macOS's Cocoa event handling must be done on the initial (main) thread
of the process. Furthermore, if library or application code uses
libdispatch, the main dispatch queue must be handling events on the main
thread as well.
So far, this has affected Qemu in both the Cocoa and SDL UIs, although
in different ways: the Cocoa UI replaces the default qemu_main function
with one that spins Qemu's internal main event loop off onto a
background thread. SDL (which uses Cocoa internally) on the other hand
uses a polling approach within Qemu's main event loop. Events are
polled during the SDL UI's dpy_refresh callback, which happens to run
on the main thread by default.
As UIs are mutually exclusive, this works OK as long as nothing else
needs platform-native event handling. In the next patch, a new device is
introduced based on the ParavirtualizedGraphics.framework in macOS.
This uses libdispatch internally, and only works when events are being
handled on the main runloop. With the current system, it works when
using either the Cocoa or the SDL UI. However, it does not when running
headless. Moreover, any attempt to install a similar scheme to the
Cocoa UI's main thread replacement fails when combined with the SDL
UI.
This change tidies up main thread management to be more flexible.
* The qemu_main global function pointer is a custom function for the
main thread, and it may now be NULL. When it is, the main thread
runs the main Qemu loop. This represents the traditional setup.
* When non-null, spawning the main Qemu event loop on a separate
thread is now done centrally rather than inside the Cocoa UI code.
* For most platforms, qemu_main is indeed NULL by default, but on
Darwin, it defaults to a function that runs the CFRunLoop.
* The Cocoa UI sets qemu_main to a function which runs the
NSApplication event handling runloop, as is usual for a Cocoa app.
* The SDL UI overrides the qemu_main function to NULL, thus
specifying that Qemu's main loop must run on the main
thread.
* The GTK UI also overrides the qemu_main function to NULL.
* For other UIs, or in the absence of UIs, the platform's default
behaviour is followed.
This means that on macOS, the platform's runloop events are always
handled, regardless of chosen UI. The new PV graphics device will
thus work in all configurations. There is no functional change on other
operating systems.
Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>