On 3/15/22 17:15, Daniel P. Berrangé wrote:
Bear with me as I suggest something potentially/probably silly
given my limited knowledge of C++ coroutines.
Given a function I know about:
void coroutine_fn qio_channel_yield(QIOChannel *ioc,
GIOCondition condition);
IIUC, you previously indicated that the header file declaration,
the implementation and any callers of this would need to be in
C++ source files.
The caller is what I'm most curious about, because I feel that
is where the big ripple effects come into play that cause large
parts of QEMU to become C++ code. [...]
I presume there is something special about the CoroutineFn<void>
prototype preventing that from working as needed, thus requiring
the caller to be compiled as C++ ? IIUC compiling as C++ though
is not neccessarily the same as using C++ linkage.
Yes, the CoroutineFn<void> function must either be passed to
qemu_coroutine_create() or called as "co_await f()". If you call it as
"f()" it does nothing except leak the memory needed by its stack frame,
so that only leaves passing the function to qemu_coroutine_create().
I suppose you could do some games with typedefs, like
#ifdef __cplusplus
typedef CoroutineFn<void> VoidCoroutine
#else
typedef struct VoidCoroutine VoidCoroutine;
#endif
to be able to declare a function that returns CoroutineFn<void> but I'm
not sure of the advantage.
So I'm assuming the caller as C++ requirement is not recursive,
otherwise it would immediately mean all of QEMU needs to be C++.
Right, qemu_coroutine_create() must be called from C++ but the caller of
qemu_coroutine_create() can be extern "C". In the particular case of
the block layer, callers of qemu_coroutine_create() include
callback-based functions such as bdrv_aio_readv(), and synchronous
functions such as bdrv_flush(). Both of these can be called from C.
IOW, can we get it such that the C++ bit is just a thin shim
"C -> C++ wrapper -> C++ CoroutineFn -> C", enabling all the
C++ bits to be well encapsulated and thus prevent arbitrary
usage of C++ features leaking all across the codebase ?
No, unfortunately not. But in particular, even though the block layer
would be C++, device models that use it would not.
Paolo
