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diff --git a/docs/components/xlat-tables-lib-v2-design.rst b/docs/components/xlat-tables-lib-v2-design.rst new file mode 100644 index 0000000..cac32f5 --- /dev/null +++ b/docs/components/xlat-tables-lib-v2-design.rst @@ -0,0 +1,442 @@ +Translation (XLAT) Tables Library +================================= + +This document describes the design of the translation tables library (version 2) +used by Trusted Firmware-A (TF-A). This library provides APIs to create page +tables based on a description of the memory layout, as well as setting up system +registers related to the Memory Management Unit (MMU) and performing the +required Translation Lookaside Buffer (TLB) maintenance operations. + +More specifically, some use cases that this library aims to support are: + +#. Statically allocate translation tables and populate them (at run-time) based + upon a description of the memory layout. The memory layout is typically + provided by the platform port as a list of memory regions; + +#. Support for generating translation tables pertaining to a different + translation regime than the exception level the library code is executing at; + +#. Support for dynamic mapping and unmapping of regions, even while the MMU is + on. This can be used to temporarily map some memory regions and unmap them + later on when no longer needed; + +#. Support for non-identity virtual to physical mappings to compress the virtual + address space; + +#. Support for changing memory attributes of memory regions at run-time. + + +About version 1, version 2 and MPU libraries +-------------------------------------------- + +This document focuses on version 2 of the library, whose sources are available +in the ``lib/xlat_tables_v2`` directory. Version 1 of the library can still be +found in ``lib/xlat_tables`` directory but it is less flexible and doesn't +support dynamic mapping. ``lib/xlat_mpu``, which configures Arm's MPU +equivalently, is also addressed here. The ``lib/xlat_mpu`` is experimental, +meaning that its API may change. It currently strives for consistency and +code-reuse with xlat_tables_v2. Future versions may be more MPU-specific (e.g., +removing all mentions of virtual addresses). Although potential bug fixes will +be applied to all versions of the xlat_* libs, future feature enhancements will +focus on version 2 and might not be back-ported to version 1 and MPU versions. +Therefore, it is recommended to use version 2, especially for new platform +ports (unless the platform uses an MPU). + +However, please note that version 2 and the MPU version are still in active +development and is not considered stable yet. Hence, compatibility breaks might +be introduced. + +From this point onwards, this document will implicitly refer to version 2 of the +library, unless stated otherwise. + + +Design concepts and interfaces +------------------------------ + +This section presents some of the key concepts and data structures used in the +translation tables library. + +`mmap` regions +~~~~~~~~~~~~~~ + +An ``mmap_region`` is an abstract, concise way to represent a memory region to +map. It is one of the key interfaces to the library. It is identified by: + +- its physical base address; +- its virtual base address; +- its size; +- its attributes; +- its mapping granularity (optional). + +See the ``struct mmap_region`` type in ``xlat_tables_v2.h``. + +The user usually provides a list of such mmap regions to map and lets the +library transpose that in a set of translation tables. As a result, the library +might create new translation tables, update or split existing ones. + +The region attributes specify the type of memory (for example device or cached +normal memory) as well as the memory access permissions (read-only or +read-write, executable or not, secure or non-secure, and so on). In the case of +the EL1&0 translation regime, the attributes also specify whether the region is +a User region (EL0) or Privileged region (EL1). See the ``MT_xxx`` definitions +in ``xlat_tables_v2.h``. Note that for the EL1&0 translation regime the Execute +Never attribute is set simultaneously for both EL1 and EL0. + +The granularity controls the translation table level to go down to when mapping +the region. For example, assuming the MMU has been configured to use a 4KB +granule size, the library might map a 2MB memory region using either of the two +following options: + +- using a single level-2 translation table entry; +- using a level-2 intermediate entry to a level-3 translation table (which + contains 512 entries, each mapping 4KB). + +The first solution potentially requires less translation tables, hence +potentially less memory. However, if part of this 2MB region is later remapped +with different memory attributes, the library might need to split the existing +page tables to refine the mappings. If a single level-2 entry has been used +here, a level-3 table will need to be allocated on the fly and the level-2 +modified to point to this new level-3 table. This has a performance cost at +run-time. + +If the user knows upfront that such a remapping operation is likely to happen +then they might enforce a 4KB mapping granularity for this 2MB region from the +beginning; remapping some of these 4KB pages on the fly then becomes a +lightweight operation. + +The region's granularity is an optional field; if it is not specified the +library will choose the mapping granularity for this region as it sees fit (more +details can be found in `The memory mapping algorithm`_ section below). + +The MPU library also uses ``struct mmap_region`` to specify translations, but +the MPU's translations are limited to specification of valid addresses and +access permissions. If the requested virtual and physical addresses mismatch +the system will panic. Being register-based for deterministic memory-reference +timing, the MPU hardware does not involve memory-resident translation tables. + +Currently, the MPU library is also limited to MPU translation at EL2 with no +MMU translation at other ELs. These limitations, however, are expected to be +overcome in future library versions. + +Translation Context +~~~~~~~~~~~~~~~~~~~ + +The library can create or modify translation tables pertaining to a different +translation regime than the exception level the library code is executing at. +For example, the library might be used by EL3 software (for instance BL31) to +create translation tables pertaining to the S-EL1&0 translation regime. + +This flexibility comes from the use of *translation contexts*. A *translation +context* constitutes the superset of information used by the library to track +the status of a set of translation tables for a given translation regime. + +The library internally allocates a default translation context, which pertains +to the translation regime of the current exception level. Additional contexts +may be explicitly allocated and initialized using the +``REGISTER_XLAT_CONTEXT()`` macro. Separate APIs are provided to act either on +the default translation context or on an alternative one. + +To register a translation context, the user must provide the library with the +following information: + +* A name. + + The resulting translation context variable will be called after this name, to + which ``_xlat_ctx`` is appended. For example, if the macro name parameter is + ``foo``, the context variable name will be ``foo_xlat_ctx``. + +* The maximum number of `mmap` regions to map. + + Should account for both static and dynamic regions, if applicable. + +* The number of sub-translation tables to allocate. + + Number of translation tables to statically allocate for this context, + excluding the initial lookup level translation table, which is always + allocated. For example, if the initial lookup level is 1, this parameter would + specify the number of level-2 and level-3 translation tables to pre-allocate + for this context. + +* The size of the virtual address space. + + Size in bytes of the virtual address space to map using this context. This + will incidentally determine the number of entries in the initial lookup level + translation table : the library will allocate as many entries as is required + to map the entire virtual address space. + +* The size of the physical address space. + + Size in bytes of the physical address space to map using this context. + +The default translation context is internally initialized using information +coming (for the most part) from platform-specific defines: + +- name: hard-coded to ``tf`` ; hence the name of the default context variable is + ``tf_xlat_ctx``; +- number of `mmap` regions: ``MAX_MMAP_REGIONS``; +- number of sub-translation tables: ``MAX_XLAT_TABLES``; +- size of the virtual address space: ``PLAT_VIRT_ADDR_SPACE_SIZE``; +- size of the physical address space: ``PLAT_PHY_ADDR_SPACE_SIZE``. + +Please refer to the :ref:`Porting Guide` for more details about these macros. + + +Static and dynamic memory regions +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The library optionally supports dynamic memory mapping. This feature may be +enabled using the ``PLAT_XLAT_TABLES_DYNAMIC`` platform build flag. + +When dynamic memory mapping is enabled, the library categorises mmap regions as +*static* or *dynamic*. + +- *Static regions* are fixed for the lifetime of the system. They can only be + added early on, before the translation tables are created and populated. They + cannot be removed afterwards. + +- *Dynamic regions* can be added or removed any time. + +When the dynamic memory mapping feature is disabled, only static regions exist. + +The dynamic memory mapping feature may be used to map and unmap transient memory +areas. This is useful when the user needs to access some memory for a fixed +period of time, after which the memory may be discarded and reclaimed. For +example, a memory region that is only required at boot time while the system is +initializing, or to temporarily share a memory buffer between the normal world +and trusted world. Note that it is up to the caller to ensure that these regions +are not accessed concurrently while the regions are being added or removed. + +Although this feature provides some level of dynamic memory allocation, this +does not allow dynamically allocating an arbitrary amount of memory at an +arbitrary memory location. The user is still required to declare at compile-time +the limits of these allocations ; the library will deny any mapping request that +does not fit within this pre-allocated pool of memory. + + +Library APIs +------------ + +The external APIs exposed by this library are declared and documented in the +``xlat_tables_v2.h`` header file. This should be the reference point for +getting information about the usage of the different APIs this library +provides. This section just provides some extra details and clarifications. + +Although the ``mmap_region`` structure is a publicly visible type, it is not +recommended to populate these structures by hand. Instead, wherever APIs expect +function arguments of type ``mmap_region_t``, these should be constructed using +the ``MAP_REGION*()`` family of helper macros. This is to limit the risk of +compatibility breaks, should the ``mmap_region`` structure type evolve in the +future. + +The ``MAP_REGION()`` and ``MAP_REGION_FLAT()`` macros do not allow specifying a +mapping granularity, which leaves the library implementation free to choose +it. However, in cases where a specific granularity is required, the +``MAP_REGION2()`` macro might be used instead. Using ``MAP_REGION_FLAT()`` only +to define regions for the MPU library is strongly recommended. + +As explained earlier in this document, when the dynamic mapping feature is +disabled, there is no notion of dynamic regions. Conceptually, there are only +static regions. For this reason (and to retain backward compatibility with the +version 1 of the library), the APIs that map static regions do not embed the +word *static* in their functions names (for example ``mmap_add_region()``), in +contrast with the dynamic regions APIs (for example +``mmap_add_dynamic_region()``). + +Although the definition of static and dynamic regions is not based on the state +of the MMU, the two are still related in some way. Static regions can only be +added before ``init_xlat_tables()`` is called and ``init_xlat_tables()`` must be +called while the MMU is still off. As a result, static regions cannot be added +once the MMU has been enabled. Dynamic regions can be added with the MMU on or +off. In practice, the usual call flow would look like this: + +#. The MMU is initially off. + +#. Add some static regions, add some dynamic regions. + +#. Initialize translation tables based on the list of mmap regions (using one of + the ``init_xlat_tables*()`` APIs). + +#. At this point, it is no longer possible to add static regions. Dynamic + regions can still be added or removed. + +#. Enable the MMU. + +#. Dynamic regions can continue to be added or removed. + +Because static regions are added early on at boot time and are all in the +control of the platform initialization code, the ``mmap_add*()`` family of APIs +are not expected to fail. They do not return any error code. + +Nonetheless, these APIs will check upfront whether the region can be +successfully added before updating the translation context structure. If the +library detects that there is insufficient memory to meet the request, or that +the new region will overlap another one in an invalid way, or if any other +unexpected error is encountered, they will print an error message on the UART. +Additionally, when asserts are enabled (typically in debug builds), an assertion +will be triggered. Otherwise, the function call will just return straight away, +without adding the offending memory region. + + +Library limitations +------------------- + +Dynamic regions are not allowed to overlap each other. Static regions are +allowed to overlap as long as one of them is fully contained inside the other +one. This is allowed for backwards compatibility with the previous behaviour in +the version 1 of the library. + + +Implementation details +---------------------- + +Code structure +~~~~~~~~~~~~~~ + +The library is divided into 4 modules: + +- **Core module** + + Provides the main functionality of the library, such as the initialization of + translation tables contexts and mapping/unmapping memory regions. This module + provides functions such as ``mmap_add_region_ctx`` that let the caller specify + the translation tables context affected by them. + + See ``xlat_tables_core.c``. + +- **Active context module** + + Instantiates the context that is used by the current BL image and provides + helpers to manipulate it, abstracting it from the rest of the code. + This module provides functions such as ``mmap_add_region``, that directly + affect the BL image using them. + + See ``xlat_tables_context.c``. + +- **Utilities module** + + Provides additional functionality like debug print of the current state of the + translation tables and helpers to query memory attributes and to modify them. + + See ``xlat_tables_utils.c``. + +- **Architectural module** + + Provides functions that are dependent on the current execution state + (AArch32/AArch64), such as the functions used for TLB invalidation, setup the + MMU, or calculate the Physical Address Space size. They do not need a + translation context to work on. + + See ``aarch32/xlat_tables_arch.c`` and ``aarch64/xlat_tables_arch.c``. + +From mmap regions to translation tables +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +A translation context contains a list of ``mmap_region_t``, which holds the +information of all the regions that are mapped at any given time. Whenever there +is a request to map (resp. unmap) a memory region, it is added to (resp. removed +from) the ``mmap_region_t`` list. + +The mmap regions list is a conceptual way to represent the memory layout. At +some point, the library has to convert this information into actual translation +tables to program into the MMU. + +Before the ``init_xlat_tables()`` API is called, the library only acts on the +mmap regions list. Adding a static or dynamic region at this point through one +of the ``mmap_add*()`` APIs does not affect the translation tables in any way, +they only get registered in the internal mmap region list. It is only when the +user calls the ``init_xlat_tables()`` that the translation tables are populated +in memory based on the list of mmap regions registered so far. This is an +optimization that allows creation of the initial set of translation tables in +one go, rather than having to edit them every time while the MMU is disabled. + +After the ``init_xlat_tables()`` API has been called, only dynamic regions can +be added. Changes to the translation tables (as well as the mmap regions list) +will take effect immediately. + +The memory mapping algorithm +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The mapping function is implemented as a recursive algorithm. It is however +bound by the level of depth of the translation tables (the Armv8-A architecture +allows up to 4 lookup levels). + +By default [#granularity]_, the algorithm will attempt to minimize the +number of translation tables created to satisfy the user's request. It will +favour mapping a region using the biggest possible blocks, only creating a +sub-table if it is strictly necessary. This is to reduce the memory footprint of +the firmware. + +The most common reason for needing a sub-table is when a specific mapping +requires a finer granularity. Misaligned regions also require a finer +granularity than what the user may had originally expected, using a lot more +memory than expected. The reason is that all levels of translation are +restricted to address translations of the same granularity as the size of the +blocks of that level. For example, for a 4 KiB page size, a level 2 block entry +can only translate up to a granularity of 2 MiB. If the Physical Address is not +aligned to 2 MiB then additional level 3 tables are also needed. + +Note that not every translation level allows any type of descriptor. Depending +on the page size, levels 0 and 1 of translation may only allow table +descriptors. If a block entry could be able to describe a translation, but that +level does not allow block descriptors, a table descriptor will have to be used +instead, as well as additional tables at the next level. + +|Alignment Example| + +The mmap regions are sorted in a way that simplifies the code that maps +them. Even though this ordering is only strictly needed for overlapping static +regions, it must also be applied for dynamic regions to maintain a consistent +order of all regions at all times. As each new region is mapped, existing +entries in the translation tables are checked to ensure consistency. Please +refer to the comments in the source code of the core module for more details +about the sorting algorithm in use. + +This mapping algorithm does not apply to the MPU library, since the MPU hardware +directly maps regions by "base" and "limit" (bottom and top) addresses. + +TLB maintenance operations +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The library takes care of performing TLB maintenance operations when required. +For example, when the user requests removing a dynamic region, the library +invalidates all TLB entries associated to that region to ensure that these +changes are visible to subsequent execution, including speculative execution, +that uses the changed translation table entries. + +A counter-example is the initialization of translation tables. In this case, +explicit TLB maintenance is not required. The Armv8-A architecture guarantees +that all TLBs are disabled from reset and their contents have no effect on +address translation at reset [#tlb-reset-ref]_. Therefore, the TLBs invalidation +is deferred to the ``enable_mmu*()`` family of functions, just before the MMU is +turned on. + +Regarding enabling and disabling memory management, for the MPU library, to +reduce confusion, calls to enable or disable the MPU use ``mpu`` in their names +in place of ``mmu``. For example, the ``enable_mmu_el2()`` call is changed to +``enable_mpu_el2()``. + +TLB invalidation is not required when adding dynamic regions either. Dynamic +regions are not allowed to overlap existing memory region. Therefore, if the +dynamic mapping request is deemed legitimate, it automatically concerns memory +that was not mapped in this translation regime and the library will have +initialized its corresponding translation table entry to an invalid +descriptor. Given that the TLBs are not architecturally permitted to hold any +invalid translation table entry [#tlb-no-invalid-entry]_, this means that this +mapping cannot be cached in the TLBs. + +.. rubric:: Footnotes + +.. [#granularity] That is, when mmap regions do not enforce their mapping + granularity. + +.. [#tlb-reset-ref] See section D4.9 ``Translation Lookaside Buffers (TLBs)``, + subsection ``TLB behavior at reset`` in Armv8-A, rev C.a. + +.. [#tlb-no-invalid-entry] See section D4.10.1 ``General TLB maintenance + requirements`` in Armv8-A, rev C.a. + +-------------- + +*Copyright (c) 2017-2021, Arm Limited and Contributors. All rights reserved.* + +.. |Alignment Example| image:: ../resources/diagrams/xlat_align.png |