Re: [PATCH v6 01/15] ui & main loop: Redesign of system-specific main thread event handling

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2024/11/04 0:00, Phil Dennis-Jordan wrote:
> 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.
>   * 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>