qemu

block-coroutine-wrapper.rst (2122B)

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 =======================
 block-coroutine-wrapper
 =======================
 
 A lot of functions in QEMU block layer (see ``block/*``) can only be
 called in coroutine context. Such functions are normally marked by the
 coroutine_fn specifier. Still, sometimes we need to call them from
 non-coroutine context; for this we need to start a coroutine, run the
 needed function from it and wait for the coroutine to finish in a
 BDRV_POLL_WHILE() loop. To run a coroutine we need a function with one
 void* argument. So for each coroutine_fn function which needs a
 non-coroutine interface, we should define a structure to pack the
 parameters, define a separate function to unpack the parameters and
 call the original function and finally define a new interface function
 with same list of arguments as original one, which will pack the
 parameters into a struct, create a coroutine, run it and wait in
 BDRV_POLL_WHILE() loop. It's boring to create such wrappers by hand,
 so we have a script to generate them.
 
 Usage
 =====
 
 Assume we have defined the ``coroutine_fn`` function
 ``bdrv_co_foo(<some args>)`` and need a non-coroutine interface for it,
 called ``bdrv_foo(<same args>)``. In this case the script can help. To
 trigger the generation:
 
 1. You need ``bdrv_foo`` declaration somewhere (for example, in
    ``block/coroutines.h``) with the ``generated_co_wrapper`` mark,
    like this:
 
 .. code-block:: c
 
     int generated_co_wrapper bdrv_foo(<some args>);
 
 2. You need to feed this declaration to block-coroutine-wrapper script.
    For this, add the .h (or .c) file with the declaration to the
    ``input: files(...)`` list of ``block_gen_c`` target declaration in
    ``block/meson.build``
 
 You are done. During the build, coroutine wrappers will be generated in
 ``<BUILD_DIR>/block/block-gen.c``.
 
 Links
 =====
 
 1. The script location is ``scripts/block-coroutine-wrapper.py``.
 
 2. Generic place for private ``generated_co_wrapper`` declarations is
    ``block/coroutines.h``, for public declarations:
    ``include/block/block.h``
 
 3. The core API of generated coroutine wrappers is placed in
    (not generated) ``block/block-gen.h``