[Qemu-devel] Docs for and debugging of Asynchronous I/O

[Ot ten Thije]

Hello,

I am working on fixing the savevm/loadvm functionality in the Android emulator, and the two issues I've encountered so far both appear to stem from the asynchronous I/O (AIO) code. In both cases, the emulator busy-waits indefinitely for an operation that never signals completion.

Unfortunately I am not really familiar with AIO, so I was hoping one of the emulator devs could point me some resources (design docs, general introduction, etc.). I've done some searching myself and found some docs for the Linux kernel AIO implementation (http://lse.sourceforge.net/io/aio.html), but I'm not sure to what extent it applies to the QEMU code.

Tips for debugging AIO would also be greatly appreciated. I can trace the execution until I am within the (emulated) device driver (i.e. block/qcow2.c:qcow_aio_writev()), but haven't been able to pinpoint the exact location where the actual async call is made. This makes it difficult to identify the code that should signal completion back to the main process (and apparently fails to do so). I know this code is called though, because some asynchronous calls *do* signal completion.

I realize that the Android emulator is a rather heavy fork of QEMU, so giving specific advice will probably be difficult. However, the overall approach is still the same, so I hope you can help me get a better understanding of that.

Ot ten Thije