On Tue, Jul 20, 2010 at 7:34 PM, Ot ten Thije <ottenth...@google.com> wrote: > 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.
qemu.git has support for Linux AIO, which you linked to. The code for that is in linux-aio.c. But by default it uses a thread pool to perform the normal blocking syscalls out-of-line with respect to the VM, see posix-aio-compat.c. Both of these aio backends are driven from the QEMU block layer, which provides a block device interface that several drivers implement (qcow2, raw, ...). bdrv_aio_readv() and bdrv_aio_writev() are the main entry points. I wonder if you can track down the sequence of block interface calls that causes the hang. Then it should be possible to construct a simple test case and even try it again qemu.git/master. > 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. Have you checked QEMU's git log to see if there are fixes in qemu.git that could fix this for the Android emulator? Stefan > > Ot ten Thije
