ARM Trusted Firmware Design

Contents :

Introduction
Cold boot
EL3 runtime services framework
Power State Coordination Interface
Secure-EL1 Payloads and Dispatchers
Crash Reporting in BL3-1
Memory layout on FVP platforms
Firmware Image Package (FIP)
Code Structure
References
Introduction

The ARM Trusted Firmware implements a subset of the Trusted Board Boot Requirements (TBBR) Platform Design Document (PDD) [1] for ARM reference platforms. The TBB sequence starts when the platform is powered on and runs up to the stage where it hands-off control to firmware running in the normal world in DRAM. This is the cold boot path.

The ARM Trusted Firmware also implements the Power State Coordination Interface (PSCI) PDD [2] as a runtime service. PSCI is the interface from normal world software to firmware implementing power management use-cases (for example, secondary CPU boot, hotplug and idle). Normal world software can access ARM Trusted Firmware runtime services via the ARM SMC (Secure Monitor Call) instruction. The SMC instruction must be used as mandated by the SMC Calling Convention PDD [3].

Cold boot

The cold boot path starts when the platform is physically turned on. One of the CPUs released from reset is chosen as the primary CPU, and the remaining CPUs are considered secondary CPUs. The primary CPU is chosen through platform-specific means. The cold boot path is mainly executed by the primary CPU, other than essential CPU initialization executed by all CPUs. The secondary CPUs are kept in a safe platform-specific state until the primary CPU has performed enough initialization to boot them.

The cold boot path in this implementation of the ARM Trusted Firmware is divided into five steps (in order of execution):

Boot Loader stage 1 (BL1) AP Trusted ROM
Boot Loader stage 2 (BL2) Trusted Boot Firmware
Boot Loader stage 3-1 (BL3-1) EL3 Runtime Firmware
Boot Loader stage 3-2 (BL3-2) Secure-EL1 Payload (optional)
Boot Loader stage 3-3 (BL3-3) Non-trusted Firmware

The ARM Fixed Virtual Platforms (FVPs) provide trusted ROM, trusted SRAM and trusted DRAM regions. Each boot loader stage uses one or more of these memories for its code and data.

The sections below provide the following details:

initialization and execution of the first three stages during cold boot
specification of the BL3-1 entrypoint requirements for use by alternative Trusted Boot Firmware in place of the provided BL1 and BL2
changes in BL3-1 behavior when using the RESET_TO_BL31 option which allows BL3-1 to run without BL1 and BL2

BL1

This stage begins execution from the platform's reset vector in trusted ROM at EL3. BL1 code starts at 0x00000000 (trusted ROM) in the FVP memory map. The BL1 data section is placed at the start of trusted SRAM, 0x04000000. The functionality implemented by this stage is as follows.

Determination of boot path

Whenever a CPU is released from reset, BL1 needs to distinguish between a warm boot and a cold boot. This is done using a platform-specific mechanism. The ARM FVPs implement a simple power controller at 0x1c100000. The PSYS register (0x10) is used to distinguish between a cold and warm boot. This information is contained in the PSYS.WK[25:24] field. Additionally, a per-CPU mailbox is maintained in trusted DRAM (0x00600000), to which BL1 writes an entrypoint. Each CPU jumps to this entrypoint upon warm boot. During cold boot, BL1 places the secondary CPUs in a safe platform-specific state while the primary CPU executes the remaining cold boot path as described in the following sections.

Architectural initialization

BL1 performs minimal architectural initialization as follows.

Exception vectors

BL1 sets up simple exception vectors for both synchronous and asynchronous exceptions. The default behavior upon receiving an exception is to set a status code. In the case of the FVP this code is written to the Versatile Express System LED register in the following format:

SYS_LED[0]   - Security state (Secure=0/Non-Secure=1)
SYS_LED[2:1] - Exception Level (EL3=0x3, EL2=0x2, EL1=0x1, EL0=0x0)
SYS_LED[7:3] - Exception Class (Sync/Async & origin). The values for
               each exception class are:

0x0 : Synchronous exception from Current EL with SP_EL0
0x1 : IRQ exception from Current EL with SP_EL0
0x2 : FIQ exception from Current EL with SP_EL0
0x3 : System Error exception from Current EL with SP_EL0
0x4 : Synchronous exception from Current EL with SP_ELx
0x5 : IRQ exception from Current EL with SP_ELx
0x6 : FIQ exception from Current EL with SP_ELx
0x7 : System Error exception from Current EL with SP_ELx
0x8 : Synchronous exception from Lower EL using aarch64
0x9 : IRQ exception from Lower EL using aarch64
0xa : FIQ exception from Lower EL using aarch64
0xb : System Error exception from Lower EL using aarch64
0xc : Synchronous exception from Lower EL using aarch32
0xd : IRQ exception from Lower EL using aarch32
0xe : FIQ exception from Lower EL using aarch32
0xf : System Error exception from Lower EL using aarch32

A write to the LED register reflects in the System LEDs (S6LED0..7) in the CLCD window of the FVP. This behavior is because this boot loader stage does not expect to receive any exceptions other than the SMC exception. For the latter, BL1 installs a simple stub. The stub expects to receive only a single type of SMC (determined by its function ID in the general purpose register X0). This SMC is raised by BL2 to make BL1 pass control to BL3-1 (loaded by BL2) at EL3. Any other SMC leads to an assertion failure.

MMU setup
BL1 sets up EL3 memory translation by creating page tables to cover the first 4GB of physical address space. This covers all the memories and peripherals needed by BL1.
Control register setup
- SCTLR_EL3. Instruction cache is enabled by setting the SCTLR_EL3.I bit. Alignment and stack alignment checking is enabled by setting the SCTLR_EL3.A and SCTLR_EL3.SA bits. Exception endianness is set to little-endian by clearing the SCTLR_EL3.EE bit.
- CPUECTLR. When the FVP includes a model of a specific ARM processor implementation (for example A57 or A53), then intra-cluster coherency is enabled by setting the CPUECTLR.SMPEN bit. The AEMv8 Base FVP is inherently coherent so does not implement CPUECTLR.
- SCR. Use of the HVC instruction from EL1 is enabled by setting the SCR.HCE bit. FIQ exceptions are configured to be taken in EL3 by setting the SCR.FIQ bit. The register width of the next lower exception level is set to AArch64 by setting the SCR.RW bit. External Aborts and SError Interrupts are configured to be taken in EL3 by setting the SCR.EA bit.
- CPTR_EL3. Accesses to the CPACR_EL1 register from EL1 or EL2, or the CPTR_EL2 register from EL2 are configured to not trap to EL3 by clearing the CPTR_EL3.TCPAC bit. Access to the trace functionality is configured not to trap to EL3 by clearing the CPTR_EL3.TTA bit. Instructions that access the registers associated with Floating Point and Advanced SIMD execution are configured to not trap to EL3 by clearing the CPTR_EL3.TFP bit.

Platform initialization

BL1 enables issuing of snoop and DVM (Distributed Virtual Memory) requests from the CCI-400 slave interface corresponding to the cluster that includes the primary CPU. BL1 also initializes UART0 (PL011 console), which enables access to the printf family of functions in BL1.

BL2 image load and execution

BL1 execution continues as follows:

BL1 determines the amount of free trusted SRAM memory available by calculating the extent of its own data section, which also resides in trusted SRAM. BL1 loads a BL2 raw binary image from platform storage, at a platform-specific base address. If the BL2 image file is not present or if there is not enough free trusted SRAM the following error message is printed:
```
"Failed to load boot loader stage 2 (BL2) firmware."
```
If the load is successful, BL1 updates the limits of the remaining free trusted SRAM. It also populates information about the amount of trusted SRAM used by the BL2 image. The exact load location of the image is provided as a base address in the platform header. Further description of the memory layout can be found later in this document.
BL1 prints the following string from the primary CPU to indicate successful execution of the BL1 stage:
```
"Booting trusted firmware boot loader stage 1"
```
BL1 passes control to the BL2 image at Secure EL1, starting from its load address.
BL1 also passes information about the amount of trusted SRAM used and available for use. This information is populated at a platform-specific memory address.

BL2

BL1 loads and passes control to BL2 at Secure-EL1. BL2 is linked against and loaded at a platform-specific base address (more information can be found later in this document). The functionality implemented by BL2 is as follows.

Architectural initialization

BL2 performs minimal architectural initialization required for subsequent stages of the ARM Trusted Firmware and normal world software. It sets up Secure EL1 memory translation by creating page tables to address the first 4GB of the physical address space in a similar way to BL1. EL1 and EL0 are given access to Floating Point & Advanced SIMD registers by clearing the CPACR.FPEN bits.

Platform initialization

BL2 copies the information regarding the trusted SRAM populated by BL1 using a platform-specific mechanism. It calculates the limits of DRAM (main memory) to determine whether there is enough space to load the BL3-3 image. A platform defined base address is used to specify the load address for the BL3-1 image. It also defines the extents of memory available for use by the BL3-2 image. BL2 also initializes UART0 (PL011 console), which enables access to the printf family of functions in BL2. Platform security is initialized to allow access to access controlled components. On the Base FVP a TrustZone controller (TZC-400) is configured to give full access to the platform DRAM. The storage abstraction layer is initialized which is used to load further bootloader images.

BL3-0 (System Control Processor Firmware) image load

Some systems have a separate System Control Processor (SCP) for power, clock, reset and system control. BL2 loads the optional BL3-0 image from platform storage into a platform-specific region of secure memory. The subsequent handling of BL3-0 is platform specific. Typically the image is transferred into SCP memory using a platform-specific protocol. The SCP executes BL3-0 and signals to the Application Processor (AP) for BL2 execution to continue.

BL3-1 (EL3 Runtime Firmware) image load

BL2 loads the BL3-1 image from platform storage into a platform-specific address in trusted SRAM. If there is not enough memory to load the image or image is missing it leads to an assertion failure. If the BL3-1 image loads successfully, BL2 updates the amount of trusted SRAM used and available for use by BL3-1. This information is populated at a platform-specific memory address.

BL3-2 (Secure-EL1 Payload) image load

BL2 loads the optional BL3-2 image from platform storage into a platform- specific region of secure memory. The image executes in the secure world. BL2 relies on BL3-1 to pass control to the BL3-2 image, if present. Hence, BL2 populates a platform-specific area of memory with the entrypoint/load-address of the BL3-2 image. The value of the Saved Processor Status Register (SPSR) for entry into BL3-2 is not determined by BL2, it is initialized by the Secure-EL1 Payload Dispatcher (see later) within BL3-1, which is responsible for managing interaction with BL3-2. This information is passed to BL3-1.

BL3-3 (Non-trusted Firmware) image load

BL2 loads the BL3-3 image (e.g. UEFI or other test or boot software) from platform storage into non-secure memory as defined by the platform (0x88000000 for FVPs).

BL2 relies on BL3-1 to pass control to BL3-3 once secure state initialization is complete. Hence, BL2 populates a platform-specific area of memory with the entrypoint and Saved Program Status Register (SPSR) of the normal world software image. The entrypoint is the load address of the BL3-3 image. The SPSR is determined as specified in Section 5.13 of the [PSCI PDD] PSCI. This information is passed to BL3-1.

BL3-1 (EL3 Runtime Firmware) execution

BL2 execution continues as follows:

BL2 passes control back to BL1 by raising an SMC, providing BL1 with the BL3-1 entrypoint. The exception is handled by the SMC exception handler installed by BL1.
BL1 turns off the MMU and flushes the caches. It clears the SCTLR_EL3.M/I/C bits, flushes the data cache to the point of coherency and invalidates the TLBs.
BL1 passes control to BL3-1 at the specified entrypoint at EL3.

BL3-1

The image for this stage is loaded by BL2 and BL1 passes control to BL3-1 at EL3. BL3-1 executes solely in trusted SRAM. BL3-1 is linked against and loaded at a platform-specific base address (more information can be found later in this document). The functionality implemented by BL3-1 is as follows.

Architectural initialization

Currently, BL3-1 performs a similar architectural initialization to BL1 as far as system register settings are concerned. Since BL1 code resides in ROM, architectural initialization in BL3-1 allows override of any previous initialization done by BL1. BL3-1 creates page tables to address the first 4GB of physical address space and initializes the MMU accordingly. It initializes a buffer of frequently used pointers, called per-cpu pointer cache, in memory for faster access. Currently the per-cpu pointer cache contains only the pointer to crash stack. It then replaces the exception vectors populated by BL1 with its own. BL3-1 exception vectors implement more elaborate support for handling SMCs since this is the only mechanism to access the runtime services implemented by BL3-1 (PSCI for example). BL3-1 checks each SMC for validity as specified by the SMC calling convention PDD before passing control to the required SMC handler routine. BL3-1 programs the CNTFRQ_EL0 register with the clock frequency of the system counter, which is provided by the platform.

Platform initialization

BL3-1 performs detailed platform initialization, which enables normal world software to function correctly. It also retrieves entrypoint information for the BL3-3 image loaded by BL2 from the platform defined memory address populated by BL2. BL3-1 also initializes UART0 (PL011 console), which enables access to the printf family of functions in BL3-1. It enables the system level implementation of the generic timer through the memory mapped interface.

GICv2 initialization:
- Enable group0 interrupts in the GIC CPU interface.
- Configure group0 interrupts to be asserted as FIQs.
- Disable the legacy interrupt bypass mechanism.
- Configure the priority mask register to allow interrupts of all priorities to be signaled to the CPU interface.
- Mark SGIs 8-15, the secure physical timer interrupt (#29) and the trusted watchdog interrupt (#56) as group0 (secure).
- Target the trusted watchdog interrupt to CPU0.
- Enable these group0 interrupts in the GIC distributor.
- Configure all other interrupts as group1 (non-secure).
- Enable signaling of group0 interrupts in the GIC distributor.
GICv3 initialization:
If a GICv3 implementation is available in the platform, BL3-1 initializes the GICv3 in GICv2 emulation mode with settings as described for GICv2 above.
Power management initialization:
BL3-1 implements a state machine to track CPU and cluster state. The state can be one of OFF, ON_PENDING, SUSPEND or ON. All secondary CPUs are initially in the OFF state. The cluster that the primary CPU belongs to is ON; any other cluster is OFF. BL3-1 initializes the data structures that implement the state machine, including the locks that protect them. BL3-1 accesses the state of a CPU or cluster immediately after reset and before the MMU is enabled in the warm boot path. It is not currently possible to use 'exclusive' based spinlocks, therefore BL3-1 uses locks based on Lamport's Bakery algorithm instead. BL3-1 allocates these locks in device memory. They are accessible irrespective of MMU state.
Runtime services initialization:
The runtime service framework and its initialization is described in the "EL3 runtime services framework" section below.
Details about the PSCI service are provided in the "Power State Coordination Interface" section below.
BL3-2 (Secure-EL1 Payload) image initialization
If a BL3-2 image is present then there must be a matching Secure-EL1 Payload Dispatcher (SPD) service (see later for details). During initialization that service must register a function to carry out initialization of BL3-2 once the runtime services are fully initialized. BL3-1 invokes such a registered function to initialize BL3-2 before running BL3-3.
Details on BL3-2 initialization and the SPD's role are described in the "Secure-EL1 Payloads and Dispatchers" section below.
BL3-3 (Non-trusted Firmware) execution
BL3-1 initializes the EL2 or EL1 processor context for normal-world cold boot, ensuring that no secure state information finds its way into the non-secure execution state. BL3-1 uses the entrypoint information provided by BL2 to jump to the Non-trusted firmware image (BL3-3) at the highest available Exception Level (EL2 if available, otherwise EL1).

Using alternative Trusted Boot Firmware in place of BL1 and BL2

Some platforms have existing implementations of Trusted Boot Firmware that would like to use ARM Trusted Firmware BL3-1 for the EL3 Runtime Firmware. To enable this firmware architecture it is important to provide a fully documented and stable interface between the Trusted Boot Firmware and BL3-1.

Future changes to the BL3-1 interface will be done in a backwards compatible way, and this enables these firmware components to be independently enhanced/ updated to develop and exploit new functionality.

Required CPU state when calling `bl31_entrypoint()` during cold boot

This function must only be called by the primary CPU, if this is called by any other CPU the firmware will abort.

On entry to this function the calling primary CPU must be executing in AArch64 EL3, little-endian data access, and all interrupt sources masked:

PSTATE.EL = 3
PSTATE.RW = 1
PSTATE.DAIF = 0xf
CTLR_EL3.EE = 0

X0 and X1 can be used to pass information from the Trusted Boot Firmware to the platform code in BL3-1:

X0 : Reserved for common Trusted Firmware information
X1 : Platform specific information

BL3-1 zero-init sections (e.g. .bss) should not contain valid data on entry, these will be zero filled prior to invoking platform setup code.

Use of the X0 and X1 parameters

The parameters are platform specific and passed from bl31_entrypoint() to bl31_early_platform_setup(). The value of these parameters is never directly used by the common BL3-1 code.

The convention is that X0 conveys information regarding the BL3-1, BL3-2 and BL3-3 images from the Trusted Boot firmware and X1 can be used for other platform specific purpose. This convention allows platforms which use ARM Trusted Firmware's BL1 and BL2 images to transfer additional platform specific information from Secure Boot without conflicting with future evolution of the Trusted Firmware using X0 to pass a bl31_params structure.

BL3-1 common and SPD initialization code depends on image and entrypoint information about BL3-3 and BL3-2, which is provided via BL3-1 platform APIs. This information is required until the start of execution of BL3-3. This information can be provided in a platform defined manner, e.g. compiled into the platform code in BL3-1, or provided in a platform defined memory location by the Trusted Boot firmware, or passed from the Trusted Boot Firmware via the Cold boot Initialization parameters. This data may need to be cleaned out of the CPU caches if it is provided by an earlier boot stage and then accessed by BL3-1 platform code before the caches are enabled.

ARM Trusted Firmware's BL2 implementation passes a bl31_params structure in X0 and the FVP port interprets this in the BL3-1 platform code.

MMU, Data caches & Coherency

BL3-1 does not depend on the enabled state of the MMU, data caches or interconnect coherency on entry to bl31_entrypoint(). If these are disabled on entry, these should be enabled during bl31_plat_arch_setup().

Data structures used in the BL3-1 cold boot interface

These structures are designed to support compatibility and independent evolution of the structures and the firmware images. For example, a version of BL3-1 that can interpret the BL3-x image information from different versions of BL2, a platform that uses an extended entry_point_info structure to convey additional register information to BL3-1, or a ELF image loader that can convey more details about the firmware images.

To support these scenarios the structures are versioned and sized, which enables BL3-1 to detect which information is present and respond appropriately. The param_header is defined to capture this information:

typedef struct param_header {
    uint8_t type;       /* type of the structure */
    uint8_t version;    /* version of this structure */
    uint16_t size;      /* size of this structure in bytes */
    uint32_t attr;      /* attributes: unused bits SBZ */
} param_header_t;

The structures using this format are entry_point_info, image_info and bl31_params. The code that allocates and populates these structures must set the header fields appropriately, and the SET_PARA_HEAD() a macro is defined to simplify this action.

Required CPU state for BL3-1 Warm boot initialization

When requesting a CPU power-on, or suspending a running CPU, ARM Trusted Firmware provides the platform power management code with a Warm boot initialization entry-point, to be invoked by the CPU immediately after the reset handler. On entry to the Warm boot initialization function the calling CPU must be in AArch64 EL3, little-endian data access and all interrupt sources masked:

PSTATE.EL = 3
PSTATE.RW = 1
PSTATE.DAIF = 0xf
SCTLR_EL3.EE = 0

The PSCI implementation will initialize the processor state and ensure that the platform power management code is then invoked as required to initialize all necessary system, cluster and CPU resources.

Using BL3-1 as the CPU reset vector

On some platforms the runtime firmware (BL3-x images) for the application processors are loaded by trusted firmware running on a secure system processor on the SoC, rather than by BL1 and BL2 running on the primary application processor. For this type of SoC it is desirable for the application processor to always reset to BL3-1 which eliminates the need for BL1 and BL2.

ARM Trusted Firmware provides a build-time option RESET_TO_BL31 that includes some additional logic in the BL3-1 entrypoint to support this use case.

In this configuration, the platform's Trusted Boot Firmware must ensure that BL3-1 is loaded to its runtime address, which must match the CPU's RVBAR reset vector address, before the application processor is powered on. Additionally, platform software is responsible for loading the other BL3-x images required and providing entry point information for them to BL3-1. Loading these images might be done by the Trusted Boot Firmware or by platform code in BL3-1.

The ARM FVP port supports the RESET_TO_BL31 configuration, in which case the bl31.bin image must be loaded to its run address in Trusted SRAM and all CPU reset vectors be changed from the default 0x0 to this run address. See the User Guide for details of running the FVP models in this way.

This configuration requires some additions and changes in the BL3-1 functionality:

Determination of boot path

In this configuration, BL3-1 uses the same reset framework and code as the one described for BL1 above. On a warm boot a CPU is directed to the PSCI implementation via a platform defined mechanism. On a cold boot, the platform must place any secondary CPUs into a safe state while the primary CPU executes a modified BL3-1 initialization, as described below.

Architectural initialization

As the first image to execute in this configuration BL3-1 must ensure that interconnect coherency is enabled (if required) before enabling the MMU.

Platform initialization

In this configuration, when the CPU resets to BL3-1 there are no parameters that can be passed in registers by previous boot stages. Instead, the platform code in BL3-1 needs to know, or be able to determine, the location of the BL3-2 (if required) and BL3-3 images and provide this information in response to the bl31_plat_get_next_image_ep_info() function.

As the first image to execute in this configuration BL3-1 must also ensure that any security initialisation, for example programming a TrustZone address space controller, is carried out during early platform initialisation.

EL3 runtime services framework

Software executing in the non-secure state and in the secure state 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 returned.

The EL3 runtime services framework enables the development of services by different providers that can be easily integrated into final product firmware. The following sections describe the framework which facilitates the registration, initialization and use of runtime services in EL3 Runtime Firmware (BL3-1).

The design 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 more detailed explanation of these terms.

The following runtime services are expected to be implemented first. They have not all been instantiated in the current implementation.

Standard service calls
This service is for management of the entire system. The Power State Coordination Interface (PSCI) is the first set of standard service calls defined by ARM (see PSCI section later).
NOTE: Currently this service is called PSCI since there are no other defined standard service calls.
Secure-EL1 Payload Dispatcher service
If a system runs a Trusted OS or other Secure-EL1 Payload (SP) then it also requires a Secure Monitor at EL3 to switch the EL1 processor context between the normal world (EL1/EL2) and trusted world (Secure-EL1). The Secure Monitor will make these world switches in response to SMCs. The SMCCC provides for such SMCs with the Trusted OS Call and Trusted Application Call OEN ranges.
The interface between the EL3 Runtime Firmware and the Secure-EL1 Payload is not defined by the SMCCC or any other standard. As a result, each Secure-EL1 Payload requires a specific Secure Monitor that runs as a runtime service - within ARM Trusted Firmware this service is referred to as the Secure-EL1 Payload Dispatcher (SPD).
ARM Trusted Firmware provides a Test Secure-EL1 Payload (TSP) and its associated Dispatcher (TSPD). Details of SPD design and TSP/TSPD operation are described in the "Secure-EL1 Payloads and Dispatchers" section below.
CPU implementation service
This service will provide an interface to CPU implementation specific services for a given platform e.g. access to processor errata workarounds. This service is currently unimplemented.

Additional services for ARM Architecture, SiP and OEM calls can be implemented. Each implemented service handles a range of SMC function identifiers as described in the SMCCC.

Registration

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. This macro instantiates a const struct rt_svc_desc for the service with these details (see runtime_svc.h). This structure is allocated in a special ELF section rt_svc_descs, enabling the framework to find all service descriptors included into BL3-1.

The specific service for a SMC Function is selected based on the OEN and call type of the Function ID, and the framework uses that information in the service descriptor to identify the handler for the SMC Call.

The service descriptors do not include information to identify the precise set of SMC function identifiers supported by this service implementation, the security state from which such calls are valid nor the capability to support 64-bit and/or 32-bit callers (using SMC32 or SMC64). Responding appropriately to these aspects of a SMC call is the responsibility of the service implementation, the framework is focused on integration of services from different providers and minimizing the time taken by the framework before the service handler is invoked.

Details of the parameters, requirements and behavior of the initialization and call handling functions are provided in the following sections.

Initialization

runtime_svc_init() in runtime_svc.c initializes the runtime services framework running on the primary CPU during cold boot as part of the BL3-1 initialization. This happens prior to initializing a Trusted OS and running Normal world boot firmware that might in turn use these services. Initialization involves validating each of the declared runtime service descriptors, calling the service initialization function and populating the index used for runtime lookup of the service.

The BL3-1 linker script collects all of the declared service descriptors into a single array and defines symbols that allow the framework to locate and traverse the array, and determine its size.

The framework does basic validation of each descriptor to halt firmware initialization if service declaration errors are detected. The framework does not check descriptors for the following error conditions, and may behave in an unpredictable manner under such scenarios:

Overlapping OEN ranges
Multiple descriptors for the same range of OENs and call_type
Incorrect range of owning entity numbers for a given call_type

Once validated, the service init() callback is invoked. This function carries out any essential EL3 initialization before servicing requests. The init() function is only invoked on the primary CPU during cold boot. 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. If init() returns anything other than 0, this is treated as an initialization error and the service is ignored: this does not cause the firmware to halt.

The OEN and call type fields present in the SMC Function ID cover a total of 128 distinct services, but in practice a single descriptor can cover a range of OENs, e.g. SMCs to call a Trusted OS function. To optimize the lookup of a service handler, the framework uses an array of 128 indices that map every distinct OEN/call-type combination either to one of the declared services or to indicate the service is not handled. This rt_svc_descs_indices[] array is populated for all of the OENs covered by a service after the service init() function has reported success. So a service that fails to initialize will never have it's handle() function invoked.

The following figure shows how the rt_svc_descs_indices[] index maps the SMC Function ID call type and OEN onto a specific service handler in the rt_svc_descs[] array.

Handling an SMC

When the EL3 runtime services framework receives a Secure Monitor Call, the SMC Function ID is passed in W0 from the lower exception level (as per the SMCCC). If the calling register width is AArch32, it is invalid to invoke an SMC Function which indicates the SMC64 calling convention: such calls are ignored and return the Unknown SMC Function Identifier result code 0xFFFFFFFF in R0/X0.

Bit[31] (fast/standard call) and bits[29:24] (owning entity number) of the SMC Function ID are combined to index into the rt_svc_descs_indices[] array. The resulting value might indicate a service that has no handler, in this case the framework will also report an Unknown SMC Function ID. Otherwise, the value is used as a further index into the rt_svc_descs[] array to locate the required service and handler.

The service's handle() callback is provided with five of the SMC parameters directly, the others are saved into memory for retrieval (if needed) by the handler. The handler is also provided with an opaque handle for use with the supporting library for parameter retrieval, setting return values and context manipulation; and with flags indicating the security state of the caller. The framework finally sets up the execution stack for the handler, and invokes the 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.

Power State Coordination Interface

TODO: Provide design walkthrough of PSCI implementation.

The complete PSCI API is not yet implemented. The following functions are currently implemented:

PSCI_VERSION
CPU_OFF
CPU_ON
CPU_SUSPEND
AFFINITY_INFO

The CPU_ON, CPU_OFF and CPU_SUSPEND functions implement the warm boot path in ARM Trusted Firmware. CPU_ON and CPU_OFF have undergone testing on all the supported FVPs. CPU_SUSPEND & AFFINITY_INFO have undergone testing only on the AEM v8 Base FVP. Support for AFFINITY_INFO is still experimental. Support for CPU_SUSPEND is stable for entry into power down states. Standby states are currently not supported. PSCI_VERSION is present but completely untested in this version of the software.

Unsupported PSCI functions can be divided into ones that can return execution to the caller and ones that cannot. The following functions return with a error code as documented in the [Power State Coordination Interface PDD] PSCI.

MIGRATE : -1 (NOT_SUPPORTED)
MIGRATE_INFO_TYPE : 2 (Trusted OS is either not present or does not require migration)
MIGRATE_INFO_UP_CPU : 0 (Return value is UNDEFINED)

The following unsupported functions do not return and signal an assertion failure if invoked.

SYSTEM_OFF
SYSTEM_RESET

Secure-EL1 Payloads and Dispatchers

On a production system that includes a Trusted OS running in Secure-EL1/EL0, the Trusted OS is coupled with a companion runtime service in the BL3-1 firmware. This service is responsible for the initialisation of the Trusted OS and all communications with it. The Trusted OS is the BL3-2 stage of the boot flow in ARM Trusted Firmware. The firmware will attempt to locate, load and execute a BL3-2 image.

ARM Trusted Firmware uses a more general term for the BL3-2 software that runs at Secure-EL1 - the Secure-EL1 Payload - as it is not always a Trusted OS.

The ARM Trusted Firmware provides a Test Secure-EL1 Payload (TSP) and a Test Secure-EL1 Payload Dispatcher (TSPD) service as an example of how a Trusted OS is supported on a production system using the Runtime Services Framework. On such a system, the Test BL3-2 image and service are replaced by the Trusted OS and its dispatcher service.

The TSP runs in Secure-EL1. It is designed to demonstrate synchronous communication with the normal-world software running in EL1/EL2. Communication is initiated by the normal-world software

either directly through a Fast SMC (as defined in the SMCCC)
or indirectly through a PSCI SMC. The PSCI implementation in turn informs the TSPD about the requested power management operation. This allows the TSP to prepare for or respond to the power state change

The TSPD service is responsible for.

Initializing the TSP
Routing requests and responses between the secure and the non-secure states during the two types of communications just described

Initializing a BL3-2 Image

The Secure-EL1 Payload Dispatcher (SPD) service is responsible for initializing the BL3-2 image. It needs access to the information passed by BL2 to BL3-1 to do so. This is provided by:

entry_point_info_t *bl31_plat_get_next_image_ep_info(uint32_t);

which returns a reference to the entry_point_info structure corresponding to the image which will be run in the specified security state. The SPD uses this API to get entry point information for the SECURE image, BL3-2.

In the absence of a BL3-2 image, BL3-1 passes control to the normal world bootloader image (BL3-3). When the BL3-2 image is present, it is typical that the SPD wants control to be passed to BL3-2 first and then later to BL3-3.

To do this the SPD has to register a BL3-2 initialization function during initialization of the SPD service. The BL3-2 initialization function has this prototype:

int32_t init();

and is registered using the bl31_register_bl32_init() function.

Trusted Firmware supports two approaches for the SPD to pass control to BL3-2 before returning through EL3 and running the non-trusted firmware (BL3-3):

In the BL3-2 initialization function, set up a secure context (see below for more details of CPU context support) for this CPU and use bl31_set_next_image_type() to request that the exit from bl31_main() is to the BL3-2 entrypoint in Secure-EL1.
When the BL3-2 has completed initialization at Secure-EL1, it returns to BL3-1 by issuing an SMC, using a Function ID allocated to the SPD. On receipt of this SMC, the SPD service handler should switch the CPU context from trusted to normal world and use the bl31_set_next_image_type() and bl31_prepare_next_image_entry() functions to set up the initial return to the normal world firmware BL3-3. On return from the handler the framework will exit to EL2 and run BL3-3.
In the BL3-2 initialization function, use an SPD-defined mechanism to invoke a 'world-switch synchronous call' to Secure-EL1 to run the BL3-2 entrypoint. NOTE: The Test SPD service included with the Trusted Firmware provides one implementation of such a mechanism.
On completion BL3-2 returns control to BL3-1 via a SMC, and on receipt the SPD service handler invokes the synchronous call return mechanism to return to the BL3-2 initialization function. On return from this function, bl31_main() will set up the return to the normal world firmware BL3-3 and continue the boot process in the normal world.
Crash Reporting in BL3-1

The BL3-1 implements a scheme for reporting the processor state when an unhandled exception is encountered. The reporting mechanism attempts to preserve all the register contents and report it via the default serial output. The general purpose registers, EL3, Secure EL1 and some EL2 state registers are reported.

A dedicated per-cpu crash stack is maintained by BL3-1 and this is retrieved via the per-cpu pointer cache. The implementation attempts to minimise the memory required for this feature. The file crash_reporting.S contains the implementation for crash reporting.

The sample crash output is shown below.

x0	:0x000000004F00007C
x1	:0x0000000007FFFFFF
x2	:0x0000000004014D50
x3	:0x0000000000000000
x4	:0x0000000088007998
x5	:0x00000000001343AC
x6	:0x0000000000000016
x7	:0x00000000000B8A38
x8	:0x00000000001343AC
x9	:0x00000000000101A8
x10	:0x0000000000000002
x11	:0x000000000000011C
x12	:0x00000000FEFDC644
x13	:0x00000000FED93FFC
x14	:0x0000000000247950
x15	:0x00000000000007A2
x16	:0x00000000000007A4
x17	:0x0000000000247950
x18	:0x0000000000000000
x19	:0x00000000FFFFFFFF
x20	:0x0000000004014D50
x21	:0x000000000400A38C
x22	:0x0000000000247950
x23	:0x0000000000000010
x24	:0x0000000000000024
x25	:0x00000000FEFDC868
x26	:0x00000000FEFDC86A
x27	:0x00000000019EDEDC
x28	:0x000000000A7CFDAA
x29	:0x0000000004010780
x30	:0x000000000400F004
scr_el3	:0x0000000000000D3D
sctlr_el3	:0x0000000000C8181F
cptr_el3	:0x0000000000000000
tcr_el3	:0x0000000080803520
daif	:0x00000000000003C0
mair_el3	:0x00000000000004FF
spsr_el3	:0x00000000800003CC
elr_el3	:0x000000000400C0CC
ttbr0_el3	:0x00000000040172A0
esr_el3	:0x0000000096000210
sp_el3	:0x0000000004014D50
far_el3	:0x000000004F00007C
spsr_el1	:0x0000000000000000
elr_el1	:0x0000000000000000
spsr_abt	:0x0000000000000000
spsr_und	:0x0000000000000000
spsr_irq	:0x0000000000000000
spsr_fiq	:0x0000000000000000
sctlr_el1	:0x0000000030C81807
actlr_el1	:0x0000000000000000
cpacr_el1	:0x0000000000300000
csselr_el1	:0x0000000000000002
sp_el1	:0x0000000004028800
esr_el1	:0x0000000000000000
ttbr0_el1	:0x000000000402C200
ttbr1_el1	:0x0000000000000000
mair_el1	:0x00000000000004FF
amair_el1	:0x0000000000000000
tcr_el1	:0x0000000000003520
tpidr_el1	:0x0000000000000000
tpidr_el0	:0x0000000000000000
tpidrro_el0	:0x0000000000000000
dacr32_el2	:0x0000000000000000
ifsr32_el2	:0x0000000000000000
par_el1	:0x0000000000000000
far_el1	:0x0000000000000000
afsr0_el1	:0x0000000000000000
afsr1_el1	:0x0000000000000000
contextidr_el1	:0x0000000000000000
vbar_el1	:0x0000000004027000
cntp_ctl_el0	:0x0000000000000000
cntp_cval_el0	:0x0000000000000000
cntv_ctl_el0	:0x0000000000000000
cntv_cval_el0	:0x0000000000000000
cntkctl_el1	:0x0000000000000000
fpexc32_el2	:0x0000000004000700
sp_el0	:0x0000000004010780

Memory layout on FVP platforms

On FVP platforms, we use the Trusted ROM and Trusted SRAM to store the trusted firmware binaries. BL1 is originally sitting in the Trusted ROM at address 0x0. Its read-write data are relocated at the base of the Trusted SRAM at runtime. BL1 loads BL2 image near the top of the trusted SRAM. BL2 loads BL3-1 image between BL1 and BL2. Optionally, BL2 then loads the TSP as the BL3-2 image. By default it is loaded in Trusted SRAM, in this case it sits between BL3-1 and BL2. This memory layout is illustrated by the following diagram.

Trusted SRAM
+----------+ 0x04040000
|          |
|----------|
|   BL2    |
|----------|
|          |
|----------|
|   BL32   | (optional)
|----------|
|          |
|----------|
|   BL31   |
|----------|
|          |
|----------|
| BL1 (rw) |
+----------+ 0x04000000

Trusted ROM
+----------+ 0x04000000
| BL1 (ro) |
+----------+ 0x00000000

The TSP image may be loaded in Trusted DRAM instead. This doesn't change the memory layout of the other boot loader images in Trusted SRAM.

Although the goal at long term is to give complete flexibility over the memory layout, all platforms should conform to this layout at the moment. This is because of some limitations in the implementation of the image loader in the Trusted Firmware. Refer to the "Limitations of the image loader" section below.

Each bootloader stage image layout is described by its own linker script. The linker scripts export some symbols into the program symbol table. Their values correspond to particular addresses. The trusted firmware code can refer to these symbols to figure out the image memory layout.

Linker symbols follow the following naming convention in the trusted firmware.

__<SECTION>_START__
Start address of a given section named <SECTION>.
__<SECTION>_END__
End address of a given section named <SECTION>. If there is an alignment constraint on the section's end address then __<SECTION>_END__ corresponds to the end address of the section's actual contents, rounded up to the right boundary. Refer to the value of __<SECTION>_UNALIGNED_END__ to know the actual end address of the section's contents.
__<SECTION>_UNALIGNED_END__
End address of a given section named <SECTION> without any padding or rounding up due to some alignment constraint.
__<SECTION>_SIZE__
Size (in bytes) of a given section named <SECTION>. If there is an alignment constraint on the section's end address then __<SECTION>_SIZE__ corresponds to the size of the section's actual contents, rounded up to the right boundary. In other words, __<SECTION>_SIZE__ = __<SECTION>_END__ - _<SECTION>_START__. Refer to the value of __<SECTION>_UNALIGNED_SIZE__ to know the actual size of the section's contents.
__<SECTION>_UNALIGNED_SIZE__
Size (in bytes) of a given section named <SECTION> without any padding or rounding up due to some alignment constraint. In other words, __<SECTION>_UNALIGNED_SIZE__ = __<SECTION>_UNALIGNED_END__ - __<SECTION>_START__.

Some of the linker symbols are mandatory as the trusted firmware code relies on them to be defined. They are listed in the following subsections. Some of them must be provided for each bootloader stage and some are specific to a given bootloader stage.

The linker scripts define some extra, optional symbols. They are not actually used by any code but they help in understanding the bootloader images' memory layout as they are easy to spot in the link map files.

Common linker symbols

Early setup code needs to know the extents of the BSS section to zero-initialise it before executing any C code. The following linker symbols are defined for this purpose:

__BSS_START__ This address must be aligned on a 16-byte boundary.
__BSS_SIZE__

Similarly, the coherent memory section must be zero-initialised. Also, the MMU setup code needs to know the extents of this section to set the right memory attributes for it. The following linker symbols are defined for this purpose:

__COHERENT_RAM_START__ This address must be aligned on a page-size boundary.
__COHERENT_RAM_END__ This address must be aligned on a page-size boundary.
__COHERENT_RAM_UNALIGNED_SIZE__

BL1's linker symbols

BL1's early setup code needs to know the extents of the .data section to relocate it from ROM to RAM before executing any C code. The following linker symbols are defined for this purpose:

__DATA_ROM_START__ This address must be aligned on a 16-byte boundary.
__DATA_RAM_START__ This address must be aligned on a 16-byte boundary.
__DATA_SIZE__

BL1's platform setup code needs to know the extents of its read-write data region to figure out its memory layout. The following linker symbols are defined for this purpose:

__BL1_RAM_START__ This is the start address of BL1 RW data.
__BL1_RAM_END__ This is the end address of BL1 RW data.

BL2's, BL3-1's and TSP's linker symbols

BL2, BL3-1 and TSP need to know the extents of their read-only section to set the right memory attributes for this memory region in their MMU setup code. The following linker symbols are defined for this purpose:

__RO_START__
__RO_END__

How to choose the right base addresses for each bootloader stage image

There is currently no support for dynamic image loading in the Trusted Firmware. This means that all bootloader images need to be linked against their ultimate runtime locations and the base addresses of each image must be chosen carefully such that images don't overlap each other in an undesired way. As the code grows, the base addresses might need adjustments to cope with the new memory layout.

The memory layout is completely specific to the platform and so there is no general recipe for choosing the right base addresses for each bootloader image. However, there are tools to aid in understanding the memory layout. These are the link map files: build/<platform>/<build-type>/bl<x>/bl<x>.map, with <x> being the stage bootloader. They provide a detailed view of the memory usage of each image. Among other useful information, they provide the end address of each image.

bl1.map link map file provides __BL1_RAM_END__ address.
bl2.map link map file provides __BL2_END__ address.
bl31.map link map file provides __BL31_END__ address.
bl32.map link map file provides __BL32_END__ address.

For each bootloader image, the platform code must provide its start address as well as a limit address that it must not overstep. The latter is used in the linker scripts to check that the image doesn't grow past that address. If that happens, the linker will issue a message similar to the following:

aarch64-none-elf-ld: BLx has exceeded its limit.

On FVP platforms, the base addresses have been chosen such that all images can reside concurrently in Trusted RAM without overlapping each other. Note that this is not a requirement, as not all images live in memory at the same time. For example, when the BL3-1 image takes over execution, BL1 and BL2 images are not needed anymore.

Limitations of the image loader

The current implementation of the image loader can result in wasted space because of the simplified data structure used to represent the extents of free memory. For example, to load BL2 at address 0x0402D000, the resulting memory layout should be as follows:

------------ 0x04040000
|          |  <- Free space (1)
|----------|
|   BL2    |
|----------| BL2_BASE (0x0402D000)
|          |  <- Free space (2)
|----------|
|   BL1    |
------------ 0x04000000

In the current implementation, we need to specify whether BL2 is loaded at the top or bottom of the free memory. BL2 is top-loaded so in the example above, the free space (1) above BL2 is hidden, resulting in the following view of memory:

------------ 0x04040000
|          |
|          |
|   BL2    |
|----------| BL2_BASE (0x0402D000)
|          |  <- Free space (2)
|----------|
|   BL1    |
------------ 0x04000000

BL3-1 is bottom-loaded above BL1. For example, if BL3-1 is bottom-loaded at 0x0400E000, the memory layout should look like this:

------------ 0x04040000
|          |
|          |
|   BL2    |
|----------| BL2_BASE (0x0402D000)
|          |  <- Free space (2)
|          |
|----------|
|          |
|   BL31   |
|----------|  BL31_BASE (0x0400E000)
|          |  <- Free space (3)
|----------|
|   BL1    |
------------ 0x04000000

But the free space (3) between BL1 and BL3-1 is wasted, resulting in the following view:

------------ 0x04040000
|          |
|          |
|   BL2    |
|----------| BL2_BASE (0x0402D000)
|          |  <- Free space (2)
|          |
|----------|
|          |
|          |
|   BL31   | BL31_BASE (0x0400E000)
|          |
|----------|
|   BL1    |
------------ 0x04000000

Firmware Image Package (FIP)

Using a Firmware Image Package (FIP) allows for packing bootloader images (and potentially other payloads) into a single archive that can be loaded by the ARM Trusted Firmware from non-volatile platform storage. A driver to load images from a FIP has been added to the storage layer and allows a package to be read from supported platform storage. A tool to create Firmware Image Packages is also provided and described below.

Firmware Image Package layout

The FIP layout consists of a table of contents (ToC) followed by payload data. The ToC itself has a header followed by one or more table entries. The ToC is terminated by an end marker entry. All ToC entries describe some payload data that has been appended to the end of the binary package. With the information provided in the ToC entry the corresponding payload data can be retrieved.

------------------
| ToC Header     |
|----------------|
| ToC Entry 0    |
|----------------|
| ToC Entry 1    |
|----------------|
| ToC End Marker |
|----------------|
|                |
|     Data 0     |
|                |
|----------------|
|                |
|     Data 1     |
|                |
------------------

The ToC header and entry formats are described in the header file include/firmware_image_package.h. This file is used by both the tool and the ARM Trusted firmware.

The ToC header has the following fields: name: The name of the ToC. This is currently used to validate the header. serial_number: A non-zero number provided by the creation tool flags: Flags associated with this data. None are yet defined.

A ToC entry has the following fields: uuid: All files are referred to by a pre-defined Universally Unique IDentifier UUID . The UUIDs are defined in include/firmware_image_package. The platform translates the requested image name into the corresponding UUID when accessing the package. offset_address: The offset address at which the corresponding payload data can be found. The offset is calculated from the ToC base address. size: The size of the corresponding payload data in bytes. flags: Flags associated with this entry. Non are yet defined.

Firmware Image Package creation tool

The FIP creation tool can be used to pack specified images into a binary package that can be loaded by the ARM Trusted Firmware from platform storage. The tool currently only supports packing bootloader images. Additional image definitions can be added to the tool as required.

The tool can be found in tools/fip_create.

Loading from a Firmware Image Package (FIP)

The Firmware Image Package (FIP) driver can load images from a binary package on non-volatile platform storage. For the FVPs this is currently NOR FLASH.

Bootloader images are loaded according to the platform policy as specified in plat/<platform>/plat_io_storage.c. For the FVPs this means the platform will attempt to load images from a Firmware Image Package located at the start of NOR FLASH0.

Currently the FVP's policy only allows loading of a known set of images. The platform policy can be modified to allow additional images.

Code Structure

Trusted Firmware code is logically divided between the three boot loader stages mentioned in the previous sections. The code is also divided into the following categories (present as directories in the source code):

Architecture specific. This could be AArch32 or AArch64.
Platform specific. Choice of architecture specific code depends upon the platform.
Common code. This is platform and architecture agnostic code.
Library code. This code comprises of functionality commonly used by all other code.
Stage specific. Code specific to a boot stage.
Drivers.
Services. EL3 runtime services, e.g. PSCI or SPD. Specific SPD services reside in the services/spd directory (e.g. services/spd/tspd).

Each boot loader stage uses code from one or more of the above mentioned categories. Based upon the above, the code layout looks like this:

Directory    Used by BL1?    Used by BL2?    Used by BL3-1?
bl1          Yes             No              No
bl2          No              Yes             No
bl31         No              No              Yes
arch         Yes             Yes             Yes
plat         Yes             Yes             Yes
drivers      Yes             No              Yes
common       Yes             Yes             Yes
lib          Yes             Yes             Yes
services     No              No              Yes

All assembler files have the .S extension. The linker source files for each boot stage have the extension .ld.S. These are processed by GCC to create the linker scripts which have the extension .ld.

FDTs provide a description of the hardware platform and are used by the Linux kernel at boot time. These can be found in the fdts directory.

References

Trusted Board Boot Requirements CLIENT PDD (ARM DEN 0006B-5). Available under NDA through your ARM account representative.
Power State Coordination Interface PDD (ARM DEN 0022B.b).
SMC Calling Convention PDD (ARM DEN 0028A).

ARM Trusted Firmware Design

BL1

Determination of boot path

Architectural initialization

Platform initialization

BL2 image load and execution

BL2

Architectural initialization

Platform initialization

BL3-0 (System Control Processor Firmware) image load

BL3-1 (EL3 Runtime Firmware) image load

BL3-2 (Secure-EL1 Payload) image load

BL3-3 (Non-trusted Firmware) image load

BL3-1 (EL3 Runtime Firmware) execution

BL3-1

Architectural initialization

Platform initialization

Using alternative Trusted Boot Firmware in place of BL1 and BL2

Required CPU state when calling bl31_entrypoint() during cold boot

Use of the X0 and X1 parameters

MMU, Data caches & Coherency

Data structures used in the BL3-1 cold boot interface

Required CPU state for BL3-1 Warm boot initialization

Using BL3-1 as the CPU reset vector

Determination of boot path

Architectural initialization

Platform initialization

Registration

Initialization

Handling an SMC

Initializing a BL3-2 Image

Common linker symbols

BL1's linker symbols

BL2's, BL3-1's and TSP's linker symbols

How to choose the right base addresses for each bootloader stage image

Limitations of the image loader

Firmware Image Package layout

Firmware Image Package creation tool

Loading from a Firmware Image Package (FIP)

Required CPU state when calling `bl31_entrypoint()` during cold boot