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We have a very frequent pattern of creating a coroutine from a function
with several arguments:
- create a structure to pack parameters
- create _entry function to call original function taking parameters
from struct
- do different magic to handle completion: set ret to NOT_DONE or
EINPROGRESS or use separate bool field
- fill the struct and create coroutine from _entry function with this
struct as a parameter
- do coroutine enter and BDRV_POLL_WHILE loop
Let's reduce code duplication by generating coroutine wrappers.
This patch adds scripts/block-coroutine-wrapper.py together with some
friends, which will generate functions with declared prototypes marked
by the 'generated_co_wrapper' specifier.
The usage of new code generation is as follows:
1. define the coroutine function somewhere
int coroutine_fn bdrv_co_NAME(...) {...}
2. declare in some header file
int generated_co_wrapper bdrv_NAME(...);
with same list of parameters (generated_co_wrapper is
defined in "include/block/block.h").
3. Make sure the block_gen_c declaration in block/meson.build
mentions the file with your marker function.
Still, no function is now marked, this work is for the following
commit.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200924185414.28642-5-vsementsov@virtuozzo.com>
[Added encoding='utf-8' to open() calls as requested by Vladimir. Fixed
typo and grammar issues pointed out by Eric Blake. Removed clang-format
dependency that caused build test issues.
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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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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