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diff --git a/docs/design/interrupt-framework-design.rst b/docs/design/interrupt-framework-design.rst new file mode 100644 index 0000000..dfb2eac --- /dev/null +++ b/docs/design/interrupt-framework-design.rst @@ -0,0 +1,1021 @@ +Interrupt Management Framework +============================== + +This framework is responsible for managing interrupts routed to EL3. It also +allows EL3 software to configure the interrupt routing behavior. Its main +objective is to implement the following two requirements. + +#. It should be possible to route interrupts meant to be handled by secure + software (Secure interrupts) to EL3, when execution is in non-secure state + (normal world). The framework should then take care of handing control of + the interrupt to either software in EL3 or Secure-EL1 depending upon the + software configuration and the GIC implementation. This requirement ensures + that secure interrupts are under the control of the secure software with + respect to their delivery and handling without the possibility of + intervention from non-secure software. + +#. It should be possible to route interrupts meant to be handled by + non-secure software (Non-secure interrupts) to the last executed exception + level in the normal world when the execution is in secure world at + exception levels lower than EL3. This could be done with or without the + knowledge of software executing in Secure-EL1/Secure-EL0. The choice of + approach should be governed by the secure software. This requirement + ensures that non-secure software is able to execute in tandem with the + secure software without overriding it. + +Concepts +-------- + +Interrupt types +~~~~~~~~~~~~~~~ + +The framework categorises an interrupt to be one of the following depending upon +the exception level(s) it is handled in. + +#. Secure EL1 interrupt. This type of interrupt can be routed to EL3 or + Secure-EL1 depending upon the security state of the current execution + context. It is always handled in Secure-EL1. + +#. Non-secure interrupt. This type of interrupt can be routed to EL3, + Secure-EL1, Non-secure EL1 or EL2 depending upon the security state of the + current execution context. It is always handled in either Non-secure EL1 + or EL2. + +#. EL3 interrupt. This type of interrupt can be routed to EL3 or Secure-EL1 + depending upon the security state of the current execution context. It is + always handled in EL3. + +The following constants define the various interrupt types in the framework +implementation. + +.. code:: c + + #define INTR_TYPE_S_EL1 0 + #define INTR_TYPE_EL3 1 + #define INTR_TYPE_NS 2 + +Routing model +~~~~~~~~~~~~~ + +A type of interrupt can be either generated as an FIQ or an IRQ. The target +exception level of an interrupt type is configured through the FIQ and IRQ bits +in the Secure Configuration Register at EL3 (``SCR_EL3.FIQ`` and ``SCR_EL3.IRQ`` +bits). When ``SCR_EL3.FIQ``\ =1, FIQs are routed to EL3. Otherwise they are routed +to the First Exception Level (FEL) capable of handling interrupts. When +``SCR_EL3.IRQ``\ =1, IRQs are routed to EL3. Otherwise they are routed to the +FEL. This register is configured independently by EL3 software for each security +state prior to entry into a lower exception level in that security state. + +A routing model for a type of interrupt (generated as FIQ or IRQ) is defined as +its target exception level for each security state. It is represented by a +single bit for each security state. A value of ``0`` means that the interrupt +should be routed to the FEL. A value of ``1`` means that the interrupt should be +routed to EL3. A routing model is applicable only when execution is not in EL3. + +The default routing model for an interrupt type is to route it to the FEL in +either security state. + +Valid routing models +~~~~~~~~~~~~~~~~~~~~ + +The framework considers certain routing models for each type of interrupt to be +incorrect as they conflict with the requirements mentioned in Section 1. The +following sub-sections describe all the possible routing models and specify +which ones are valid or invalid. EL3 interrupts are currently supported only +for GIC version 3.0 (Arm GICv3) and only the Secure-EL1 and Non-secure interrupt +types are supported for GIC version 2.0 (Arm GICv2) (see `Assumptions in +Interrupt Management Framework`_). The terminology used in the following +sub-sections is explained below. + +#. **CSS**. Current Security State. ``0`` when secure and ``1`` when non-secure + +#. **TEL3**. Target Exception Level 3. ``0`` when targeted to the FEL. ``1`` when + targeted to EL3. + +Secure-EL1 interrupts +^^^^^^^^^^^^^^^^^^^^^ + +#. **CSS=0, TEL3=0**. Interrupt is routed to the FEL when execution is in + secure state. This is a valid routing model as secure software is in + control of handling secure interrupts. + +#. **CSS=0, TEL3=1**. Interrupt is routed to EL3 when execution is in secure + state. This is a valid routing model as secure software in EL3 can + handover the interrupt to Secure-EL1 for handling. + +#. **CSS=1, TEL3=0**. Interrupt is routed to the FEL when execution is in + non-secure state. This is an invalid routing model as a secure interrupt + is not visible to the secure software which violates the motivation behind + the Arm Security Extensions. + +#. **CSS=1, TEL3=1**. Interrupt is routed to EL3 when execution is in + non-secure state. This is a valid routing model as secure software in EL3 + can handover the interrupt to Secure-EL1 for handling. + +Non-secure interrupts +^^^^^^^^^^^^^^^^^^^^^ + +#. **CSS=0, TEL3=0**. Interrupt is routed to the FEL when execution is in + secure state. This allows the secure software to trap non-secure + interrupts, perform its book-keeping and hand the interrupt to the + non-secure software through EL3. This is a valid routing model as secure + software is in control of how its execution is preempted by non-secure + interrupts. + +#. **CSS=0, TEL3=1**. Interrupt is routed to EL3 when execution is in secure + state. This is a valid routing model as secure software in EL3 can save + the state of software in Secure-EL1/Secure-EL0 before handing the + interrupt to non-secure software. This model requires additional + coordination between Secure-EL1 and EL3 software to ensure that the + former's state is correctly saved by the latter. + +#. **CSS=1, TEL3=0**. Interrupt is routed to FEL when execution is in + non-secure state. This is a valid routing model as a non-secure interrupt + is handled by non-secure software. + +#. **CSS=1, TEL3=1**. Interrupt is routed to EL3 when execution is in + non-secure state. This is an invalid routing model as there is no valid + reason to route the interrupt to EL3 software and then hand it back to + non-secure software for handling. + +.. _EL3 interrupts: + +EL3 interrupts +^^^^^^^^^^^^^^ + +#. **CSS=0, TEL3=0**. Interrupt is routed to the FEL when execution is in + Secure-EL1/Secure-EL0. This is a valid routing model as secure software + in Secure-EL1/Secure-EL0 is in control of how its execution is preempted + by EL3 interrupt and can handover the interrupt to EL3 for handling. + + 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. See :ref:`exception + handling<interrupt-handling>` documentation. + +#. **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 + in EL3 can handle the interrupt. + +#. **CSS=1, TEL3=0**. Interrupt is routed to the FEL when execution is in + non-secure state. This is an invalid routing model as a secure interrupt + is not visible to the secure software which violates the motivation behind + the Arm Security Extensions. + +#. **CSS=1, TEL3=1**. Interrupt is routed to EL3 when execution is in + non-secure state. This is a valid routing model as secure software in EL3 + can handle the interrupt. + +Mapping of interrupt type to signal +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The framework is meant to work with any interrupt controller implemented by a +platform. A interrupt controller could generate a type of interrupt as either an +FIQ or IRQ signal to the CPU depending upon the current security state. The +mapping between the type and signal is known only to the platform. The framework +uses this information to determine whether the IRQ or the FIQ bit should be +programmed in ``SCR_EL3`` while applying the routing model for a type of +interrupt. The platform provides this information through the +``plat_interrupt_type_to_line()`` API (described in the +:ref:`Porting Guide`). For example, on the FVP port when the platform uses an +Arm GICv2 interrupt controller, Secure-EL1 interrupts are signaled through the +FIQ signal while Non-secure interrupts are signaled through the IRQ signal. +This applies when execution is in either security state. + +Effect of mapping of several interrupt types to one signal +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +It should be noted that if more than one interrupt type maps to a single +interrupt signal, and if any one of the interrupt type sets **TEL3=1** for a +particular security state, then interrupt signal will be routed to EL3 when in +that security state. This means that all the other interrupt types using the +same interrupt signal will be forced to the same routing model. This should be +borne in mind when choosing the routing model for an interrupt type. + +For example, in Arm GICv3, when the execution context is Secure-EL1/ +Secure-EL0, both the EL3 and the non secure interrupt types map to the FIQ +signal. So if either one of the interrupt type sets the routing model so +that **TEL3=1** when **CSS=0**, the FIQ bit in ``SCR_EL3`` will be programmed to +route the FIQ signal to EL3 when executing in Secure-EL1/Secure-EL0, thereby +effectively routing the other interrupt type also to EL3. + +Assumptions in Interrupt Management Framework +--------------------------------------------- + +The framework makes the following assumptions to simplify its implementation. + +#. Although the framework has support for 2 types of secure interrupts (EL3 + and Secure-EL1 interrupt), only interrupt controller architectures + like Arm GICv3 has architectural support for EL3 interrupts in the form of + Group 0 interrupts. In Arm GICv2, all secure interrupts are assumed to be + handled in Secure-EL1. They can be delivered to Secure-EL1 via EL3 but they + cannot be handled in EL3. + +#. Interrupt exceptions (``PSTATE.I`` and ``F`` bits) are masked during execution + in EL3. + +#. Interrupt management: the following sections describe how interrupts are + managed by the interrupt handling framework. This entails: + + #. Providing an interface to allow registration of a handler and + specification of the routing model for a type of interrupt. + + #. Implementing support to hand control of an interrupt type to its + registered handler when the interrupt is generated. + +Both aspects of interrupt management involve various components in the secure +software stack spanning from EL3 to Secure-EL1. These components are described +in the section `Software components`_. The framework stores information +associated with each type of interrupt in the following data structure. + +.. code:: c + + typedef struct intr_type_desc { + interrupt_type_handler_t handler; + uint32_t flags; + uint32_t scr_el3[2]; + } intr_type_desc_t; + +The ``flags`` field stores the routing model for the interrupt type in +bits[1:0]. Bit[0] stores the routing model when execution is in the secure +state. Bit[1] stores the routing model when execution is in the non-secure +state. As mentioned in Section `Routing model`_, a value of ``0`` implies that +the interrupt should be targeted to the FEL. A value of ``1`` implies that it +should be targeted to EL3. The remaining bits are reserved and SBZ. The helper +macro ``set_interrupt_rm_flag()`` should be used to set the bits in the +``flags`` parameter. + +The ``scr_el3[2]`` field also stores the routing model but as a mapping of the +model in the ``flags`` field to the corresponding bit in the ``SCR_EL3`` for each +security state. + +The framework also depends upon the platform port to configure the interrupt +controller to distinguish between secure and non-secure interrupts. The platform +is expected to be aware of the secure devices present in the system and their +associated interrupt numbers. It should configure the interrupt controller to +enable the secure interrupts, ensure that their priority is always higher than +the non-secure interrupts and target them to the primary CPU. It should also +export the interface described in the :ref:`Porting Guide` to enable +handling of interrupts. + +In the remainder of this document, for the sake of simplicity a Arm GICv2 system +is considered and it is assumed that the FIQ signal is used to generate Secure-EL1 +interrupts and the IRQ signal is used to generate non-secure interrupts in either +security state. EL3 interrupts are not considered. + +Software components +------------------- + +Roles and responsibilities for interrupt management are sub-divided between the +following components of software running in EL3 and Secure-EL1. Each component is +briefly described below. + +#. EL3 Runtime Firmware. This component is common to all ports of TF-A. + +#. Secure Payload Dispatcher (SPD) service. This service interfaces with the + Secure Payload (SP) software which runs in Secure-EL1/Secure-EL0 and is + responsible for switching execution between secure and non-secure states. + A switch is triggered by a Secure Monitor Call and it uses the APIs + exported by the Context management library to implement this functionality. + Switching execution between the two security states is a requirement for + interrupt management as well. This results in a significant dependency on + the SPD service. TF-A implements an example Test Secure Payload Dispatcher + (TSPD) service. + + An SPD service plugs into the EL3 runtime firmware and could be common to + some ports of TF-A. + +#. Secure Payload (SP). On a production system, the Secure Payload corresponds + to a Secure OS which runs in Secure-EL1/Secure-EL0. It interfaces with the + SPD service to manage communication with non-secure software. TF-A + implements an example secure payload called Test Secure Payload (TSP) + which runs only in Secure-EL1. + + A Secure payload implementation could be common to some ports of TF-A, + just like the SPD service. + +Interrupt registration +---------------------- + +This section describes in detail the role of each software component (see +`Software components`_) during the registration of a handler for an interrupt +type. + +.. _el3-runtime-firmware: + +EL3 runtime firmware +~~~~~~~~~~~~~~~~~~~~ + +This component declares the following prototype for a handler of an interrupt type. + +.. code:: c + + typedef uint64_t (*interrupt_type_handler_t)(uint32_t id, + uint32_t flags, + void *handle, + void *cookie); + +The ``id`` is parameter is reserved and could be used in the future for passing +the interrupt id of the highest pending interrupt only if there is a foolproof +way of determining the id. Currently it contains ``INTR_ID_UNAVAILABLE``. + +The ``flags`` parameter contains miscellaneous information as follows. + +#. Security state, bit[0]. This bit indicates the security state of the lower + exception level when the interrupt was generated. A value of ``1`` means + that it was in the non-secure state. A value of ``0`` indicates that it was + in the secure state. This bit can be used by the handler to ensure that + interrupt was generated and routed as per the routing model specified + during registration. + +#. Reserved, bits[31:1]. The remaining bits are reserved for future use. + +The ``handle`` parameter points to the ``cpu_context`` structure of the current CPU +for the security state specified in the ``flags`` parameter. + +Once the handler routine completes, execution will return to either the secure +or non-secure state. The handler routine must return a pointer to +``cpu_context`` structure of the current CPU for the target security state. On +AArch64, this return value is currently ignored by the caller as the +appropriate ``cpu_context`` to be used is expected to be set by the handler +via the context management library APIs. +A portable interrupt handler implementation must set the target context both in +the structure pointed to by the returned pointer and via the context management +library APIs. The handler should treat all error conditions as critical errors +and take appropriate action within its implementation e.g. use assertion +failures. + +The runtime firmware provides the following API for registering a handler for a +particular type of interrupt. A Secure Payload Dispatcher service should use +this API to register a handler for Secure-EL1 and optionally for non-secure +interrupts. This API also requires the caller to specify the routing model for +the type of interrupt. + +.. code:: c + + int32_t register_interrupt_type_handler(uint32_t type, + interrupt_type_handler handler, + uint64_t flags); + +The ``type`` parameter can be one of the three interrupt types listed above i.e. +``INTR_TYPE_S_EL1``, ``INTR_TYPE_NS`` & ``INTR_TYPE_EL3``. The ``flags`` parameter +is as described in Section 2. + +The function will return ``0`` upon a successful registration. It will return +``-EALREADY`` in case a handler for the interrupt type has already been +registered. If the ``type`` is unrecognised or the ``flags`` or the ``handler`` are +invalid it will return ``-EINVAL``. + +Interrupt routing is governed by the configuration of the ``SCR_EL3.FIQ/IRQ`` bits +prior to entry into a lower exception level in either security state. The +context management library maintains a copy of the ``SCR_EL3`` system register for +each security state in the ``cpu_context`` structure of each CPU. It exports the +following APIs to let EL3 Runtime Firmware program and retrieve the routing +model for each security state for the current CPU. The value of ``SCR_EL3`` stored +in the ``cpu_context`` is used by the ``el3_exit()`` function to program the +``SCR_EL3`` register prior to returning from the EL3 exception level. + +.. code:: c + + uint32_t cm_get_scr_el3(uint32_t security_state); + void cm_write_scr_el3_bit(uint32_t security_state, + uint32_t bit_pos, + uint32_t value); + +``cm_get_scr_el3()`` returns the value of the ``SCR_EL3`` register for the specified +security state of the current CPU. ``cm_write_scr_el3_bit()`` writes a ``0`` or ``1`` +to the bit specified by ``bit_pos``. ``register_interrupt_type_handler()`` invokes +``set_routing_model()`` API which programs the ``SCR_EL3`` according to the routing +model using the ``cm_get_scr_el3()`` and ``cm_write_scr_el3_bit()`` APIs. + +It is worth noting that in the current implementation of the framework, the EL3 +runtime firmware is responsible for programming the routing model. The SPD is +responsible for ensuring that the routing model has been adhered to upon +receiving an interrupt. + +.. _spd-int-registration: + +Secure payload dispatcher +~~~~~~~~~~~~~~~~~~~~~~~~~ + +A SPD service is responsible for determining and maintaining the interrupt +routing model supported by itself and the Secure Payload. It is also responsible +for ferrying interrupts between secure and non-secure software depending upon +the routing model. It could determine the routing model at build time or at +runtime. It must use this information to register a handler for each interrupt +type using the ``register_interrupt_type_handler()`` API in EL3 runtime firmware. + +If the routing model is not known to the SPD service at build time, then it must +be provided by the SP as the result of its initialisation. The SPD should +program the routing model only after SP initialisation has completed e.g. in the +SPD initialisation function pointed to by the ``bl32_init`` variable. + +The SPD should determine the mechanism to pass control to the Secure Payload +after receiving an interrupt from the EL3 runtime firmware. This information +could either be provided to the SPD service at build time or by the SP at +runtime. + +Test secure payload dispatcher behavior +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. note:: + Where this document discusses ``TSP_NS_INTR_ASYNC_PREEMPT`` as being + ``1``, the same results also apply when ``EL3_EXCEPTION_HANDLING`` is ``1``. + +The TSPD only handles Secure-EL1 interrupts and is provided with the following +routing model at build time. + +- Secure-EL1 interrupts are routed to EL3 when execution is in non-secure + state and are routed to the FEL when execution is in the secure state + i.e **CSS=0, TEL3=0** & **CSS=1, TEL3=1** for Secure-EL1 interrupts + +- When the build flag ``TSP_NS_INTR_ASYNC_PREEMPT`` is zero, the default routing + model is used for non-secure interrupts. They are routed to the FEL in + either security state i.e **CSS=0, TEL3=0** & **CSS=1, TEL3=0** for + Non-secure interrupts. + +- When the build flag ``TSP_NS_INTR_ASYNC_PREEMPT`` is defined to 1, then the + non secure interrupts are routed to EL3 when execution is in secure state + i.e **CSS=0, TEL3=1** for non-secure interrupts. This effectively preempts + Secure-EL1. The default routing model is used for non secure interrupts in + non-secure state. i.e **CSS=1, TEL3=0**. + +It performs the following actions in the ``tspd_init()`` function to fulfill the +requirements mentioned earlier. + +#. It passes control to the Test Secure Payload to perform its + initialisation. The TSP provides the address of the vector table + ``tsp_vectors`` in the SP which also includes the handler for Secure-EL1 + interrupts in the ``sel1_intr_entry`` field. The TSPD passes control to the TSP at + this address when it receives a Secure-EL1 interrupt. + + The handover agreement between the TSP and the TSPD requires that the TSPD + masks all interrupts (``PSTATE.DAIF`` bits) when it calls + ``tsp_sel1_intr_entry()``. The TSP has to preserve the callee saved general + purpose, SP_EL1/Secure-EL0, LR, VFP and system registers. It can use + ``x0-x18`` to enable its C runtime. + +#. The TSPD implements a handler function for Secure-EL1 interrupts. This + function is registered with the EL3 runtime firmware using the + ``register_interrupt_type_handler()`` API as follows + + .. code:: c + + /* Forward declaration */ + interrupt_type_handler tspd_secure_el1_interrupt_handler; + int32_t rc, flags = 0; + set_interrupt_rm_flag(flags, NON_SECURE); + rc = register_interrupt_type_handler(INTR_TYPE_S_EL1, + tspd_secure_el1_interrupt_handler, + flags); + if (rc) + panic(); + +#. When the build flag ``TSP_NS_INTR_ASYNC_PREEMPT`` is defined to 1, the TSPD + implements a handler function for non-secure interrupts. This function is + registered with the EL3 runtime firmware using the + ``register_interrupt_type_handler()`` API as follows + + .. code:: c + + /* Forward declaration */ + interrupt_type_handler tspd_ns_interrupt_handler; + int32_t rc, flags = 0; + set_interrupt_rm_flag(flags, SECURE); + rc = register_interrupt_type_handler(INTR_TYPE_NS, + tspd_ns_interrupt_handler, + flags); + if (rc) + panic(); + +.. _sp-int-registration: + +Secure payload +~~~~~~~~~~~~~~ + +A Secure Payload must implement an interrupt handling framework at Secure-EL1 +(Secure-EL1 IHF) to support its chosen interrupt routing model. Secure payload +execution will alternate between the below cases. + +#. In the code where IRQ, FIQ or both interrupts are enabled, if an interrupt + type is targeted to the FEL, then it will be routed to the Secure-EL1 + exception vector table. This is defined as the **asynchronous mode** of + handling interrupts. This mode applies to both Secure-EL1 and non-secure + interrupts. + +#. In the code where both interrupts are disabled, if an interrupt type is + targeted to the FEL, then execution will eventually migrate to the + non-secure state. Any non-secure interrupts will be handled as described + in the routing model where **CSS=1 and TEL3=0**. Secure-EL1 interrupts + will be routed to EL3 (as per the routing model where **CSS=1 and + TEL3=1**) where the SPD service will hand them to the SP. This is defined + as the **synchronous mode** of handling interrupts. + +The interrupt handling framework implemented by the SP should support one or +both these interrupt handling models depending upon the chosen routing model. + +The following list briefly describes how the choice of a valid routing model +(see `Valid routing models`_) effects the implementation of the Secure-EL1 +IHF. If the choice of the interrupt routing model is not known to the SPD +service at compile time, then the SP should pass this information to the SPD +service at runtime during its initialisation phase. + +As mentioned earlier, an Arm GICv2 system is considered and it is assumed that +the FIQ signal is used to generate Secure-EL1 interrupts and the IRQ signal +is used to generate non-secure interrupts in either security state. + +Secure payload IHF design w.r.t secure-EL1 interrupts +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +#. **CSS=0, TEL3=0**. If ``PSTATE.F=0``, Secure-EL1 interrupts will be + triggered at one of the Secure-EL1 FIQ exception vectors. The Secure-EL1 + IHF should implement support for handling FIQ interrupts asynchronously. + + If ``PSTATE.F=1`` then Secure-EL1 interrupts will be handled as per the + synchronous interrupt handling model. The SP could implement this scenario + by exporting a separate entrypoint for Secure-EL1 interrupts to the SPD + service during the registration phase. The SPD service would also need to + know the state of the system, general purpose and the ``PSTATE`` registers + in which it should arrange to return execution to the SP. The SP should + provide this information in an implementation defined way during the + registration phase if it is not known to the SPD service at build time. + +#. **CSS=1, TEL3=1**. Interrupts are routed to EL3 when execution is in + non-secure state. They should be handled through the synchronous interrupt + handling model as described in 1. above. + +#. **CSS=0, TEL3=1**. Secure-EL1 interrupts are routed to EL3 when execution + is in secure state. They will not be visible to the SP. The ``PSTATE.F`` bit + in Secure-EL1/Secure-EL0 will not mask FIQs. The EL3 runtime firmware will + call the handler registered by the SPD service for Secure-EL1 interrupts. + Secure-EL1 IHF should then handle all Secure-EL1 interrupt through the + synchronous interrupt handling model described in 1. above. + +Secure payload IHF design w.r.t non-secure interrupts +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +#. **CSS=0, TEL3=0**. If ``PSTATE.I=0``, non-secure interrupts will be + triggered at one of the Secure-EL1 IRQ exception vectors . The Secure-EL1 + IHF should co-ordinate with the SPD service to transfer execution to the + non-secure state where the interrupt should be handled e.g the SP could + allocate a function identifier to issue a SMC64 or SMC32 to the SPD + service which indicates that the SP execution has been preempted by a + non-secure interrupt. If this function identifier is not known to the SPD + service at compile time then the SP could provide it during the + registration phase. + + If ``PSTATE.I=1`` then the non-secure interrupt will pend until execution + resumes in the non-secure state. + +#. **CSS=0, TEL3=1**. Non-secure interrupts are routed to EL3. They will not + be visible to the SP. The ``PSTATE.I`` bit in Secure-EL1/Secure-EL0 will + have not effect. The SPD service should register a non-secure interrupt + handler which should save the SP state correctly and resume execution in + the non-secure state where the interrupt will be handled. The Secure-EL1 + IHF does not need to take any action. + +#. **CSS=1, TEL3=0**. Non-secure interrupts are handled in the FEL in + non-secure state (EL1/EL2) and are not visible to the SP. This routing + model does not affect the SP behavior. + +A Secure Payload must also ensure that all Secure-EL1 interrupts are correctly +configured at the interrupt controller by the platform port of the EL3 runtime +firmware. It should configure any additional Secure-EL1 interrupts which the EL3 +runtime firmware is not aware of through its platform port. + +Test secure payload behavior +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The routing model for Secure-EL1 and non-secure interrupts chosen by the TSP is +described in Section `Secure Payload Dispatcher`__. It is known to the TSPD +service at build time. + +.. __: #spd-int-registration + +The TSP implements an entrypoint (``tsp_sel1_intr_entry()``) for handling Secure-EL1 +interrupts taken in non-secure state and routed through the TSPD service +(synchronous handling model). It passes the reference to this entrypoint via +``tsp_vectors`` to the TSPD service. + +The TSP also replaces the default exception vector table referenced through the +``early_exceptions`` variable, with a vector table capable of handling FIQ and IRQ +exceptions taken at the same (Secure-EL1) exception level. This table is +referenced through the ``tsp_exceptions`` variable and programmed into the +VBAR_EL1. It caters for the asynchronous handling model. + +The TSP also programs the Secure Physical Timer in the Arm Generic Timer block +to raise a periodic interrupt (every half a second) for the purpose of testing +interrupt management across all the software components listed in `Software +components`_. + +Interrupt handling +------------------ + +This section describes in detail the role of each software component (see +Section `Software components`_) in handling an interrupt of a particular type. + +EL3 runtime firmware +~~~~~~~~~~~~~~~~~~~~ + +The EL3 runtime firmware populates the IRQ and FIQ exception vectors referenced +by the ``runtime_exceptions`` variable as follows. + +#. IRQ and FIQ exceptions taken from the current exception level with + ``SP_EL0`` or ``SP_EL3`` are reported as irrecoverable error conditions. As + mentioned earlier, EL3 runtime firmware always executes with the + ``PSTATE.I`` and ``PSTATE.F`` bits set. + +#. The following text describes how the IRQ and FIQ exceptions taken from a + lower exception level using AArch64 or AArch32 are handled. + +When an interrupt is generated, the vector for each interrupt type is +responsible for: + +#. Saving the entire general purpose register context (x0-x30) immediately + upon exception entry. The registers are saved in the per-cpu ``cpu_context`` + data structure referenced by the ``SP_EL3``\ register. + +#. Saving the ``ELR_EL3``, ``SP_EL0`` and ``SPSR_EL3`` system registers in the + per-cpu ``cpu_context`` data structure referenced by the ``SP_EL3`` register. + +#. Switching to the C runtime stack by restoring the ``CTX_RUNTIME_SP`` value + from the per-cpu ``cpu_context`` data structure in ``SP_EL0`` and + executing the ``msr spsel, #0`` instruction. + +#. Determining the type of interrupt. Secure-EL1 interrupts will be signaled + at the FIQ vector. Non-secure interrupts will be signaled at the IRQ + vector. The platform should implement the following API to determine the + type of the pending interrupt. + + .. code:: c + + uint32_t plat_ic_get_interrupt_type(void); + + It should return either ``INTR_TYPE_S_EL1`` or ``INTR_TYPE_NS``. + +#. Determining the handler for the type of interrupt that has been generated. + The following API has been added for this purpose. + + .. code:: c + + interrupt_type_handler get_interrupt_type_handler(uint32_t interrupt_type); + + It returns the reference to the registered handler for this interrupt + type. The ``handler`` is retrieved from the ``intr_type_desc_t`` structure as + described in Section 2. ``NULL`` is returned if no handler has been + registered for this type of interrupt. This scenario is reported as an + irrecoverable error condition. + +#. Calling the registered handler function for the interrupt type generated. + The ``id`` parameter is set to ``INTR_ID_UNAVAILABLE`` currently. The id along + with the current security state and a reference to the ``cpu_context_t`` + structure for the current security state are passed to the handler function + as its arguments. + + The handler function returns a reference to the per-cpu ``cpu_context_t`` + structure for the target security state. + +#. Calling ``el3_exit()`` to return from EL3 into a lower exception level in + the security state determined by the handler routine. The ``el3_exit()`` + function is responsible for restoring the register context from the + ``cpu_context_t`` data structure for the target security state. + +Secure payload dispatcher +~~~~~~~~~~~~~~~~~~~~~~~~~ + +Interrupt entry +^^^^^^^^^^^^^^^ + +The SPD service begins handling an interrupt when the EL3 runtime firmware calls +the handler function for that type of interrupt. The SPD service is responsible +for the following: + +#. Validating the interrupt. This involves ensuring that the interrupt was + generated according to the interrupt routing model specified by the SPD + service during registration. It should use the security state of the + exception level (passed in the ``flags`` parameter of the handler) where + the interrupt was taken from to determine this. If the interrupt is not + recognised then the handler should treat it as an irrecoverable error + condition. + + An SPD service can register a handler for Secure-EL1 and/or Non-secure + interrupts. A non-secure interrupt should never be routed to EL3 from + from non-secure state. Also if a routing model is chosen where Secure-EL1 + interrupts are routed to S-EL1 when execution is in Secure state, then a + S-EL1 interrupt should never be routed to EL3 from secure state. The handler + could use the security state flag to check this. + +#. Determining whether a context switch is required. This depends upon the + routing model and interrupt type. For non secure and S-EL1 interrupt, + if the security state of the execution context where the interrupt was + generated is not the same as the security state required for handling + the interrupt, a context switch is required. The following 2 cases + require a context switch from secure to non-secure or vice-versa: + + #. A Secure-EL1 interrupt taken from the non-secure state should be + routed to the Secure Payload. + + #. A non-secure interrupt taken from the secure state should be routed + to the last known non-secure exception level. + + The SPD service must save the system register context of the current + security state. It must then restore the system register context of the + target security state. It should use the ``cm_set_next_eret_context()`` API + to ensure that the next ``cpu_context`` to be restored is of the target + security state. + + If the target state is secure then execution should be handed to the SP as + per the synchronous interrupt handling model it implements. A Secure-EL1 + interrupt can be routed to EL3 while execution is in the SP. This implies + that SP execution can be preempted while handling an interrupt by a + another higher priority Secure-EL1 interrupt or a EL3 interrupt. The SPD + service should be able to handle this preemption or manage secure interrupt + priorities before handing control to the SP. + +#. Setting the return value of the handler to the per-cpu ``cpu_context`` if + the interrupt has been successfully validated and ready to be handled at a + lower exception level. + +The routing model allows non-secure interrupts to interrupt Secure-EL1 when in +secure state if it has been configured to do so. The SPD service and the SP +should implement a mechanism for routing these interrupts to the last known +exception level in the non-secure state. The former should save the SP context, +restore the non-secure context and arrange for entry into the non-secure state +so that the interrupt can be handled. + +Interrupt exit +^^^^^^^^^^^^^^ + +When the Secure Payload has finished handling a Secure-EL1 interrupt, it could +return control back to the SPD service through a SMC32 or SMC64. The SPD service +should handle this secure monitor call so that execution resumes in the +exception level and the security state from where the Secure-EL1 interrupt was +originally taken. + +Test secure payload dispatcher Secure-EL1 interrupt handling +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The example TSPD service registers a handler for Secure-EL1 interrupts taken +from the non-secure state. During execution in S-EL1, the TSPD expects that the +Secure-EL1 interrupts are handled in S-EL1 by TSP. Its handler +``tspd_secure_el1_interrupt_handler()`` expects only to be invoked for Secure-EL1 +originating from the non-secure state. It takes the following actions upon being +invoked. + +#. It uses the security state provided in the ``flags`` parameter to ensure + that the secure interrupt originated from the non-secure state. It asserts + if this is not the case. + +#. It saves the system register context for the non-secure state by calling + ``cm_el1_sysregs_context_save(NON_SECURE);``. + +#. It sets the ``ELR_EL3`` system register to ``tsp_sel1_intr_entry`` and sets the + ``SPSR_EL3.DAIF`` bits in the secure CPU context. It sets ``x0`` to + ``TSP_HANDLE_SEL1_INTR_AND_RETURN``. If the TSP was preempted earlier by a non + secure interrupt during ``yielding`` SMC processing, save the registers that + will be trashed, which is the ``ELR_EL3`` and ``SPSR_EL3``, in order to be able + to re-enter TSP for Secure-EL1 interrupt processing. It does not need to + save any other secure context since the TSP is expected to preserve it + (see section `Test secure payload dispatcher behavior`_). + +#. It restores the system register context for the secure state by calling + ``cm_el1_sysregs_context_restore(SECURE);``. + +#. It ensures that the secure CPU context is used to program the next + exception return from EL3 by calling ``cm_set_next_eret_context(SECURE);``. + +#. It returns the per-cpu ``cpu_context`` to indicate that the interrupt can + now be handled by the SP. ``x1`` is written with the value of ``elr_el3`` + register for the non-secure state. This information is used by the SP for + debugging purposes. + +The figure below describes how the interrupt handling is implemented by the TSPD +when a Secure-EL1 interrupt is generated when execution is in the non-secure +state. + +|Image 1| + +The TSP issues an SMC with ``TSP_HANDLED_S_EL1_INTR`` as the function identifier to +signal completion of interrupt handling. + +The TSPD service takes the following actions in ``tspd_smc_handler()`` function +upon receiving an SMC with ``TSP_HANDLED_S_EL1_INTR`` as the function identifier: + +#. It ensures that the call originated from the secure state otherwise + execution returns to the non-secure state with ``SMC_UNK`` in ``x0``. + +#. It restores the saved ``ELR_EL3`` and ``SPSR_EL3`` system registers back to + the secure CPU context (see step 3 above) in case the TSP had been preempted + by a non secure interrupt earlier. + +#. It restores the system register context for the non-secure state by + calling ``cm_el1_sysregs_context_restore(NON_SECURE)``. + +#. It ensures that the non-secure CPU context is used to program the next + exception return from EL3 by calling ``cm_set_next_eret_context(NON_SECURE)``. + +#. ``tspd_smc_handler()`` returns a reference to the non-secure ``cpu_context`` + as the return value. + +Test secure payload dispatcher non-secure interrupt handling +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The TSP in Secure-EL1 can be preempted by a non-secure interrupt during +``yielding`` SMC processing or by a higher priority EL3 interrupt during +Secure-EL1 interrupt processing. When ``EL3_EXCEPTION_HANDLING`` is ``0``, only +non-secure interrupts can cause preemption of TSP since there are no EL3 +interrupts in the system. With ``EL3_EXCEPTION_HANDLING=1`` however, any EL3 +interrupt may preempt Secure execution. + +It should be noted that while TSP is preempted, the TSPD only allows entry into +the TSP either for Secure-EL1 interrupt handling or for resuming the preempted +``yielding`` SMC in response to the ``TSP_FID_RESUME`` SMC from the normal world. +(See Section `Implication of preempted SMC on Non-Secure Software`_). + +The non-secure interrupt triggered in Secure-EL1 during ``yielding`` SMC +processing can be routed to either EL3 or Secure-EL1 and is controlled by build +option ``TSP_NS_INTR_ASYNC_PREEMPT`` (see Section `Test secure payload +dispatcher behavior`_). If the build option is set, the TSPD will set the +routing model for the non-secure interrupt to be routed to EL3 from secure state +i.e. **TEL3=1, CSS=0** and registers ``tspd_ns_interrupt_handler()`` as the +non-secure interrupt handler. The ``tspd_ns_interrupt_handler()`` on being +invoked ensures that the interrupt originated from the secure state and disables +routing of non-secure interrupts from secure state to EL3. This is to prevent +further preemption (by a non-secure interrupt) when TSP is reentered for +handling Secure-EL1 interrupts that triggered while execution was in the normal +world. The ``tspd_ns_interrupt_handler()`` then invokes +``tspd_handle_sp_preemption()`` for further handling. + +If the ``TSP_NS_INTR_ASYNC_PREEMPT`` build option is zero (default), the default +routing model for non-secure interrupt in secure state is in effect +i.e. **TEL3=0, CSS=0**. During ``yielding`` SMC processing, the IRQ +exceptions are unmasked i.e. ``PSTATE.I=0``, and a non-secure interrupt will +trigger at Secure-EL1 IRQ exception vector. The TSP saves the general purpose +register context and issues an SMC with ``TSP_PREEMPTED`` as the function +identifier to signal preemption of TSP. The TSPD SMC handler, +``tspd_smc_handler()``, ensures that the SMC call originated from the +secure state otherwise execution returns to the non-secure state with +``SMC_UNK`` in ``x0``. It then invokes ``tspd_handle_sp_preemption()`` for +further handling. + +The ``tspd_handle_sp_preemption()`` takes the following actions upon being +invoked: + +#. It saves the system register context for the secure state by calling + ``cm_el1_sysregs_context_save(SECURE)``. + +#. It restores the system register context for the non-secure state by + calling ``cm_el1_sysregs_context_restore(NON_SECURE)``. + +#. It ensures that the non-secure CPU context is used to program the next + exception return from EL3 by calling ``cm_set_next_eret_context(NON_SECURE)``. + +#. ``SMC_PREEMPTED`` is set in x0 and return to non secure state after + restoring non secure context. + +The Normal World is expected to resume the TSP after the ``yielding`` SMC +preemption by issuing an SMC with ``TSP_FID_RESUME`` as the function identifier +(see section `Implication of preempted SMC on Non-Secure Software`_). The TSPD +service takes the following actions in ``tspd_smc_handler()`` function upon +receiving this SMC: + +#. It ensures that the call originated from the non secure state. An + assertion is raised otherwise. + +#. Checks whether the TSP needs a resume i.e check if it was preempted. It + then saves the system register context for the non-secure state by calling + ``cm_el1_sysregs_context_save(NON_SECURE)``. + +#. Restores the secure context by calling + ``cm_el1_sysregs_context_restore(SECURE)`` + +#. It ensures that the secure CPU context is used to program the next + exception return from EL3 by calling ``cm_set_next_eret_context(SECURE)``. + +#. ``tspd_smc_handler()`` returns a reference to the secure ``cpu_context`` as the + return value. + +The figure below describes how the TSP/TSPD handle a non-secure interrupt when +it is generated during execution in the TSP with ``PSTATE.I`` = 0 when the +``TSP_NS_INTR_ASYNC_PREEMPT`` build flag is 0. + +|Image 2| + +.. _sp-synchronous-int: + +Secure payload interrupt handling +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The SP should implement one or both of the synchronous and asynchronous +interrupt handling models depending upon the interrupt routing model it has +chosen (as described in section :ref:`Secure Payload <sp-int-registration>`). + +In the synchronous model, it should begin handling a Secure-EL1 interrupt after +receiving control from the SPD service at an entrypoint agreed upon during build +time or during the registration phase. Before handling the interrupt, the SP +should save any Secure-EL1 system register context which is needed for resuming +normal execution in the SP later e.g. ``SPSR_EL1``, ``ELR_EL1``. After handling +the interrupt, the SP could return control back to the exception level and +security state where the interrupt was originally taken from. The SP should use +an SMC32 or SMC64 to ask the SPD service to do this. + +In the asynchronous model, the Secure Payload is responsible for handling +non-secure and Secure-EL1 interrupts at the IRQ and FIQ vectors in its exception +vector table when ``PSTATE.I`` and ``PSTATE.F`` bits are 0. As described earlier, +when a non-secure interrupt is generated, the SP should coordinate with the SPD +service to pass control back to the non-secure state in the last known exception +level. This will allow the non-secure interrupt to be handled in the non-secure +state. + +Test secure payload behavior +^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The TSPD hands control of a Secure-EL1 interrupt to the TSP at the +``tsp_sel1_intr_entry()``. The TSP handles the interrupt while ensuring that the +handover agreement described in Section `Test secure payload dispatcher +behavior`_ is maintained. It updates some statistics by calling +``tsp_update_sync_sel1_intr_stats()``. It then calls +``tsp_common_int_handler()`` which. + +#. Checks whether the interrupt is the secure physical timer interrupt. It + uses the platform API ``plat_ic_get_pending_interrupt_id()`` to get the + interrupt number. If it is not the secure physical timer interrupt, then + that means that a higher priority interrupt has preempted it. Invoke + ``tsp_handle_preemption()`` to handover control back to EL3 by issuing + an SMC with ``TSP_PREEMPTED`` as the function identifier. + +#. Handles the secure timer interrupt interrupt by acknowledging it using the + ``plat_ic_acknowledge_interrupt()`` platform API, calling + ``tsp_generic_timer_handler()`` to reprogram the secure physical generic + timer and calling the ``plat_ic_end_of_interrupt()`` platform API to signal + end of interrupt processing. + +The TSP passes control back to the TSPD by issuing an SMC64 with +``TSP_HANDLED_S_EL1_INTR`` as the function identifier. + +The TSP handles interrupts under the asynchronous model as follows. + +#. Secure-EL1 interrupts are handled by calling the ``tsp_common_int_handler()`` + function. The function has been described above. + +#. Non-secure interrupts are handled by calling the ``tsp_common_int_handler()`` + function which ends up invoking ``tsp_handle_preemption()`` and issuing an + SMC64 with ``TSP_PREEMPTED`` as the function identifier. Execution resumes at + the instruction that follows this SMC instruction when the TSPD hands control + to the TSP in response to an SMC with ``TSP_FID_RESUME`` as the function + identifier from the non-secure state (see section `Test secure payload + dispatcher non-secure interrupt handling`_). + +Other considerations +-------------------- + +Implication of preempted SMC on Non-Secure Software +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +A ``yielding`` SMC call to Secure payload can be preempted by a non-secure +interrupt and the execution can return to the non-secure world for handling +the interrupt (For details on ``yielding`` SMC refer `SMC calling convention`_). +In this case, the SMC call has not completed its execution and the execution +must return back to the secure payload to resume the preempted SMC call. +This can be achieved by issuing an SMC call which instructs to resume the +preempted SMC. + +A ``fast`` SMC cannot be preempted and hence this case will not happen for +a fast SMC call. + +In the Test Secure Payload implementation, ``TSP_FID_RESUME`` is designated +as the resume SMC FID. It is important to note that ``TSP_FID_RESUME`` is a +``yielding`` SMC which means it too can be be preempted. The typical non +secure software sequence for issuing a ``yielding`` SMC would look like this, +assuming ``P.STATE.I=0`` in the non secure state : + +.. code:: c + + int rc; + rc = smc(TSP_YIELD_SMC_FID, ...); /* Issue a Yielding SMC call */ + /* The pending non-secure interrupt is handled by the interrupt handler + and returns back here. */ + while (rc == SMC_PREEMPTED) { /* Check if the SMC call is preempted */ + rc = smc(TSP_FID_RESUME); /* Issue resume SMC call */ + } + +The ``TSP_YIELD_SMC_FID`` is any ``yielding`` SMC function identifier and the smc() +function invokes a SMC call with the required arguments. The pending non-secure +interrupt causes an IRQ exception and the IRQ handler registered at the +exception vector handles the non-secure interrupt and returns. The return value +from the SMC call is tested for ``SMC_PREEMPTED`` to check whether it is +preempted. If it is, then the resume SMC call ``TSP_FID_RESUME`` is issued. The +return value of the SMC call is tested again to check if it is preempted. +This is done in a loop till the SMC call succeeds or fails. If a ``yielding`` +SMC is preempted, until it is resumed using ``TSP_FID_RESUME`` SMC and +completed, the current TSPD prevents any other SMC call from re-entering +TSP by returning ``SMC_UNK`` error. + +-------------- + +*Copyright (c) 2014-2020, Arm Limited and Contributors. All rights reserved.* + +.. _SMC calling convention: https://developer.arm.com/docs/den0028/latest + +.. |Image 1| image:: ../resources/diagrams/sec-int-handling.png +.. |Image 2| image:: ../resources/diagrams/non-sec-int-handling.png |