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:
int generated_co_wrapper bdrv_foo(<some args>);
  1. 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.