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