architecture/core.rst

.. _core:

####
Core
####

.. _interrupt_handling:

Interrupt handling
******************
This section describes how :ref:`optee_os` handles switches of world execution
context based on :ref:`SMC` exceptions and interrupt notifications. Interrupt
notifications are IRQ/FIQ exceptions which may also imply switching of world
execution context: normal world to secure world, or secure world to normal
world.

Use cases of world context switch
=================================
This section lists all the cases where OP-TEE OS is involved in world context
switches. Optee_os executes in the secure world. World switch is done by the
core's secure monitor level/mode, referred below as the Monitor.

When the normal world invokes the secure world, the normal world executes a SMC
instruction. The SMC exception is always trapped by the Monitor. If the related
service targets the trusted OS, the Monitor will switch to OP-TEE OS world
execution. When the secure world returns to the normal world, OP-TEE OS executes
a SMC that is caught by the Monitor which switches back to the normal world.

When a secure interrupt is signaled by the Arm GIC, it shall reach the OP-TEE OS
interrupt exception vector. If the secure world is executing, OP-TEE OS will
handle interrupt straight from its exception vector. If the normal world is
executing when the secure interrupt raises, the Monitor vector must handle the
exception and invoke OP-TEE OS to serve the interrupt.

When a non-secure interrupt is signaled by the Arm GIC, it shall reach the
normal world interrupt exception vector. If the normal world is executing, it
will handle straight the exception from its exception vector. If the secure
world is executing when the non-secure interrupt raises, OP-TEE OS will
temporarily return back to normal world via the Monitor to let normal world
serve the interrupt.

Core exception vectors
======================
Monitor vector is ``VBAR_EL3`` in AArch64 and ``MVBAR`` in Armv7-A/AArch32.
Monitor can be reached while normal world or secure world is executing. The
executing secure state is known to the Monitor through the ``SCR_NS``.

Monitor can be reached from a SMC exception, an IRQ or FIQ exception (so-called
interrupts) and from asynchronous aborts. Obviously monitor aborts (data,
prefetch, undef) are local to the Monitor execution.

The Monitor can be external to OP-TEE OS (case ``CFG_WITH_ARM_TRUSTED_FW=y``).
If not, provides a local secure monitor ``core/arch/arm/sm``. Armv7-A platforms
should use the OP-TEE OS secure monitor. Armv8-A platforms are likely to rely on
an `Trusted Firmware A`_.

When executing outside the Monitor, the system is executing either in the
normal world (``SCR_NS=1``) or in the secure world (``SCR_NS=0``). Each world
owns its own exception vector table (state vector):

    - ``VBAR_EL2`` or ``VBAR_EL1`` non-secure or ``VBAR_EL1`` secure for
      AArch64.
    - ``HVBAR`` or ``VBAR`` non-secure or ``VBAR`` secure for Armv7-A and
      AArch32.

All SMC exceptions are trapped in the Monitor vector. IRQ/FIQ exceptions can be
trapped either in the Monitor vector or in the state vector of the executing
world.

When the normal world is executing, the system is configured to route:

    - secure interrupts to the Monitor that will forward to OP-TEE OS
    - non-secure interrupts to the executing world exception vector.

When the secure world is executing, the system is configured to route:

    - secure and non-secure interrupts to the executing OP-TEE OS exception
      vector. OP-TEE OS shall forward the non-secure interrupts to the normal
      world.

Optee_os non-secure interrupts are always trapped in the state vector of the
executing world. This is reflected by a static value of ``SCR_(IRQ|FIQ)``.

.. _native_foreign_irqs:

Native and foreign interrupts
=============================
Two types of interrupt are defined from OP-TEE OS point of view.

    - **Native interrupt** - The interrupt handled by OP-TEE OS, secure
      interrupts targetting S-EL1 or secure privileged mode
    - **Foreign interrupt** - The interrupt not handled by OP-TEE OS, non-secure
      interrupts targetting normal world or secure interrupts targetting EL3.

For Arm **GICv2** mode, a native interrupt is signalled with a FIQ and a
foreign interrupt is signalled with an IRQ. For Arm **GICv3** mode, a
foreign interrupts is signalled as a FIQ which could be handled by either
secure world (aarch32 Monitor mode or aarch64 EL3) or normal world.

Arm GICv3 mode can be enabled by setting ``CFG_ARM_GICV3=y``.
Native interrupts must be securely routed to OP-TEE OS. Foreign interrupts, when
trapped during secure world execution might need to be efficiently routed to
the normal world.

IRQ and FIQ keeps their meaning in normal world so for clarity we will keep
using those names in the normal world context.

Normal World invokes OP-TEE OS using SMC
========================================

**Entering the Secure Monitor**

The monitor manages all entries and exits of secure world. To enter secure
world from normal world the monitor saves the state of normal world (general
purpose registers and system registers which are not banked) and restores the
previous state of secure world. Then a return from exception is performed and
the restored secure state is resumed. Exit from secure world to normal world is
the reverse.

Some general purpose registers are not saved and restored on entry and exit,
those are used to pass parameters between secure and normal world (see
ARM_DEN0028A_SMC_Calling_Convention_ for details).

**Entry and exit of Trusted OS**

On entry and exit of Trusted OS each CPU is uses a separate entry stack and runs
with IRQ and FIQ masked. SMCs are categorised in two flavors: **fast** and
**yielding**.

    - For **fast** SMCs, OP-TEE OS will execute on the entry stack with IRQ/FIQ
      masked until the execution returns to normal world.

    - For **yielding** SMCs, OP-TEE OS will at some point execute the requested
      service with interrupts unmasked. In order to handle interrupts, mainly
      forwarding of foreign interrupts, OP-TEE OS assigns a trusted thread
      (`core/arch/arm/kernel/thread.c`_) to the SMC request. The trusted thread
      stores the execution context of the requested service. This context can be
      suspended and resumed as the requested service executes and is
      interrupted. The trusted thread is released only once the service
      execution returns with a completion status.

      For **yielding** SMCs, OP-TEE OS allocates or resumes a trusted thread
      then unmasks the IRQ and FIQ lines. When the OP-TEE OS needs to invoke the
      normal world from a foreign interrupt or a remote service call, OP-TEE OS
      masks IRQ and FIQ and suspends the trusted thread. When suspending,
      OP-TEE OS gets back to the entry stack.

    - **Both** fast and yielding SMCs end on the entry stack with IRQ and
      FIQ masked and OP-TEE OS invokes the Monitor through a SMC to return
      to the normal world.

.. uml::
    :align: center
    :caption: SMC entry to secure world

    participant "Normal World" as nwd
    participant "Secure Monitor" as smon
    participant "OP-TEE OS entry" as entry
    participant "OP-TEE OS" as optee
    == IRQ and FIQ unmasked ==
    nwd -> smon : smc: TEE_FUNC_INVOKE
    smon -> smon : Save non-secure context
    smon -> smon : Restore secure context
    smon --> entry : eret: TEE_FUNC_INVOKE
    entry -> entry : assign thread
    entry -> optee : TEE_FUNC_INVOKE
    == IRQ and FIQ unmasked ==
    optee -> optee : process
    == IRQ and FIQ masked ==
    optee --> entry : SMC_CALL_RETURN
    entry -> smon : smc: SMC_CALL_RETURN
    smon -> smon : Save secure context
    smon -> smon : Restore non-secure context
    == IRQ and FIQ unmasked ==
    smon --> nwd : eret: return

Deliver non-secure interrupts to Normal World
=============================================

**Forward a Foreign Interrupt from Secure World to Normal World**

When a foreign interrupt is received in secure world as an IRQ or FIQ
exception then secure world:

    1. Saves trusted thread context (entire state of all processor modes for
       Armv7-A)

    2. Masks all interrupts (IRQ and FIQ)

    3. Switches to entry stack

    4. Issues an SMC with a value to indicates to normal world that an IRQ has
       been detected and last SMC call should be continued

The monitor restores normal world context with a return code indicating that an
IRQ is about to be delivered. Normal world issues a new SMC indicating that it
should continue last SMC.

The monitor restores secure world context which locates the previously
saved context and checks that it is a return from a foreign interrupt that
is requested before restoring the context and lets the secure world foreign
interrupt handler return from exception where the execution would be
resumed.

Note that the monitor itself does not know or care that it has just forwarded
a foreign interrupt to normal world. The bookkeeping is done in the trusted
thread handling in OP-TEE OS. Normal world is responsible to decide when
the secure world thread should resume execution (for details, see
:ref:`thread_handling`).

.. uml::
    :align: center
    :caption: Foreign interrupt received in secure world and forwarded to
              normal world

    participant "Normal World" as nwd
    participant "Secure Monitor" as smon
    participant "OP-TEE OS entry" as entry
    participant "OP-TEE OS" as optee
    == IRQ and FIQ unmasked ==
    optee -> optee : process
    == IRQ and FIQ unmasked,\nForeign interrupt received ==
    optee -> optee : suspend thread
    optee -> entry : forward foreign interrupt
    entry -> smon : smc: forward foreign interrupt
    smon -> smon: Save secure context
    smon -> smon: Restore non-secure context
    == IRQ and FIQ unmasked ==
    smon --> nwd : eret: IRQ forwarded
    == FIQ unmasked, IRQ received ==
    nwd -> nwd : process IRQ
    == IRQ and FIQ unmasked ==
    nwd -> smon : smc: return from IRQ
    == IRQ and FIQ masked ==
    smon -> smon : Save non-secure context
    smon -> smon : Restore secure context
    smon --> entry : eret: return from foreign interrupt
    entry -> entry : find thread
    entry --> optee : resume execution
    == IRQ and FIQ unmasked ==
    optee -> optee : process

**Deliver a foreign interrupt to normal world when ``SCR_NS`` is set**

Since ``SCR_IRQ`` is cleared, an IRQ will be delivered using the exception
vector (``VBAR``) in the normal world. The IRQ is received as any other
exception by normal world, the monitor and the OP-TEE OS are not involved
at all.

Deliver secure interrupts to Secure World
=========================================
A secure (foreign) interrupt can be received during two different states,
either in normal world (``SCR_NS`` is set) or in secure world (``SCR_NS``
is cleared). When the secure monitor is active (Armv8-A EL3 or Armv7-A
Monitor mode) FIQ and IRQ are masked. FIQ reception in the two different
states is described below.

**Deliver secure interrupt to secure world when SCR_NS is set**

When the monitor traps a secure interrupt it:

    1. Saves normal world context and restores secure world context from last
       secure world exit (which will have IRQ and FIQ blocked)
    2. Clears ``SCR_FIQ`` when clearing ``SCR_NS``
    3. Does a return from exception into OP-TEE OS via the secure interrupt
       entry point
    4. OP-TEE OS handles the native interrupt directly in the entry point
    5. OP-TEE OS issues an SMC to return to normal world
    6. The monitor saves the secure world context and restores the normal world context
    7. Does a return from exception into the restored context

.. uml::
    :align: center
    :caption: Secure interrupt received when SCR_NS is set

    participant "Normal World" as nwd
    participant "Secure Monitor" as smon
    participant "OP-TEE OS entry" as entry
    participant "OP-TEE OS" as optee
    == IRQ and FIQ unmasked ==
    == Running in non-secure world (SCR_NS set) ==
    nwd -> nwd : process
    == IRQ and FIQ masked,\nSecure interrupt received ==
    smon -> smon : Save non-secure context
    smon -> smon : Restore secure context
    smon --> entry : eret: native interrupt entry point
    entry -> entry: process received native interrupt
    entry -> smon: smc: return
    smon -> smon : Save secure context
    smon -> smon : Restore non-secure context
    smon --> nwd : eret: return to Normal world
    == IRQ and FIQ unmasked ==
    nwd -> nwd : process

**Deliver FIQ to secure world when SCR_NS is cleared**

.. uml::
    :align: center
    :caption: FIQ received while processing an IRQ forwarded from secure world

    participant "Normal World" as nwd
    participant "Secure Monitor" as smon
    participant "OP-TEE OS entry" as entry
    participant "OP-TEE OS" as optee
    == IRQ and FIQ unmasked ==
    optee -> optee : process
    == IRQ and FIQ unmasked,\nForeign interrupt received ==
    optee -> optee : suspend thread
    optee -> entry : forward foreign interrupt
    entry -> smon : smc: forward foreign interrupt
    smon -> smon: Save secure context
    smon -> smon: Restore non-secure context
    == IRQ and FIQ unmasked ==
    smon --> nwd : eret: IRQ forwarded
    == FIQ unmasked, IRQ received ==
    nwd -> nwd : process IRQ
    == IRQ and FIQ masked,\nSecure interrupt received ==
    smon -> smon : Save non-secure context
    smon -> smon : Restore secure context
    smon --> entry : eret: native interrupt entry point
    entry -> entry : process received native interrupt
    entry -> smon: smc: return
    smon -> smon : Save secure context
    smon -> smon : Restore non-secure context
    smon --> nwd : eret: return to Normal world
    == FIQ unmasked\nIRQ still being processed ==
    nwd -> nwd : process IRQ
    == IRQ and FIQ unmasked ==
    nwd -> smon : smc: return from IRQ
    == IRQ and FIQ masked ==
    smon -> smon : Save non-secure context
    smon -> smon : Restore secure context
    smon --> entry : eret: return from foreign interrupt
    entry -> entry : find thread
    entry --> optee : resume execution
    == IRQ and FIQ unmasked ==
    optee -> optee : process

Trusted thread scheduling
=========================
**Trusted thread for standard services**

OP-TEE yielding services are carried through standard SMC. Execution of these
services can be interrupted by foreign interrupts. To suspend and restore the
service execution, optee_os assigns a trusted thread at yielding SMC entry.

The trusted thread terminates when optee_os returns to the normal world with a
service completion status.

A trusted thread execution can be interrupted by a native interrupt. In this
case the native interrupt is handled by the interrupt exception handlers and
once served, optee_os returns to the execution trusted thread.

A trusted thread execution can be interrupted by a foreign interrupt. In this
case, optee_os suspends the trusted thread and invokes the normal world through
the Monitor (optee_os so-called RPC services). The trusted threads will resume
only once normal world invokes the optee_os with the RPC service status.

A trusted thread execution can lead optee_os to invoke a service in normal
world: access a file, get the REE current time, etc. The trusted thread is
first suspended then resumed during remote service execution.

**Scheduling considerations**

When a trusted thread is interrupted by a foreign interrupt and when optee_os
invokes a normal world service, the normal world gets the opportunity to
reschedule the running applications. The trusted thread will resume only once
the client application is scheduled back. Thus, a trusted thread execution
follows the scheduling of the normal world caller context.

Optee_os does not implement any thread scheduling. Each trusted thread is
expected to track a service that is invoked from the normal world and should
return to it with an execution status.

The OP-TEE Linux driver (as implemented in `drivers/tee/optee`_ since Linux
kernel 4.12) is designed so that the Linux thread invoking OP-TEE gets assigned
a trusted thread on TEE side. The execution of the trusted thread is tied to the
execution of the caller Linux thread which is under the Linux kernel scheduling
decision. This means trusted threads are scheduled by the Linux kernel.

**Trusted thread constraints**

TEE core handles a static number of trusted threads, see ``CFG_NUM_THREADS``.

Trusted threads are expensive on memory constrained system, mainly
because of the execution stack size.

On SMP systems, optee_os can execute several trusted threads in parallel if the
normal world supports scheduling of processes. Even on UP systems, supporting
several trusted threads in optee_os helps normal world scheduler to be
efficient.

----

.. _memory_objects:

Memory objects
**************
A memory object, **MOBJ**, describes a piece of memory. The interface provided
is mostly abstract when it comes to using the MOBJ to populate translation
tables etc. There are different kinds of MOBJs describing:

    - Physically contiguous memory
        - created with ``mobj_phys_alloc(...)``.

    - Virtual memory
        - one instance with the name ``mobj_virt`` available.
        - spans the entire virtual address space.

    - Physically contiguous memory allocated from a ``tee_mm_pool_t *``
        - created with ``mobj_mm_alloc(...)``.

    - Paged memory
        - created with ``mobj_paged_alloc(...)``.
        - only contains the supplied size and makes ``mobj_is_paged(...)``
          return true if supplied as argument.

    - Secure copy paged shared memory
        - created with ``mobj_seccpy_shm_alloc(...)``.
        - makes ``mobj_is_paged(...)`` and ``mobj_is_secure(...)`` return true
          if supplied as argument.

----

.. _mmu:

MMU
***
Translation tables
==================

OP-TEE supports two translation table formats:

1. Short-descriptor translation table format, available on ARMv7-A and
   ARMv8-A AArch32
2. Long-descriptor translation format, available on ARMv7-A with LPAE and
   ARMv8-A

ARMv7-A without LPAE (Large Physical Address Extension) must use the
short-descriptor translation table format only. ARMv8-A AArch64 must use
the long-descriptor translation format only.

Translation table format is a static build time configuration option,
``CFG_WITH_LPAE``. The design around the translation table handling has
been centered around these factors:

1. Share translation tables between CPUs when possible to save memory
   and simplify paging
2. Support non-global CPU specific mappings to allow executing different
   TAs in parallel.

Short-descriptor translation table format
-----------------------------------------

Several L1 translation tables are used, one large spanning 4 GiB and two or
more small tables spanning 32 MiB. The large translation table handles kernel
mode mapping and matches all addresses not covered by the small translation
tables. The small translation tables are assigned per thread and covers the
mapping of the virtual memory space for one TA context.

Memory space between small and large translation table is configured by TTBRC.
TTBR1 always points to the large translation table. TTBR0 points to the a small
translation table when user mapping is active and to the large translation table
when no user mapping is currently active. For details about registers etc,
please refer to a Technical Reference Manual for your architecture, for example
`Cortex-A53 TRM`_.

The translation tables has certain alignment constraints, the alignment (of the
physical address) has to be the same as the size of the translation table. The
translation tables are statically allocated to avoid fragmentation of memory due
to the alignment constraints.

Each thread has one small L1 translation table of its own. Each TA context has a
compact representation of its L1 translation table. The compact representation
is used to initialize the thread specific L1 translation table when the TA
context is activated.

.. graphviz::
    :align: center

    digraph xlat_table {
        graph [
            rankdir = "LR"
        ];
        node [
            shape = "ellipse"
        ];
        edge [
        ];
        "node_ttb" [
            label = "<f0> TTBR0 | <f1> TTBR1"
            shape = "record"
        ];
        "node_large_l1" [
            label = "<f0> Large L1\nSpans 4 GiB"
            shape = "record"
        ];
        "node_small_l1" [
            label = "Small L1\nSpans 32 MiB\nper entry | <f0> 0 | <f1> 1 | ... | <fn> n"
            shape = "record"
        ];

        "node_ttb":f0 -> "node_small_l1":f0 [ label = "Thread 0 ctx active" ];
        "node_ttb":f0 -> "node_small_l1":f1 [ label = "Thread 1 ctx active" ];
        "node_ttb":f0 -> "node_small_l1":fn [ label = "Thread n ctx active" ];
        "node_ttb":f0 -> "node_large_l1" [ label="No active ctx" ];
        "node_ttb":f1 -> "node_large_l1";
    }

Long-descriptor translation table format
----------------------------------------

Each CPU is assigned a L1 translation table which is programmed into
Translation Table Base Register 0 (``TTBR0`` or ``TTBR0_EL1`` as
appropriate).

L1 and L2 translation tables are statically allocated and initialized at
boot. Normally there is only one shared L2 table, but with ASLR enabled the
virtual address space used for the shared mapping may need to use two
tables. An unused entry in the L1 table is selected to point to the per
thread L2 table. With ASLR configured this means that different per thread
entry may be selected each time the system boots. Note that this entry will
only point to a table when the per thread mapping is activated.

The L2 translation tables in their turn point to L3 tables which use the
small page granularity of 4 KiB. The shared mappings has the L3 tables
initialized too at boot, but the per thread L3 tables are dynamic and are
only assigned when the mapping is activated.

.. graphviz::
    :align: center
    :caption: Example translation table setup with 4GiB virtual address space
              with L3 tables excluded

    digraph xlat_table {
        graph [ rankdir = "LR" ];
        node [ ];
        edge [ ];

        "ttbr0" [
            label = "TTBR0"
            shape = "record"
        ];
        "node_l1" [
            label = "<h> Per CPU L1 table | <f0> 0 | <f1> 1 | <f2> 2 | <f3> 3"
            shape = "record"
        ];
        "shared_l2_n" [
            label = "<h> Shared L2 table n | 0 | ... | 512"
            shape = "record"
        ]
        "shared_l2_m" [
            label = "<h> Shared L2 table m | 0 | ... | 512"
            shape = "record"
        ]
        "per_thread_l2" [
            label = "<h> Per thread L2 table | 0 | ... | 512"
            shape = "record"
        ]
        "ttbr0" -> "node_l1":h;
        "node_l1":f2 -> "shared_l2_n":h;
        "node_l1":f3 -> "shared_l2_m":h;
        "node_l1":f0 -> "per_thread_l2":h;
    }


Page table cache
================
Page tables used to map TAs are managed with the page table cache. When the
context of a TA is unmapped, all its page tables are released with a call
to ``pgt_free()``. All page tables needed when mapping a TA are allocated
using ``pgt_alloc()``.

A fixed maximum number of translation tables are available in a pool. One
thread may execute a TA which needs all or almost all tables. This can
block TAs from being executed by other threads. To ensure that all TAs
eventually will be permitted to execute ``pgt_alloc()`` temporarily frees
eventual tables allocated before waiting for tables to become available.

The page table cache behaves differently depending on configuration
options.

Without paging (``CFG_WITH_PAGER=n``)
-------------------------------------
This is the easiest configuration. All page tables are statically allocated
in the ``.nozi.pgt_cache`` section. ``pgt_alloc()`` allocates tables from the
free-list and ``pgt_free()`` returns the tables directly to the free-list.

With paging enabled (``CFG_WITH_PAGER=y``)
------------------------------------------

Page tables are allocated as zero initialized locked pages during boot
using ``tee_pager_alloc()``. Locked pages are populated with physical pages
on demand from the pager. The physical page can be released when not needed
any longer with ``tee_pager_release_phys()``.

With ``CFG_WITH_LPAE=y`` each translation table has the same size as a
physical page which makes it easy to release the physical page when the
translation table isn't needed any longer. With the short-descriptor table
format (``CFG_WITH_LPAE=n``) it becomes more complicated as four
translation tables are stored in each page. Additional bookkeeping is used
to tell when the page for used by four separate translation tables can be
released.

With paging of user TA enabled (``CFG_PAGED_USER_TA=y``)
--------------------------------------------------------
With paging of user TAs enabled a cache of recently used translation tables
is used. This can save us from a storm of page faults when restoring the
mappings of a recently unmapped TA. Which translation tables should be
cached is indicated with reference counting by the pager on used tables.
When a table needs to be forcefully freed
``tee_pager_pgt_save_and_release_entries()`` is called to let the pager
know that the table can't be used any longer.

When a mapping in a TA is removed it also needs to be purged from cached
tables with ``pgt_flush_ctx_range()`` to prevent old mappings from being
accidentally reused.

Switching to user mode
======================
This section only applies with following configuration flags:

    - ``CFG_WITH_LPAE=n``
    - ``CFG_CORE_UNMAP_CORE_AT_EL0=y``

When switching to user mode only a minimal kernel mode mapping is kept. This is
achieved by selecting a zeroed out big L1 translation in TTBR1 when
transitioning to user mode. When returning back to kernel mode the original L1
translation table is restored in TTBR1.

Switching to normal world
=========================
When switching to normal world either via a foreign interrupt (see
:ref:`native_foreign_irqs` or RPC there is a chance that secure world will
resume execution on a different CPU. This means that the new CPU need to be
configured with the context of the currently active TA. This is solved by always
setting the TA context in the CPU when resuming execution.

----

.. _pager:

Pager
*****
OP-TEE currently requires >256 KiB RAM for OP-TEE kernel memory. This is not a
problem if OP-TEE uses TrustZone protected DDR, but for security reasons OP-TEE
may need to use TrustZone protected SRAM instead. The amount of available SRAM
varies between platforms, from just a few KiB up to over 512 KiB. Platforms with
just a few KiB of SRAM cannot be expected to be able to run a complete TEE
solution in SRAM. But those with 128 to 256 KiB of SRAM can be expected to have
a capable TEE solution in SRAM. The pager provides a solution to this by demand
paging parts of OP-TEE using virtual memory.

Secure memory
=============
TrustZone protected SRAM is generally considered more secure than TrustZone
protected DRAM as there is usually more attack vectors on DRAM. The attack
vectors are hardware dependent and can be different for different platforms.

Backing store
=============
TrustZone protected DRAM or in some cases non-secure DRAM is used as backing
store. The data in the backing store is integrity protected with one hash
(SHA-256) per page (4KiB). Readonly pages are not encrypted since the OP-TEE
binary itself is not encrypted.

Partitioning of memory
======================
The code that handles demand paging must always be available as it would
otherwise lead to deadlock. The virtual memory is partitioned as:

    +--------------+-------------------+
    | Type         | Sections          |
    +==============+===================+
    | unpaged      | | text            |
    |              | | rodata          |
    |              | | data            |
    |              | | bss             |
    |              | | heap1           |
    |              | | nozi            |
    |              | | heap2           |
    +--------------+-------------------+
    | init / paged | | text_init       |
    |              | | rodata_init     |
    +--------------+-------------------+
    | paged        | | text_pageable   |
    |              | | rodata_pageable |
    +--------------+-------------------+
    | demand alloc |                   |
    +--------------+-------------------+

Where ``nozi`` stands for "not zero initialized", this section contains entry
stacks (thread stack when TEE pager is not enabled) and translation tables (TEE
pager cached translation table when the pager is enabled and LPAE MMU is used).

The ``init`` area is available when OP-TEE is initializing and contains
everything that is needed to initialize the pager. After the pager has been
initialized this area will be used for demand paged instead.

The ``demand alloc`` area is a special area where the pages are allocated and
removed from the pager on demand. Those pages are returned when OP-TEE does not
need them any longer. The thread stacks currently belongs this area. This means
that when a stack is not used the physical pages can be used by the pager for
better performance.

The technique to gather code in the different area is based on compiling all
functions and data into separate sections. The unpaged text and rodata is then
gathered by linking all object files with ``--gc-sections`` to eliminate
sections that are outside the dependency graph of the entry functions for
unpaged functions. A script analyzes this ELF file and generates the bits of the
final link script. The process is repeated for init text and rodata. What is
not "unpaged" or "init" becomes "paged".

Partitioning of the binary
==========================
.. note::
    The struct definitions provided in this section are explicitly covered by
    the following dual license:

    .. code-block:: none

        SPDX-License-Identifier: (BSD-2-Clause OR GPL-2.0)

The binary is partitioned into four parts as:


    +----------+
    | Binary   |
    +==========+
    | Header   |
    +----------+
    | Init     |
    +----------+
    | Hashes   |
    +----------+
    | Pageable |
    +----------+

The header is defined as:

.. code-block:: c

    #define OPTEE_MAGIC             0x4554504f
    #define OPTEE_VERSION           1
    #define OPTEE_ARCH_ARM32        0
    #define OPTEE_ARCH_ARM64        1

    struct optee_header {
            uint32_t magic;
            uint8_t version;
            uint8_t arch;
            uint16_t flags;
            uint32_t init_size;
            uint32_t init_load_addr_hi;
            uint32_t init_load_addr_lo;
            uint32_t init_mem_usage;
            uint32_t paged_size;
    };

The header is only used by the loader of OP-TEE, not OP-TEE itself. To
initialize OP-TEE the loader loads the complete binary into memory and copies
what follows the header and the following ``init_size`` bytes to
``(init_load_addr_hi << 32 | init_load_addr_lo)``. ``init_mem_usage`` is used by
the loader to be able to check that there is enough physical memory available
for OP-TEE to be able to initialize at all. The loader supplies in ``r0/x0`` the
address of the first byte following what was not copied and jumps to the load
address to start OP-TEE.

In addition to overall binary with partitions inside described as above, three
extra binaries are generated simultaneously during build process for loaders who
support loading separate binaries:

    +-----------+
    | v2 binary |
    +===========+
    | Header    |
    +-----------+

    +-----------+
    | v2 binary |
    +===========+
    | Init      |
    +-----------+
    | Hashes    |
    +-----------+

    +-----------+
    | v2 binary |
    +===========+
    | Pageable  |
    +-----------+

In this case, loaders load header binary first to get image list and information
of each image; and then load each of them into specific load address assigned in
structure. These binaries are named with `v2` suffix to distinguish from the
existing binaries. Header format is updated to help loaders loading binaries
efficiently:

.. code-block:: c

    #define OPTEE_IMAGE_ID_PAGER    0
    #define OPTEE_IMAGE_ID_PAGED    1

    struct optee_image {
            uint32_t load_addr_hi;
            uint32_t load_addr_lo;
            uint32_t image_id;
            uint32_t size;
    };

    struct optee_header_v2 {
            uint32_t magic;
            uint8_t version;
            uint8_t arch;
            uint16_t flags;
            uint32_t nb_images;
            struct optee_image optee_image[];
    };

Magic number and architecture are identical as original. Version is increased to
two. ``load_addr_hi`` and ``load_addr_lo`` may be ``0xFFFFFFFF`` for pageable
binary since pageable part may get loaded by loader into dynamic available
position. ``image_id`` indicates how loader handles current binary. Loaders who
don't support separate loading just ignore all v2 binaries.

Initializing the pager
======================
The pager is initialized as early as possible during boot in order to minimize
the "init" area. The global variable ``tee_mm_vcore`` describes the virtual
memory range that is covered by the level 2 translation table supplied to
``tee_pager_init(...)``.

Assign pageable areas
---------------------
A virtual memory range to be handled by the pager is registered with a call to
``tee_pager_add_core_area()``.

.. code-block:: c

    bool tee_pager_add_area(tee_mm_entry_t *mm,
                            uint32_t flags,
                            const void *store,
                            const void *hashes);

which takes a pointer to ``tee_mm_entry_t`` to tell the range, flags to tell how
memory should be mapped (readonly, execute etc), and pointers to backing store
and hashes of the pages.

Assign physical pages
---------------------
Physical SRAM pages are supplied by calling ``tee_pager_add_pages(...)``

.. code-block:: c

    void tee_pager_add_pages(tee_vaddr_t vaddr,
                             size_t npages,
                             bool unmap);

``tee_pager_add_pages(...)`` takes the physical address stored in the entry
mapping the virtual address ``vaddr`` and ``npages`` entries after that and uses
it to map new pages when needed. The unmap parameter tells whether the pages
should be unmapped immediately since they does not contain initialized data or
be kept mapped until they need to be recycled. The pages in the "init" area are
supplied with ``unmap == false`` since those page have valid content and are in
use.

Invocation
==========
The pager is invoked as part of the abort handler. A pool of physical pages are
used to map different virtual addresses. When a new virtual address needs to be
mapped a free physical page is mapped at the new address, if a free physical
page cannot be found the oldest physical page is selected instead. When the page
is mapped new data is copied from backing store and the hash of the page is
verified. If it is OK the pager returns from the exception to resume the
execution.

Data structures
===============
.. figure:: ../images/core/tee_pager_area.png
    :figclass: align-center

    How the main pager data structures relates to each other

``struct tee_pager_area``
-------------------------
This is a central data structure when handling paged
memory ranges. It's defined as:

.. code-block:: c

    struct tee_pager_area {
        struct fobj *fobj;
        size_t fobj_pgoffs;
        enum tee_pager_area_type type;
        uint32_t flags;
        vaddr_t base;
        size_t size;
        struct pgt *pgt;
        TAILQ_ENTRY(tee_pager_area) link;
        TAILQ_ENTRY(tee_pager_area) fobj_link;
    };

Where ``base`` and ``size`` tells the memory range and ``fobj`` and
``fobj_pgoffs`` holds the content. A ``struct tee_pager_area`` can only use
``struct fobj`` and one ``struct pgt`` (translation table) so memory ranges
spanning multiple fobjs or pgts are split into multiple areas.

``struct fobj``
---------------
This is a polymorph object, using different implmentations depending on how
it's initialized. It's defines as:

.. code-block:: c

    struct fobj_ops {
        void (*free)(struct fobj *fobj);
        TEE_Result (*load_page)(struct fobj *fobj, unsigned int page_idx,
                                void *va);
        TEE_Result (*save_page)(struct fobj *fobj, unsigned int page_idx,
                                const void *va);
    };

    struct fobj {
        const struct fobj_ops *ops;
        unsigned int num_pages;
        struct refcount refc;
        struct tee_pager_area_head areas;
    };

:``num_pages``: Tells how many pages this ``fobj`` covers.
:``refc``:      A reference counter, everyone referring to a ``fobj`` need to
                increase and decrease this as needed.
:``areas``:     A list of areas using this ``fobj``, traversed when making
                a virtual page unavailable.

``struct tee_pager_pmem``
-------------------------
This structure represents a physical page. It's defined as:

.. code-block:: c

    struct tee_pager_pmem {
        unsigned int flags;
        unsigned int fobj_pgidx;
        struct fobj *fobj;
        void *va_alias;
        TAILQ_ENTRY(tee_pager_pmem) link;
    };

:``PMEM_FLAG_DIRTY``:   Bit is set in ``flags`` when the page is mapped
                        read/write at at least one location.
:``PMEM_FLAG_HIDDEN``:  Bit is set in ``flags`` when the page is hidden, that
                        is, not accessible anywhere.
:``fobj_pgidx``:        The page at this index in the ``fobj`` is used in this
                        physical page.
:``fobj``:              The ``fobj`` backing this page.
:``va_alias``:          Virtual address where this physical page is updated
                        when loading it from backing store or when writing it
                        back.

All ``struct tee_pager_pmem`` are stored either in the global list
``tee_pager_pmem_head`` or in ``tee_pager_lock_pmem_head``. The latter is
used by pages which are mapped and then locked in memory on demand. The
pages are returned back to ``tee_pager_pmem_head`` when the pages are
exlicitly released with a call to ``tee_pager_release_phys()``.

A physical page can be used by more than one ``tee_pager_area``
simultaneously. This is also know as shared secure memory and will appear
as such for both read-only and read-write mappings.

When a page is hidden it's unmapped from all translation tables and the
``PMEM_FLAG_HIDDEN`` bit is set, but kept in memory. When a physical page
is released it's also unmapped from all translation tables and it's content
is written back to storage, then the ``fobj`` field is set to ``NULL`` to
note the physical page as unused.

Note that when ``struct tee_pager_pmem`` references a ``fobj`` it doesn't
update the reference counter since it's already guaranteed to be available
due the ``struct tee_pager_area`` which must reference the ``fobj`` too.

Paging of user TA
=================
Paging of user TAs can optionally be enabled with ``CFG_PAGED_USER_TA=y``.
Paging of user TAs is analogous to paging of OP-TEE kernel parts but with a few
differences:

    - Read/write pages are paged in addition to read-only pages
    - Page tables are managed dynamically

``tee_pager_add_uta_area(...)`` is used to setup initial read/write mapping
needed when populating the TA. When the TA is fully populated and relocated
``tee_pager_set_uta_area_attr(...)`` changes the mapping of the area to strict
permissions used when the TA is running.

Paging shared secure memory
---------------------------
Shared secure memory is achieved by letting several ``tee_pager_area``
using the same backing ``fobj``. When a ``tee_pager_area`` is allocated and
assigned a ``fobj`` it's also added to a list for ``tee_pager_areas`` using
this ``fobj``. This helps when a physical page is released.

When a fault occurs first a matching ``tee_pager_area`` is located. Then
``tee_pager_pmem_head`` is searched to see if a physical page already holds
the page of the ``fobj`` needed. If so the ``pgt`` is updated to map the
physical page at the appropriate locatation. If no physical page was holding
the page a new physical page is allocated, initialized and finally mapped.

In order to make as few updates to mappings as possible changes to less
restricted, no access -> read-only or read-only to read-write, is done only
for the virtual address was used when the page fault occurred. Changes in
the other direction has to be done in all translation tables used to map
the physical page.

----

.. _stacks:

Stacks
******
Different stacks are used during different stages. The stacks are:

    - **Secure monitor stack** (128 bytes), bound to the CPU. Only available if
      OP-TEE is compiled with a secure monitor always the case if the target is
      Armv7-A but never for Armv8-A.

    - **Temp stack** (small ~1KB), bound to the CPU. Used when transitioning
      from one state to another. Interrupts are always disabled when using this
      stack, aborts are fatal when using the temp stack.

    - **Abort stack** (medium ~2KB), bound to the CPU. Used when trapping a data
      or pre-fetch abort. Aborts from user space are never fatal the TA is only
      killed. Aborts from kernel mode are used by the pager to do the demand
      paging, if pager is disabled all kernel mode aborts are fatal.

    - **Thread stack** (large ~8KB), not bound to the CPU instead used by the
      current thread/task. Interrupts are usually enabled when using this stack.

Notes for Armv7-A/AArch32
    .. list-table::
        :header-rows: 1
        :widths: 1 5

        * - Stack
          - Comment

        * - Temp
          - Assigned to ``SP_SVC`` during entry/exit, always assigned to
            ``SP_IRQ`` and ``SP_FIQ``

        * - Abort
          - Always assigned to ``SP_ABT``

        * - Thread
          - Assigned to ``SP_SVC`` while a thread is active

Notes for AArch64
    There are only two stack pointers, ``SP_EL1`` and ``SP_EL0``, available for
    OP-TEE in AArch64. When an exception is received stack pointer is always
    ``SP_EL1`` which is used temporarily while assigning an appropriate stack
    pointer for ``SP_EL0``. ``SP_EL1`` is always assigned the value of
    ``thread_core_local[cpu_id]``. This structure has some spare space for
    temporary storage of registers and also keeps the relevant stack pointers.
    In general when we talk about assigning a stack pointer to the CPU below we
    mean ``SP_EL0``.

Boot
====
During early boot the CPU is configured with the temp stack which is used until
OP-TEE exits to normal world the first time.

Notes for AArch64
    ``SPSEL`` is always ``0`` on entry/exit to have ``SP_EL0`` acting as stack
    pointer.

Normal entry
============
Each time OP-TEE is entered from normal world the temp stack is used as the
initial stack. For fast calls, this is the only stack used. For normal calls an
empty thread slot is selected and the CPU switches to that stack.

Normal exit
===========
Normal exit occurs when a thread has finished its task and the thread is freed.
When the main thread function, ``tee_entry_std(...)``, returns interrupts are
disabled and the CPU switches to the temp stack instead. The thread is freed and
OP-TEE exits to normal world.

RPC exit
========
RPC exit occurs when OP-TEE need some service from normal world. RPC can
currently only be performed with a thread is in running state. RPC is initiated
with a call to ``thread_rpc(...)`` which saves the state in a way that when the
thread is restored it will continue at the next instruction as if this function
did a normal return. CPU switches to use the temp stack before returning to
normal world.

Foreign interrupt exit
======================
Foreign interrupt exit occurs when OP-TEE receives a foreign interrupt. For Arm
GICv2 mode, foreign interrupt is sent as IRQ which is always handled in normal
world. Foreign interrupt exit is similar to RPC exit but it is
``thread_irq_handler(...)`` and ``elx_irq(...)`` (respectively for
Armv7-A/Aarch32 and for Aarch64) that saves the thread state instead. The thread
is resumed in the same way though. For Arm GICv3 mode, foreign interrupt is sent
as FIQ which could be handled by either secure world (EL3 in AArch64) or normal
world. This mode is not supported yet.

Notes for Armv7-A/AArch32
    SP_IRQ is initialized to temp stack instead of a separate stack. Prior to
    exiting to normal world CPU state is changed to SVC and temp stack is
    selected.

Notes for AArch64
    ``SP_EL0`` is assigned temp stack and is selected during IRQ processing. The
    original ``SP_EL0`` is saved in the thread context to be restored when
    resuming.

Resume entry
============
OP-TEE is entered using the temp stack in the same way as for normal entry. The
thread to resume is looked up and the state is restored to resume execution. The
procedure to resume from an RPC exit or an foreign interrupt exit is exactly the
same.

Syscall
=======
Syscall's are executed using the thread stack.

Notes for Armv7-A/AArch32
    Nothing special ``SP_SVC`` is already set with thread stack.

Notes for syscall AArch64
    Early in the exception processing the original ``SP_EL0`` is saved in
    ``struct thread_svc_regs`` in case the TA is executed in AArch64. Current
    thread stack is assigned to ``SP_EL0`` which is then selected. When
    returning ``SP_EL0`` is assigned what is in ``struct thread_svc_regs``. This
    allows ``tee_svc_sys_return_helper(...)`` having the syscall exception
    handler return directly to ``thread_unwind_user_mode(...)``.

----

.. _shared_memory:

Shared Memory
*************
Shared Memory is a block of memory that is shared between the non-secure and the
secure world. It is used to transfer data between both worlds.

The shared memory is allocated and managed by the non-secure world, i.e. the
Linux OP-TEE driver. Secure world only considers the individual shared buffers,
not their pool. Each shared memory is referenced with associated attributes:

    - Buffer start address and byte size,
    - Cache attributes of the shared memory buffer,
    - List of chunks if mapped from noncontiguous pages.

Shared memory buffer references manipulated must fit inside one of the
shared memory areas known from the OP-TEE core. OP-TEE supports two kinds
of shared memory areas: an area for contiguous buffers and an area for
noncontiguous buffers. At least one has to be enabled.

Contiguous shared buffers
=========================
Configuration directives ``CFG_SHMEM_START`` and ``CFG_SHMEM_SIZE``
define a share memory area where shared memory buffers are contiguous.
Generic memory layout registers it as the ``MEM_AREA_NSEC_SHM`` memory area.

The non-secure world issues ``OPTEE_SMC_GET_SHM_CONFIG`` to retrieve contiguous
shared memory area configuration:

    - Physical address of the start of the pool
    - Size of the pool
    - Whether or not the memory is cached

Contiguous shared memory (also known as static or reserved shared memory)
is enabled with the configuration flag ``CFG_CORE_RESERVED_SHM=y``.

Noncontiguous shared buffers
============================
To benefit from noncontiguous shared memory buffers, secure world register
dynamic shared memory areas and non-secure world must register noncontiguous
buffers prior to referring to them using the OP-TEE API.

The OP-TEE core generic boot sequence discovers dynamic shared areas from the
device tree and/or areas explicitly registered by the platform.

Non-secure side needs to register buffers as 4kByte chunks lists into OP-TEE
core using the ``OPTEE_MSG_CMD_REGISTER_SHM`` API prior referencing to them
using the OP-TEE invocation API.

Noncontiguous shared memory (also known as dynamic shared memory) is
enabled with the configuration flag ``CFG_CORE_DYN_SHM=y``.

For performance reasons, the TEE Client Library (``libteec``) uses
noncontiguous shared memory when available since it avoids copies in some
situations.

Shared Memory Chunk Allocation
==============================
It is the Linux kernel driver for OP-TEE that is responsible for allocating
chunks of shared memory. OP-TEE linux kernel driver relies on linux kernel
generic allocation support (``CONFIG_GENERIC_ALLOCATION``) to allocation/release
of shared memory physical chunks. OP-TEE linux kernel driver relies on linux
kernel dma-buf support (``CONFIG_DMA_SHARED_BUFFER``) to track shared memory
buffers references.

Using shared memory
===================
From the Client Application
    The client application can ask for shared memory allocation using the
    GlobalPlatform Client API function ``TEEC_AllocateSharedMemory(...)``. The
    client application can also register a memory through the GlobalPlatform
    Client API function ``TEEC_RegisterSharedMemory(...)``. The shared memory
    reference can then be used as parameter when invoking a trusted application.

From the Linux Driver
    Occasionally the Linux kernel driver needs to allocate shared memory for the
    communication with secure world, for example when using buffers of type
    ``TEEC_TempMemoryReference``.

From OP-TEE core
    In case OP-TEE core needs information from TEE supplicant (dynamic TA
    loading, REE time request,...), shared memory must be allocated. Allocation
    depends on the use case. OP-TEE core asks for the following shared memory
    allocation:

        - ``optee_msg_arg`` structure, used to pass the arguments to the
          non-secure world, where the allocation will be done by sending a
          ``OPTEE_SMC_RPC_FUNC_ALLOC`` message.

        - In some cases, a payload might be needed for storing the result from
          TEE supplicant, for example when loading a Trusted Application. This
          type of allocation will be done by sending the message
          ``OPTEE_MSG_RPC_CMD_SHM_ALLOC(OPTEE_MSG_RPC_SHM_TYPE_APPL,...)``,
          which then will return:

            - the physical address of the shared memory
            - a handle to the memory, that later on will be used later on when
              freeing this memory.

From TEE Supplicant
    TEE supplicant is also working with shared memory, used to exchange data
    between normal and secure worlds. TEE supplicant receives a memory address
    from the OP-TEE core, used to store the data. This is for example the case
    when a Trusted Application is loaded. In this case, TEE supplicant must
    register the provided shared memory in the same way a client application
    would do, involving the Linux driver.

----

.. _smc:

SMC
***
SMC Interface
=============
OP-TEE's SMC interface is defined in two levels using optee_smc.h_ and
optee_msg.h_. The former file defines SMC identifiers and what is passed in the
registers for each SMC. The latter file defines the OP-TEE Message protocol
which is not restricted to only SMC even if that currently is the only option
available.

SMC communication
=================
The main structure used for the SMC communication is defined in ``struct
optee_msg_arg`` (in optee_msg.h_). If we are looking into the source code, we
could see that communication mainly is achieved using ``optee_msg_arg`` and
``thread_smc_args`` (in thread.h_), where ``optee_msg_arg`` could be seen as the
main structure. What will happen is that the :ref:`linux_kernel` driver will get
the parameters either from :ref:`optee_client` or directly from an internal
service in Linux kernel. The TEE driver will populate the struct
``optee_msg_arg`` with the parameters plus some additional bookkeeping
information.  Parameters for the SMC are passed in registers 1 to 7, register 0
holds the SMC id which among other things tells whether it is a standard or a
fast call.

----

.. _thread_handling:

Thread handling
***************
OP-TEE core uses a couple of threads to be able to support running jobs in
parallel (not fully enabled!). There are handlers for different purposes. In
thread.c_ you will find a function called ``thread_init_primary(...)`` which
assigns ``init_handlers`` (functions) that should be called when OP-TEE core
receives standard or fast calls, FIQ and PSCI calls. There are default handlers
for these services, but the platform can decide if they want to implement their
own platform specific handlers instead.

Synchronization primitives
==========================
OP-TEE has three primitives for synchronization of threads and CPUs:
*spin-lock*, *mutex*, and *condvar*.

Spin-lock
    A spin-lock is represented as an ``unsigned int``. This is the most
    primitive lock. Interrupts should be disabled before attempting to take a
    spin-lock and should remain disabled until the lock is released. A spin-lock
    is initialized with ``SPINLOCK_UNLOCK``.

    .. list-table:: Spin lock functions
        :header-rows: 1
        :widths: 1 5

        * - Function
          - Purpose

        * - ``cpu_spin_lock(...)``
          - Locks a spin-lock

        * - ``cpu_spin_trylock(...)``
          - Locks a spin-lock if unlocked and returns ``0`` else the spin-lock
            is unchanged and the function returns ``!0``

        * - ``cpu_spin_unlock(...)``
          - Unlocks a spin-lock

Mutex
    A mutex is represented by ``struct mutex``. A mutex can be locked and
    unlocked with interrupts enabled or disabled, but only from a normal thread.
    A mutex cannot be used in an interrupt handler, abort handler or before a
    thread has been selected for the CPU. A mutex is initialized with either
    ``MUTEX_INITIALIZER`` or ``mutex_init(...)``.

    .. list-table:: Mutex functions
        :header-rows: 1
        :widths: 1 5

        * - Function
          - Purpose

        * - ``mutex_lock(...)``
          - Locks a mutex. If the mutex is unlocked this is a fast operation,
            else the function issues an RPC to wait in normal world.

        * - ``mutex_unlock(...)``
          - Unlocks a mutex. If there is no waiters this is a fast operation,
            else the function issues an RPC to wake up a waiter in normal world.

        * - ``mutex_trylock(...)``
          - Locks a mutex if unlocked and returns ``true`` else the mutex is
            unchanged and the function returns ``false``.

        * - ``mutex_destroy(...)``
          - Asserts that the mutex is unlocked and there is no waiters, after
            this the memory used by the mutex can be freed.

    When a mutex is locked it is owned by the thread calling ``mutex_lock(...)``
    or ``mutex_trylock(...)``, the mutex may only be unlocked by the thread
    owning the mutex. A thread should not exit to TA user space when holding a
    mutex.

Condvar
    A condvar is represented by ``struct condvar``. A condvar is similar to a
    ``pthread_condvar_t`` in the pthreads standard, only less advanced.
    Condition variables are used to wait for some condition to be fulfilled and
    are always used together a mutex. Once a condition variable has been used
    together with a certain mutex, it must only be used with that mutex until
    destroyed. A condvar is initialized with ``CONDVAR_INITIALIZER`` or
    ``condvar_init(...)``.

    .. list-table:: Condvar functions
        :header-rows: 1
        :widths: 1 5

        * - Function
          - Purpose

        * - ``condvar_wait(...)``
          - Atomically unlocks the supplied mutex and waits in normal world via
            an RPC for the condition variable to be signaled, when the function
            returns the mutex is locked again.

        * - ``condvar_signal(...)``
          - Wakes up one waiter of the condition variable (waiting in
            ``condvar_wait(...)``).

        * - ``condvar_broadcast(...)``
          - Wake up all waiters of the condition variable.

    The caller of ``condvar_signal(...)`` or ``condvar_broadcast(...)`` should
    hold the mutex associated with the condition variable to guarantee that a
    waiter does not miss the signal.

.. _core/arch/arm/kernel/thread.c: https://github.com/OP-TEE/optee_os/blob/master/core/arch/arm/kernel/thread.c
.. _optee_msg.h: https://github.com/OP-TEE/optee_os/blob/master/core/include/optee_msg.h
.. _optee_smc.h: https://github.com/OP-TEE/optee_os/blob/master/core/arch/arm/include/sm/optee_smc.h
.. _thread.c: https://github.com/OP-TEE/optee_os/blob/master/core/arch/arm/kernel/thread.c
.. _thread.h: https://github.com/OP-TEE/optee_os/blob/master/core/arch/arm/include/kernel/thread.h

.. _ARM_DEN0028A_SMC_Calling_Convention: http://infocenter.arm.com/help/topic/com.arm.doc.den0028b/ARM_DEN0028B_SMC_Calling_Convention.pdf
.. _Cortex-A53 TRM: http://infocenter.arm.com/help/topic/com.arm.doc.ddi0500j/DDI0500J_cortex_a53_trm.pdf
.. _drivers/tee/optee: https://github.com/torvalds/linux/tree/master/drivers/tee/optee
.. _Trusted Firmware A: https://github.com/ARM-software/arm-trusted-firmware