QEMU is event-driven and suffers when blocking operations are performed because VM execution may be stopped until the operation completes. Therefore many operations that could block are performed asynchronously and a callback is invoked when the operation has completed. This allows QEMU to continue executing while the operation is pending.
The downside to callbacks is that they split up code into many smaller functions, each of which is a single step in a state machine that quickly becomes complex and hard to understand. Callback functions also result in lots of noise as variables are packed and unpacked into temporary structs that pass state to the callback function. This patch series introduces coroutines as a solution for writing asynchronous code while still having a nice sequential control flow. The semantics are explained in the second patch. The fourth patch adds automated tests. A nice feature of coroutines is that it is relatively easy to take synchronous code and lift it into a coroutine to make it asynchronous. Work has been done to move qcow2 request processing into coroutines and thereby make it asynchronous (today qcow2 will perform synchronous metadata accesses). This qcow2 work is still ongoing and not quite ready for mainline yet. v8: * Bisectability: introduce gthread implementation before ucontext/fibers v7: * Reduce ucontext and win32 fiber stack size to 1 MB * Add qemu-timer-common.o to test-coroutine dependencies for OpenBSD build v6: * Use GThread on Mac OS X, fix from Andreas Färber <andreas.faer...@web.de> * abort(3) if coroutine-ucontext.c fails to create a coroutine v5: * GThread-based implementation for platforms without makecontext(3) (Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com>) * Switch to gtester test framework v4: * Windows Fibers support (Paolo Bonzini <pbonz...@redhat.com>) * Return-after-setjmp() fix (Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com>) * Re-entrancy for multi-threaded coroutines support * qemu-coroutine.h cleanup and documentation v3: * Updated LGPL v2 license header to use web link * Removed atexit(3) pool freeing * Removed thread-local current/leader * Documented thread-safety limitation * Disabled trace events v2: * Added ./check-coroutine --lifecycle-benchmark for performance measurement * Split pooling into a separate patch with performance justification * Set maximum pool size to prevent holding onto too many free coroutines * Added atexit(3) handler to free pool * Coding style cleanups Kevin Wolf (1): coroutine: add ucontext and win32 implementations Stefan Hajnoczi (4): coroutine: add gthread dependency coroutine: introduce coroutines API coroutine: add test-coroutine automated tests coroutine: add test-coroutine --benchmark-lifecycle .gitignore | 1 + Makefile | 3 +- Makefile.objs | 11 +++ configure | 24 +++++- coroutine-gthread.c | 131 ++++++++++++++++++++++++++++ coroutine-ucontext.c | 230 ++++++++++++++++++++++++++++++++++++++++++++++++++ coroutine-win32.c | 91 ++++++++++++++++++++ qemu-coroutine-int.h | 48 +++++++++++ qemu-coroutine.c | 75 ++++++++++++++++ qemu-coroutine.h | 95 +++++++++++++++++++++ test-coroutine.c | 192 +++++++++++++++++++++++++++++++++++++++++ trace-events | 5 + 12 files changed, 902 insertions(+), 4 deletions(-) create mode 100644 coroutine-gthread.c create mode 100644 coroutine-ucontext.c create mode 100644 coroutine-win32.c create mode 100644 qemu-coroutine-int.h create mode 100644 qemu-coroutine.c create mode 100644 qemu-coroutine.h create mode 100644 test-coroutine.c -- 1.7.5.4
