1Copyright (c) 2014-2017 Red Hat Inc.
2
3This work is licensed under the terms of the GNU GPL, version 2 or later.  See
4the COPYING file in the top-level directory.
5
6
7This document explains the IOThread feature and how to write code that runs
8outside the QEMU global mutex.
9
10The main loop and IOThreads
11---------------------------
12QEMU is an event-driven program that can do several things at once using an
13event loop.  The VNC server and the QMP monitor are both processed from the
14same event loop, which monitors their file descriptors until they become
15readable and then invokes a callback.
16
17The default event loop is called the main loop (see main-loop.c).  It is
18possible to create additional event loop threads using -object
19iothread,id=my-iothread.
20
21Side note: The main loop and IOThread are both event loops but their code is
22not shared completely.  Sometimes it is useful to remember that although they
23are conceptually similar they are currently not interchangeable.
24
25Why IOThreads are useful
26------------------------
27IOThreads allow the user to control the placement of work.  The main loop is a
28scalability bottleneck on hosts with many CPUs.  Work can be spread across
29several IOThreads instead of just one main loop.  When set up correctly this
30can improve I/O latency and reduce jitter seen by the guest.
31
32The main loop is also deeply associated with the QEMU global mutex, which is a
33scalability bottleneck in itself.  vCPU threads and the main loop use the QEMU
34global mutex to serialize execution of QEMU code.  This mutex is necessary
35because a lot of QEMU's code historically was not thread-safe.
36
37The fact that all I/O processing is done in a single main loop and that the
38QEMU global mutex is contended by all vCPU threads and the main loop explain
39why it is desirable to place work into IOThreads.
40
41The experimental virtio-blk data-plane implementation has been benchmarked and
42shows these effects:
43ftp://public.dhe.ibm.com/linux/pdfs/KVM_Virtualized_IO_Performance_Paper.pdf
44
45How to program for IOThreads
46----------------------------
47The main difference between legacy code and new code that can run in an
48IOThread is dealing explicitly with the event loop object, AioContext
49(see include/block/aio.h).  Code that only works in the main loop
50implicitly uses the main loop's AioContext.  Code that supports running
51in IOThreads must be aware of its AioContext.
52
53AioContext supports the following services:
54 * File descriptor monitoring (read/write/error on POSIX hosts)
55 * Event notifiers (inter-thread signalling)
56 * Timers
57 * Bottom Halves (BH) deferred callbacks
58
59There are several old APIs that use the main loop AioContext:
60 * LEGACY qemu_aio_set_fd_handler() - monitor a file descriptor
61 * LEGACY qemu_aio_set_event_notifier() - monitor an event notifier
62 * LEGACY timer_new_ms() - create a timer
63 * LEGACY qemu_bh_new() - create a BH
64 * LEGACY qemu_aio_wait() - run an event loop iteration
65
66Since they implicitly work on the main loop they cannot be used in code that
67runs in an IOThread.  They might cause a crash or deadlock if called from an
68IOThread since the QEMU global mutex is not held.
69
70Instead, use the AioContext functions directly (see include/block/aio.h):
71 * aio_set_fd_handler() - monitor a file descriptor
72 * aio_set_event_notifier() - monitor an event notifier
73 * aio_timer_new() - create a timer
74 * aio_bh_new() - create a BH
75 * aio_poll() - run an event loop iteration
76
77The AioContext can be obtained from the IOThread using
78iothread_get_aio_context() or for the main loop using qemu_get_aio_context().
79Code that takes an AioContext argument works both in IOThreads or the main
80loop, depending on which AioContext instance the caller passes in.
81
82How to synchronize with an IOThread
83-----------------------------------
84AioContext is not thread-safe so some rules must be followed when using file
85descriptors, event notifiers, timers, or BHs across threads:
86
871. AioContext functions can always be called safely.  They handle their
88own locking internally.
89
902. Other threads wishing to access the AioContext must use
91aio_context_acquire()/aio_context_release() for mutual exclusion.  Once the
92context is acquired no other thread can access it or run event loop iterations
93in this AioContext.
94
95Legacy code sometimes nests aio_context_acquire()/aio_context_release() calls.
96Do not use nesting anymore, it is incompatible with the BDRV_POLL_WHILE() macro
97used in the block layer and can lead to hangs.
98
99There is currently no lock ordering rule if a thread needs to acquire multiple
100AioContexts simultaneously.  Therefore, it is only safe for code holding the
101QEMU global mutex to acquire other AioContexts.
102
103Side note: the best way to schedule a function call across threads is to call
104aio_bh_schedule_oneshot().  No acquire/release or locking is needed.
105
106AioContext and the block layer
107------------------------------
108The AioContext originates from the QEMU block layer, even though nowadays
109AioContext is a generic event loop that can be used by any QEMU subsystem.
110
111The block layer has support for AioContext integrated.  Each BlockDriverState
112is associated with an AioContext using bdrv_try_set_aio_context() and
113bdrv_get_aio_context().  This allows block layer code to process I/O inside the
114right AioContext.  Other subsystems may wish to follow a similar approach.
115
116Block layer code must therefore expect to run in an IOThread and avoid using
117old APIs that implicitly use the main loop.  See the "How to program for
118IOThreads" above for information on how to do that.
119
120If main loop code such as a QMP function wishes to access a BlockDriverState
121it must first call aio_context_acquire(bdrv_get_aio_context(bs)) to ensure
122that callbacks in the IOThread do not run in parallel.
123
124Code running in the monitor typically needs to ensure that past
125requests from the guest are completed.  When a block device is running
126in an IOThread, the IOThread can also process requests from the guest
127(via ioeventfd).  To achieve both objects, wrap the code between
128bdrv_drained_begin() and bdrv_drained_end(), thus creating a "drained
129section".  The functions must be called between aio_context_acquire()
130and aio_context_release().  You can freely release and re-acquire the
131AioContext within a drained section.
132
133Long-running jobs (usually in the form of coroutines) are best scheduled in
134the BlockDriverState's AioContext to avoid the need to acquire/release around
135each bdrv_*() call.  The functions bdrv_add/remove_aio_context_notifier,
136or alternatively blk_add/remove_aio_context_notifier if you use BlockBackends,
137can be used to get a notification whenever bdrv_try_set_aio_context() moves a
138BlockDriverState to a different AioContext.
139