.
interrupts will always preempt Secure EL1/EL0 execution. See `exception
handling`__ documentation.
.. __: exception-handling.rst#interrupt-handling
#. **CSS=0, TEL3=1**. Interrupt is routed to EL3 when execution is in
Secure-EL1/Secure-EL0. This is a valid routing model as secure software
......
......@@ -2646,8 +2646,8 @@ you can keep the sample implementation here (which calls ``console_flush()``).
If you're trying to debug crashes in BL1, you can call the console_xx_core_flush
function exported by some console drivers from here.
Extternal Abort handling and RAS Support
----------------------------------------
External Abort handling and RAS Support
---------------------------------------
Function : plat_ea_handler
~~~~~~~~~~~~~~~~~~~~~~~~~~
......
RAS support in Trusted Firmware-A
=================================
.. section-numbering::
:suffix: .
.. contents::
:depth: 2
.. |EHF| replace:: Exception Handling Framework
.. |TF-A| replace:: Trusted Firmware-A
This document describes |TF-A| support for Arm Reliability, Availability, and
Serviceability (RAS) extensions. RAS is a mandatory extension for Armv8.2 and
later CPUs, and also an optional extension to the base Armv8.0 architecture.
In conjunction with the |EHF|, support for RAS extension enables firmware-first
paradigm for handling platform errors, in which exceptions resulting from
errors—viz. Synchronous External Abort (SEA), Asynchronous External Abort
(signalled as SErrors), Fault Handling and Error Recovery interrupts are routed
to and handled in EL3. The |EHF| document mentions various `error handling
use-cases`__.
.. __: exception-handling.rst#delegation-use-cases
For the description of Arm RAS extensions, Standard Error Records, and the
precise definition of RAS terminology, please refer to the Arm Architecture
Reference Manual. The rest of this document assumes familiarity with
architecture and terminology.
Overview
--------
As mentioned above, the RAS support in |TF-A| enables routing to and handling of
exceptions resulting from platform errors in EL3. It allows the platform to
define an External Abort handler, and to register RAS nodes and interrupts. RAS
framework also provides `helpers`__ for accessing Standard Error Records as
introduced by the RAS extensions.
.. __: `Standard Error Record helpers`_
The build option ``RAS_EXTENSION`` when set to ``1`` includes the RAS in run
time firmware; ``EL3_EXCEPTION_HANDLING`` and ``HANDLE_EA_EL3_FIRST`` must also
be set ``1``.
.. _ras-figure:
.. image:: draw.io/ras.svg
See more on `Engaging the RAS framework`_.
Platform APIs
-------------
The RAS framework allows the platform to define handlers for External Abort,
Uncontainable Errors, Double Fault, and errors rising from EL3 execution. Please
refer to the porting guide for the `RAS platform API descriptions`__.
.. __: porting-guide.rst#external-abort-handling-and-ras-support
Registering RAS error records
-----------------------------
RAS nodes are components in the system capable of signalling errors to PEs
through one one of the notification mechanisms—SEAs, SErrors, or interrupts. RAS
nodes contain one or more error records, which are registers through which the
nodes advertise various properties of the signalled error. Arm recommends that
error records are implemented in the Standard Error Record format. The RAS
architecture allows for error records to be accessible via. system or
memory-mapped registers.
The platform should enumerate the error records providing for each of them:
- A handler to probe error records for errors;
- When the probing identifies an error, a handler to handle it;
- For memory-mapped error record, its base address and size in KB; for a system
register-accessed record, the start index of the record and number of
continuous records from that index;
- Any node-specific auxiliary data.
With this information supplied, when the run time firmware receives one of the
notification mechanisms, the RAS framework can iterate through and probe error
records for error, and invoke the appropriate handler to handle it.
The RAS framework provides the macros to populate error record information. The
macros are versioned, and the latest version as of this writing is 1. These
macros create a structure of type ``struct err_record_info`` from its arguments,
which are later passed to probe and error handlers.
For memory-mapped error records:
.. code:: c
ERR_RECORD_MEMMAP_V1(base_addr, size_num_k, probe, handler, aux)
And, for system register ones:
.. code:: c
ERR_RECORD_SYSREG_V1(idx_start, num_idx, probe, handler, aux)
The probe handler must have the following prototype:
.. code:: c
typedef int (*err_record_probe_t)(const struct err_record_info *info,
int *probe_data);
The probe handler must return a non-zero value if an error was detected, or 0
otherwise. The ``probe_data`` output parameter can be used to pass any useful
information resulting from probe to the error handler (see `below`__). For
example, it could return the index of the record.
.. __: `Standard Error Record helpers`_
The error handler must have the following prototype:
.. code:: c
typedef int (*err_record_handler_t)(const struct err_record_info *info,
int probe_data, const struct err_handler_data *const data);
The ``data`` constant parameter describes the various properties of the error,
viz. the reason for the error, exception syndrome, and also ``flags``,
``cookie``, and ``handle`` parameters from the `top-level exception handler`__.
.. __: interrupt-framework-design.rst#el3-interrupts
The platform is expected populate an array using the macros above, and register
the it with the RAS framework using the macro ``REGISTER_ERR_RECORD_INFO()``,
passing it the name of the array describing the records. Note that the macro
must be used in the same file where the array is defined.
Standard Error Record helpers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The |TF-A| RAS framework provides probe handlers for Standard Error Records, for
both memory-mapped and System Register accesses:
.. code:: c
int ras_err_ser_probe_memmap(const struct err_record_info *info,
int *probe_data);
int ras_err_ser_probe_sysreg(const struct err_record_info *info,
int *probe_data);
When the platform enumerates error records, for those records in the Standard
Error Record format, these helpers maybe used instead of rolling out their own.
Both helpers above:
- Return non-zero value when an error is detected in a Standard Error Record;
- Set ``probe_data`` to the index of the error record upon detecting an error.
Registering RAS interrupts
--------------------------
RAS nodes can signal errors to the PE by raising Fault Handling and/or Error
Recovery interrupts. For the firmware-first handling paradigm for interrupts to
work, the platform must setup and register with |EHF|. See `Interaction with
Exception Handling Framework`_.
For each RAS interrupt, the platform has to provide structure of type ``struct
ras_interrupt``:
- Interrupt number;
- The associated error record information (pointer to the corresponding
``struct err_record_info``);
- Optionally, a cookie.
The platform is expected to define an array of ``struct ras_interrupt``, and
register it with the RAS framework using the macro
``REGISTER_RAS_INTERRUPTS()``, passing it the name of the array. Note that the
macro must be used in the same file where the array is defined.
The array of ``struct ras_interrupt`` must be sorted in the increasing order of
interrupt number. This allows for fast look of handlers in order to service RAS
interrupts.
Double-fault handling
---------------------
A Double Fault condition arises when an error is signalled to the PE while
handling of a previously signalled error is still underway. When a Double Fault
condition arises, the Arm RAS extensions only require for handler to perform
orderly shutdown of the system, as recovery may be impossible.
The RAS extensions part of Armv8.4 introduced new architectural features to deal
with Double Fault conditions, specifically, the introduction of ``NMEA`` and
``EASE`` bits to ``SCR_EL3`` register. These were introduced to assist EL3
software which runs part of its entry/exit routines with exceptions momentarily
masked—meaning, in such systems, External Aborts/SErrors are not immediately
handled when they occur, but only after the exceptions are unmasked again.
|TF-A|, for legacy reasons, executes entire EL3 with all exceptions unmasked.
This means that all exceptions routed to EL3 are handled immediately. |TF-A|
thus is able to detect a Double Fault conditions in software, without needing
the intended advantages of Armv8.4 Double Fault architecture extensions.
Double faults are fatal, and terminate at the platform double fault handler, and
doesn't return.
Engaging the RAS framework
--------------------------
Enabling RAS support is a platform choice conjunctional of three distinct but
related build options:
- ``RAS_EXTENSION=1`` includes the RAS framework in the run time firmware;
- ``EL3_EXCEPTION_HANDLING=1`` enables handling of exceptions at EL3. See
`Interaction with Exception Handling Framework`_;
- ``HANDLE_EA_EL3_FIRST=1`` enables routing of External Aborts and SErrors to
EL3.
The RAS support in |TF-A| introduces a default implementation of
``plat_ea_handler``, the External Abort handler in EL3. When ``RAS_EXTENSION``
is set to ``1``, it'll first call ``ras_ea_handler()`` function, which is the
top-level RAS exception handler. ``ras_ea_handler`` is responsible for iterating
to through platform-supplied error records, probe them, and when an error is
identified, look up and invoke the corresponding error handler.
Note that, if the platform chooses to override the ``plat_ea_handler`` function
and intend to use the RAS framework, it must explicitly call
``ras_ea_handler()`` from within.
Similarly, for RAS interrupts, the framework defines
``ras_interrupt_handler()``. The RAS framework arranges for it to be invoked
when a RAS interrupt taken at EL3. The function bisects the platform-supplied
sorted array of interrupts to look up the error record information associated
with the interrupt number. That error handler for that record is then invoked to
handle the error.
Interaction with Exception Handling Framework
---------------------------------------------
As mentioned in earlier sections, RAS framework interacts with the |EHF| to
arbitrate handling of RAS exceptions with others that are routed to EL3. This
means that the platform must partition a `priority level`__ for handling RAS
exceptions. The platform must then define the macro ``PLAT_RAS_PRI`` to the
priority level used for RAS exceptions. Platforms would typically want to
allocate the highest secure priority for RAS handling.
.. __: exception-handling.rst#partitioning-priority-levels
Handling of both `interrrupt`__ and `non-interrupt`__ exceptions follow the
sequences outlined in the |EHF| documentation. I.e., for interrupts, the
priority management is implicit; but for non-interrupt exceptions, they're
explicit using `EHF APIs`__.
.. __: exception-handling.rst#interrupt-flow
.. __: exception-handling.rst#non-interrupt-flow
.. __: exception-handling.rst#activating-and-deactivating-priorities
----
*Copyright (c) 2018, Arm Limited and Contributors. All rights reserved.*
......@@ -485,15 +485,14 @@ int sdei_intr_handler(uint32_t intr_raw, uint32_t flags, void *handle,
/*
* We reach here when client completes the event.
*
* If the cause of dispatch originally interrupted the Secure world, and
* if Non-secure world wasn't allowed to preempt Secure execution,
* If the cause of dispatch originally interrupted the Secure world,
* resume Secure.
*
* No need to save the Non-secure context ahead of a world switch: the
* Non-secure context was fully saved before dispatch, and has been
* returned to its pre-dispatch state.
*/
if ((sec_state == SECURE) && (ehf_is_ns_preemption_allowed() == 0U))
if (sec_state == SECURE)
restore_and_resume_secure_context();
/*
......
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