Assembler Annotations
  =====================
  
  Copyright (c) 2017-2019 Jiri Slaby
  
  This document describes the new macros for annotation of data and code in
  assembly. In particular, it contains information about ``SYM_FUNC_START``,
  ``SYM_FUNC_END``, ``SYM_CODE_START``, and similar.
  
  Rationale
  ---------
  Some code like entries, trampolines, or boot code needs to be written in
  assembly. The same as in C, such code is grouped into functions and
  accompanied with data. Standard assemblers do not force users into precisely
  marking these pieces as code, data, or even specifying their length.
  Nevertheless, assemblers provide developers with such annotations to aid
  debuggers throughout assembly. On top of that, developers also want to mark
  some functions as *global* in order to be visible outside of their translation
  units.
  
  Over time, the Linux kernel has adopted macros from various projects (like
  ``binutils``) to facilitate such annotations. So for historic reasons,
  developers have been using ``ENTRY``, ``END``, ``ENDPROC``, and other
  annotations in assembly.  Due to the lack of their documentation, the macros
  are used in rather wrong contexts at some locations. Clearly, ``ENTRY`` was
  intended to denote the beginning of global symbols (be it data or code).
  ``END`` used to mark the end of data or end of special functions with
  *non-standard* calling convention. In contrast, ``ENDPROC`` should annotate
  only ends of *standard* functions.
  
  When these macros are used correctly, they help assemblers generate a nice
  object with both sizes and types set correctly. For example, the result of
  ``arch/x86/lib/putuser.S``::
  
     Num:    Value          Size Type    Bind   Vis      Ndx Name
      25: 0000000000000000    33 FUNC    GLOBAL DEFAULT    1 __put_user_1
      29: 0000000000000030    37 FUNC    GLOBAL DEFAULT    1 __put_user_2
      32: 0000000000000060    36 FUNC    GLOBAL DEFAULT    1 __put_user_4
      35: 0000000000000090    37 FUNC    GLOBAL DEFAULT    1 __put_user_8
  
  This is not only important for debugging purposes. When there are properly
  annotated objects like this, tools can be run on them to generate more useful
  information. In particular, on properly annotated objects, ``objtool`` can be
  run to check and fix the object if needed. Currently, ``objtool`` can report
  missing frame pointer setup/destruction in functions. It can also
  automatically generate annotations for :doc:`ORC unwinder <x86/orc-unwinder>`
  for most code. Both of these are especially important to support reliable
  stack traces which are in turn necessary for :doc:`Kernel live patching
  <livepatch/livepatch>`.
  
  Caveat and Discussion
  ---------------------
  As one might realize, there were only three macros previously. That is indeed
  insufficient to cover all the combinations of cases:
  
  * standard/non-standard function
  * code/data
  * global/local symbol
  
  There was a discussion_ and instead of extending the current ``ENTRY/END*``
  macros, it was decided that brand new macros should be introduced instead::
  
      So how about using macro names that actually show the purpose, instead
      of importing all the crappy, historic, essentially randomly chosen
      debug symbol macro names from the binutils and older kernels?
  
  .. _discussion: https://lkml.kernel.org/r/20170217104757.28588-1-jslaby@suse.cz
  
  Macros Description
  ------------------
  
  The new macros are prefixed with the ``SYM_`` prefix and can be divided into
  three main groups:
  
  1. ``SYM_FUNC_*`` -- to annotate C-like functions. This means functions with

     standard C calling conventions. For example, on x86, this means that the
     stack contains a return address at the predefined place and a return from
     the function can happen in a standard way. When frame pointers are enabled,
     save/restore of frame pointer shall happen at the start/end of a function,
     respectively, too.

  
     Checking tools like ``objtool`` should ensure such marked functions conform
     to these rules. The tools can also easily annotate these functions with
     debugging information (like *ORC data*) automatically.
  
  2. ``SYM_CODE_*`` -- special functions called with special stack. Be it
     interrupt handlers with special stack content, trampolines, or startup
     functions.
  
     Checking tools mostly ignore checking of these functions. But some debug
     information still can be generated automatically. For correct debug data,
     this code needs hints like ``UNWIND_HINT_REGS`` provided by developers.
  
  3. ``SYM_DATA*`` -- obviously data belonging to ``.data`` sections and not to
     ``.text``. Data do not contain instructions, so they have to be treated
     specially by the tools: they should not treat the bytes as instructions,
     nor assign any debug information to them.
  
  Instruction Macros
  ~~~~~~~~~~~~~~~~~~
  This section covers ``SYM_FUNC_*`` and ``SYM_CODE_*`` enumerated above.
  
  * ``SYM_FUNC_START`` and ``SYM_FUNC_START_LOCAL`` are supposed to be **the
    most frequent markings**. They are used for functions with standard calling
    conventions -- global and local. Like in C, they both align the functions to
    architecture specific ``__ALIGN`` bytes. There are also ``_NOALIGN`` variants
    for special cases where developers do not want this implicit alignment.
  
    ``SYM_FUNC_START_WEAK`` and ``SYM_FUNC_START_WEAK_NOALIGN`` markings are
    also offered as an assembler counterpart to the *weak* attribute known from
    C.
  
    All of these **shall** be coupled with ``SYM_FUNC_END``. First, it marks
    the sequence of instructions as a function and computes its size to the
    generated object file. Second, it also eases checking and processing such
    object files as the tools can trivially find exact function boundaries.
  
    So in most cases, developers should write something like in the following
    example, having some asm instructions in between the macros, of course::

      SYM_FUNC_START(memset)

          ... asm insns ...

      SYM_FUNC_END(memset)

  
    In fact, this kind of annotation corresponds to the now deprecated ``ENTRY``
    and ``ENDPROC`` macros.
  
  * ``SYM_FUNC_START_ALIAS`` and ``SYM_FUNC_START_LOCAL_ALIAS`` serve for those
    who decided to have two or more names for one function. The typical use is::
  
      SYM_FUNC_START_ALIAS(__memset)
      SYM_FUNC_START(memset)
          ... asm insns ...
      SYM_FUNC_END(memset)
      SYM_FUNC_END_ALIAS(__memset)
  
    In this example, one can call ``__memset`` or ``memset`` with the same
    result, except the debug information for the instructions is generated to
    the object file only once -- for the non-``ALIAS`` case.
  
  * ``SYM_CODE_START`` and ``SYM_CODE_START_LOCAL`` should be used only in
    special cases -- if you know what you are doing. This is used exclusively
    for interrupt handlers and similar where the calling convention is not the C
    one. ``_NOALIGN`` variants exist too. The use is the same as for the ``FUNC``
    category above::
  
      SYM_CODE_START_LOCAL(bad_put_user)
          ... asm insns ...
      SYM_CODE_END(bad_put_user)
  
    Again, every ``SYM_CODE_START*`` **shall** be coupled by ``SYM_CODE_END``.
  
    To some extent, this category corresponds to deprecated ``ENTRY`` and
    ``END``. Except ``END`` had several other meanings too.
  
  * ``SYM_INNER_LABEL*`` is used to denote a label inside some
    ``SYM_{CODE,FUNC}_START`` and ``SYM_{CODE,FUNC}_END``.  They are very similar
    to C labels, except they can be made global. An example of use::
  
      SYM_CODE_START(ftrace_caller)
          /* save_mcount_regs fills in first two parameters */
          ...
  
      SYM_INNER_LABEL(ftrace_caller_op_ptr, SYM_L_GLOBAL)
          /* Load the ftrace_ops into the 3rd parameter */
          ...
  
      SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
          call ftrace_stub
          ...
          retq
      SYM_CODE_END(ftrace_caller)
  
  Data Macros
  ~~~~~~~~~~~
  Similar to instructions, there is a couple of macros to describe data in the
  assembly.
  
  * ``SYM_DATA_START`` and ``SYM_DATA_START_LOCAL`` mark the start of some data
    and shall be used in conjunction with either ``SYM_DATA_END``, or
    ``SYM_DATA_END_LABEL``. The latter adds also a label to the end, so that
    people can use ``lstack`` and (local) ``lstack_end`` in the following
    example::
  
      SYM_DATA_START_LOCAL(lstack)
          .skip 4096
      SYM_DATA_END_LABEL(lstack, SYM_L_LOCAL, lstack_end)
  
  * ``SYM_DATA`` and ``SYM_DATA_LOCAL`` are variants for simple, mostly one-line
    data::
  
      SYM_DATA(HEAP,     .long rm_heap)
      SYM_DATA(heap_end, .long rm_stack)
  
    In the end, they expand to ``SYM_DATA_START`` with ``SYM_DATA_END``
    internally.
  
  Support Macros
  ~~~~~~~~~~~~~~
  All the above reduce themselves to some invocation of ``SYM_START``,
  ``SYM_END``, or ``SYM_ENTRY`` at last. Normally, developers should avoid using
  these.
  
  Further, in the above examples, one could see ``SYM_L_LOCAL``. There are also
  ``SYM_L_GLOBAL`` and ``SYM_L_WEAK``. All are intended to denote linkage of a
  symbol marked by them. They are used either in ``_LABEL`` variants of the
  earlier macros, or in ``SYM_START``.
  
  
  Overriding Macros
  ~~~~~~~~~~~~~~~~~
  Architecture can also override any of the macros in their own
  ``asm/linkage.h``, including macros specifying the type of a symbol
  (``SYM_T_FUNC``, ``SYM_T_OBJECT``, and ``SYM_T_NONE``).  As every macro
  described in this file is surrounded by ``#ifdef`` + ``#endif``, it is enough
  to define the macros differently in the aforementioned architecture-dependent
  header.