[Qemu-devel] [PATCH v3 0/4] Coroutines for better asynchronous programming

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

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 first patch.  The second 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.

Coroutines are also being used for virtfs (virtio-9p) so I have submitted this
patch now because virtfs patches that depend on coroutines are being published.

Other areas of QEMU that could take advantage of coroutines include the VNC
server, migration, and qemu-tools.

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