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diff --git a/docs/getting_started/rt-svc-writers-guide.rst b/docs/getting_started/rt-svc-writers-guide.rst new file mode 100644 index 0000000..5a4be4d --- /dev/null +++ b/docs/getting_started/rt-svc-writers-guide.rst @@ -0,0 +1,320 @@ +EL3 Runtime Service Writer's Guide +===================================================== + +Introduction +------------ + +This document describes how to add a runtime service to the EL3 Runtime +Firmware component of Trusted Firmware-A (TF-A), BL31. + +Software executing in the normal world and in the trusted world at exception +levels lower than EL3 will request runtime services using the Secure Monitor +Call (SMC) instruction. These requests will follow the convention described in +the SMC Calling Convention PDD (`SMCCC`_). The `SMCCC`_ assigns function +identifiers to each SMC request and describes how arguments are passed and +results are returned. + +SMC Functions are grouped together based on the implementor of the service, for +example a subset of the Function IDs are designated as "OEM Calls" (see `SMCCC`_ +for full details). The EL3 runtime services framework in BL31 enables the +independent implementation of services for each group, which are then compiled +into the BL31 image. This simplifies the integration of common software from +Arm to support `PSCI`_, Secure Monitor for a Trusted OS and SoC specific +software. The common runtime services framework ensures that SMC Functions are +dispatched to their respective service implementation - the +:ref:`Firmware Design` document provides details of how this is achieved. + +The interface and operation of the runtime services depends heavily on the +concepts and definitions described in the `SMCCC`_, in particular SMC Function +IDs, Owning Entity Numbers (OEN), Fast and Standard calls, and the SMC32 and +SMC64 calling conventions. Please refer to that document for a full explanation +of these terms. + +Owning Entities, Call Types and Function IDs +-------------------------------------------- + +The SMC Function Identifier includes a OEN field. These values and their +meaning are described in `SMCCC`_ and summarized in table 1 below. Some entities +are allocated a range of of OENs. The OEN must be interpreted in conjunction +with the SMC call type, which is either *Fast* or *Yielding*. Fast calls are +uninterruptible whereas Yielding calls can be pre-empted. The majority of +Owning Entities only have allocated ranges for Fast calls: Yielding calls are +reserved exclusively for Trusted OS providers or for interoperability with +legacy 32-bit software that predates the `SMCCC`_. + +:: + + Type OEN Service + Fast 0 Arm Architecture calls + Fast 1 CPU Service calls + Fast 2 SiP Service calls + Fast 3 OEM Service calls + Fast 4 Standard Service calls + Fast 5-47 Reserved for future use + Fast 48-49 Trusted Application calls + Fast 50-63 Trusted OS calls + + Yielding 0- 1 Reserved for existing Armv7-A calls + Yielding 2-63 Trusted OS Standard Calls + +*Table 1: Service types and their corresponding Owning Entity Numbers* + +Each individual entity can allocate the valid identifiers within the entity +range as they need - it is not necessary to coordinate with other entities of +the same type. For example, two SoC providers can use the same Function ID +within the SiP Service calls OEN range to mean different things - as these +calls should be specific to the SoC. The Standard Runtime Calls OEN is used for +services defined by Arm standards, such as `PSCI`_. + +The SMC Function ID also indicates whether the call has followed the SMC32 +calling convention, where all parameters are 32-bit, or the SMC64 calling +convention, where the parameters are 64-bit. The framework identifies and +rejects invalid calls that use the SMC64 calling convention but that originate +from an AArch32 caller. + +The EL3 runtime services framework uses the call type and OEN to identify a +specific handler for each SMC call, but it is expected that an individual +handler will be responsible for all SMC Functions within a given service type. + +Getting started +--------------- + +TF-A has a ``services`` directory in the source tree under which +each owning entity can place the implementation of its runtime service. The +`PSCI`_ implementation is located here in the ``lib/psci`` directory. + +Runtime service sources will need to include the ``runtime_svc.h`` header file. + +Registering a runtime service +----------------------------- + +A runtime service is registered using the ``DECLARE_RT_SVC()`` macro, specifying +the name of the service, the range of OENs covered, the type of service and +initialization and call handler functions. + +.. code:: c + + #define DECLARE_RT_SVC(_name, _start, _end, _type, _setup, _smch) + +- ``_name`` is used to identify the data structure declared by this macro, and + is also used for diagnostic purposes + +- ``_start`` and ``_end`` values must be based on the ``OEN_*`` values defined in + ``smccc.h`` + +- ``_type`` must be one of ``SMC_TYPE_FAST`` or ``SMC_TYPE_YIELD`` + +- ``_setup`` is the initialization function with the ``rt_svc_init`` signature: + + .. code:: c + + typedef int32_t (*rt_svc_init)(void); + +- ``_smch`` is the SMC handler function with the ``rt_svc_handle`` signature: + + .. code:: c + + typedef uintptr_t (*rt_svc_handle_t)(uint32_t smc_fid, + u_register_t x1, u_register_t x2, + u_register_t x3, u_register_t x4, + void *cookie, + void *handle, + u_register_t flags); + +Details of the requirements and behavior of the two callbacks is provided in +the following sections. + +During initialization the services framework validates each declared service +to ensure that the following conditions are met: + +#. The ``_start`` OEN is not greater than the ``_end`` OEN +#. The ``_end`` OEN does not exceed the maximum OEN value (63) +#. The ``_type`` is one of ``SMC_TYPE_FAST`` or ``SMC_TYPE_YIELD`` +#. ``_setup`` and ``_smch`` routines have been specified + +``std_svc_setup.c`` provides an example of registering a runtime service: + +.. code:: c + + /* Register Standard Service Calls as runtime service */ + DECLARE_RT_SVC( + std_svc, + OEN_STD_START, + OEN_STD_END, + SMC_TYPE_FAST, + std_svc_setup, + std_svc_smc_handler + ); + +Initializing a runtime service +------------------------------ + +Runtime services are initialized once, during cold boot, by the primary CPU +after platform and architectural initialization is complete. The framework +performs basic validation of the declared service before calling +the service initialization function (``_setup`` in the declaration). This +function must carry out any essential EL3 initialization prior to receiving a +SMC Function call via the handler function. + +On success, the initialization function must return ``0``. Any other return value +will cause the framework to issue a diagnostic: + +:: + + Error initializing runtime service <name of the service> + +and then ignore the service - the system will continue to boot but SMC calls +will not be passed to the service handler and instead return the *Unknown SMC +Function ID* result ``0xFFFFFFFF``. + +If the system must not be allowed to proceed without the service, the +initialization function must itself cause the firmware boot to be halted. + +If the service uses per-CPU data this must either be initialized for all CPUs +during this call, or be done lazily when a CPU first issues an SMC call to that +service. + +Handling runtime service requests +--------------------------------- + +SMC calls for a service are forwarded by the framework to the service's SMC +handler function (``_smch`` in the service declaration). This function must have +the following signature: + +.. code:: c + + typedef uintptr_t (*rt_svc_handle_t)(uint32_t smc_fid, + u_register_t x1, u_register_t x2, + u_register_t x3, u_register_t x4, + void *cookie, + void *handle, + u_register_t flags); + +The handler is responsible for: + +#. Determining that ``smc_fid`` is a valid and supported SMC Function ID, + otherwise completing the request with the *Unknown SMC Function ID*: + + .. code:: c + + SMC_RET1(handle, SMC_UNK); + +#. Determining if the requested function is valid for the calling security + state. SMC Calls can be made from Non-secure, Secure or Realm worlds and + the framework will forward all calls to the service handler. + + The ``flags`` parameter to this function indicates the caller security state + in bits 0 and 5. The ``is_caller_secure(flags)``, ``is_caller_non_secure(flags)`` + and ``is_caller_realm(flags)`` helper functions can be used to determine whether + the caller's security state is Secure, Non-secure or Realm respectively. + + If invalid, the request should be completed with: + + .. code:: c + + SMC_RET1(handle, SMC_UNK); + +#. Truncating parameters for calls made using the SMC32 calling convention. + Such calls can be determined by checking the CC field in bit[30] of the + ``smc_fid`` parameter, for example by using: + + :: + + if (GET_SMC_CC(smc_fid) == SMC_32) ... + + For such calls, the upper bits of the parameters x1-x4 and the saved + parameters X5-X7 are UNDEFINED and must be explicitly ignored by the + handler. This can be done by truncating the values to a suitable 32-bit + integer type before use, for example by ensuring that functions defined + to handle individual SMC Functions use appropriate 32-bit parameters. + +#. Providing the service requested by the SMC Function, utilizing the + immediate parameters x1-x4 and/or the additional saved parameters X5-X7. + The latter can be retrieved using the ``SMC_GET_GP(handle, ref)`` function, + supplying the appropriate ``CTX_GPREG_Xn`` reference, e.g. + + .. code:: c + + uint64_t x6 = SMC_GET_GP(handle, CTX_GPREG_X6); + +#. Implementing the standard SMC32 Functions that provide information about + the implementation of the service. These are the Call Count, Implementor + UID and Revision Details for each service documented in section 6 of the + `SMCCC`_. + + TF-A expects owning entities to follow this recommendation. + +#. Returning the result to the caller. Based on `SMCCC`_ spec, results are + returned in W0-W7(X0-X7) registers for SMC32(SMC64) calls from AArch64 + state. Results are returned in R0-R7 registers for SMC32 calls from AArch32 + state. The framework provides a family of macros to set the multi-register + return value and complete the handler: + + .. code:: c + + AArch64 state: + + SMC_RET1(handle, x0); + SMC_RET2(handle, x0, x1); + SMC_RET3(handle, x0, x1, x2); + SMC_RET4(handle, x0, x1, x2, x3); + SMC_RET5(handle, x0, x1, x2, x3, x4); + SMC_RET6(handle, x0, x1, x2, x3, x4, x5); + SMC_RET7(handle, x0, x1, x2, x3, x4, x5, x6); + SMC_RET8(handle, x0, x1, x2, x3, x4, x5, x6, x7); + + AArch32 state: + + SMC_RET1(handle, r0); + SMC_RET2(handle, r0, r1); + SMC_RET3(handle, r0, r1, r2); + SMC_RET4(handle, r0, r1, r2, r3); + SMC_RET5(handle, r0, r1, r2, r3, r4); + SMC_RET6(handle, r0, r1, r2, r3, r4, r5); + SMC_RET7(handle, r0, r1, r2, r3, r4, r5, r6); + SMC_RET8(handle, r0, r1, r2, r3, r4, r5, r6, r7); + +The ``cookie`` parameter to the handler is reserved for future use and can be +ignored. The ``handle`` is returned by the SMC handler - completion of the +handler function must always be via one of the ``SMC_RETn()`` macros. + +.. note:: + The PSCI and Test Secure-EL1 Payload Dispatcher services do not follow + all of the above requirements yet. + +Services that contain multiple sub-services +------------------------------------------- + +It is possible that a single owning entity implements multiple sub-services. For +example, the Standard calls service handles ``0x84000000``-``0x8400FFFF`` and +``0xC4000000``-``0xC400FFFF`` functions. Within that range, the `PSCI`_ service +handles the ``0x84000000``-``0x8400001F`` and ``0xC4000000``-``0xC400001F`` functions. +In that respect, `PSCI`_ is a 'sub-service' of the Standard calls service. In +future, there could be additional such sub-services in the Standard calls +service which perform independent functions. + +In this situation it may be valuable to introduce a second level framework to +enable independent implementation of sub-services. Such a framework might look +very similar to the current runtime services framework, but using a different +part of the SMC Function ID to identify the sub-service. TF-A does not provide +such a framework at present. + +Secure-EL1 Payload Dispatcher service (SPD) +------------------------------------------- + +Services that handle SMC Functions targeting a Trusted OS, Trusted Application, +or other Secure-EL1 Payload are special. These services need to manage the +Secure-EL1 context, provide the *Secure Monitor* functionality of switching +between the normal and secure worlds, deliver SMC Calls through to Secure-EL1 +and generally manage the Secure-EL1 Payload through CPU power-state transitions. + +TODO: Provide details of the additional work required to implement a SPD and +the BL31 support for these services. Or a reference to the document that will +provide this information.... + +-------------- + +*Copyright (c) 2014-2021, Arm Limited and Contributors. All rights reserved.* + +.. _SMCCC: https://developer.arm.com/docs/den0028/latest +.. _PSCI: http://infocenter.arm.com/help/topic/com.arm.doc.den0022c/DEN0022C_Power_State_Coordination_Interface.pdf |