On Wed, 13 Nov 2024, 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.
> * 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>
> ---
>
> v5:
>
> * Simplified the way of setting/clearing the main loop by going back
> to setting qemu_main directly, but narrowing the scope of what it
> needs to do, and it can now be NULL.
>
> v6:
>
> * Folded function qemu_run_default_main_on_new_thread's code into
> main()
> * Removed whitespace changes left over on lines near code removed
> between v4 and v5
>
> v9:
>
> * Set qemu_main to NULL for GTK UI as well.
>
> v10:
>
> * Added comments clarifying the functionality and purpose of qemu_main.
>
> include/qemu-main.h | 21 ++++++++++++++--
> include/qemu/typedefs.h | 1 +
> system/main.c | 50 ++++++++++++++++++++++++++++++++++----
> ui/cocoa.m | 54 ++++++++++-------------------------------
> ui/gtk.c | 8 ++++++
> ui/sdl2.c | 4 +++
> 6 files changed, 90 insertions(+), 48 deletions(-)
>
> diff --git a/include/qemu-main.h b/include/qemu-main.h
> index 940960a7dbc..fc70681c8b5 100644
> --- a/include/qemu-main.h
> +++ b/include/qemu-main.h
> @@ -5,7 +5,24 @@
> #ifndef QEMU_MAIN_H
> #define QEMU_MAIN_H
>
> -int qemu_default_main(void);
> -extern int (*qemu_main)(void);
> +/*
> + * The function to run on the main (initial) thread of the process.
> + * NULL means QEMU's main event loop.
> + * When non-NULL, QEMU's main event loop will run on a purposely created
> + * thread, after which the provided function pointer will be invoked on
> + * the initial thread.
> + * This is useful on platforms which treat the main thread as special
> + * (macOS/Darwin) and/or require all UI API calls to occur from a
> + * specific thread.
> + * Implementing this via a global function pointer variable is a bit
> + * ugly, but it's probably worth investigating the existing UI thread rule
> + * violations in the SDL (e.g. #2537) and GTK+ back-ends. Fixing those
> + * issues might precipitate requirements similar but not identical to those
> + * of the Cocoa UI; hopefully we'll see some kind of pattern emerge, which
> + * can then be used as a basis for an overhaul. (In fact, it may turn
> + * out to be simplest to split the UI/native platform event thread from the
> + * QEMU main event loop on all platforms, with any UI or even none at all.)
> + */
> +extern qemu_main_fn qemu_main;
qemu_main_fn is defined in typdefefs.h but not included here. Also coding
style says typedefs should be CamelCase but maybe it's just not worth to
define a type for this and you can just leave the existing line here only
removing the qemu_default_main declaration and adding the comment.
> #endif /* QEMU_MAIN_H */
> diff --git a/include/qemu/typedefs.h b/include/qemu/typedefs.h
> index 3d84efcac47..b02cfe1f328 100644
> --- a/include/qemu/typedefs.h
> +++ b/include/qemu/typedefs.h
> @@ -131,5 +131,6 @@ typedef struct IRQState *qemu_irq;
> * Function types
> */
> typedef void (*qemu_irq_handler)(void *opaque, int n, int level);
> +typedef int (*qemu_main_fn)(void);
Then drop this change...
> #endif /* QEMU_TYPEDEFS_H */
> diff --git a/system/main.c b/system/main.c
> index 9b91d21ea8c..d9397a6d5d0 100644
> --- a/system/main.c
> +++ b/system/main.c
> @@ -24,13 +24,14 @@
>
> #include "qemu/osdep.h"
> #include "qemu-main.h"
> +#include "qemu/main-loop.h"
> #include "sysemu/sysemu.h"
>
> -#ifdef CONFIG_SDL
> -#include <SDL.h>
> +#ifdef CONFIG_DARWIN
> +#include <CoreFoundation/CoreFoundation.h>
> #endif
>
> -int qemu_default_main(void)
> +static int qemu_default_main(void)
> {
> int status;
>
> @@ -40,10 +41,49 @@ int qemu_default_main(void)
> return status;
> }
>
> -int (*qemu_main)(void) = qemu_default_main;
...and leave this line here without init to define the global variable and
only put assigning the init value in the #ifdef if you don't want to
repeat the function pointer definition twice (but that wouldn't be a
problem either in my opinion to do it in the #ifdef this way).
> +/*
> + * Various macOS system libraries, including the Cocoa UI and anything using
> + * libdispatch, such as ParavirtualizedGraphics.framework, requires that the
> + * main runloop, on the main (initial) thread be running or at least regularly
> + * polled for events. A special mode is therefore supported, where the QEMU
> + * main loop runs on a separate thread and the main thread handles the
> + * CF/Cocoa runloop.
> + */
> +
> +static void *call_qemu_default_main(void *opaque)
> +{
> + int status;
> +
> + bql_lock();
> + status = qemu_default_main();
> + bql_unlock();
> +
> + exit(status);
> +}
> +
> +#ifdef CONFIG_DARWIN
> +static int os_darwin_cfrunloop_main(void)
> +{
> + CFRunLoopRun();
> + abort();
Is it better to use g_assert_not_reached() here?
> +}
> +
> +qemu_main_fn qemu_main = os_darwin_cfrunloop_main;
> +#else
> +qemu_main_fn qemu_main;
> +#endif
>
> int main(int argc, char **argv)
> {
> + QemuThread main_loop_thread;
> +
> qemu_init(argc, argv);
> - return qemu_main();
> + if (qemu_main) {
> + qemu_thread_create(&main_loop_thread, "qemu_main",
> + call_qemu_default_main, NULL, QEMU_THREAD_DETACHED);
> + bql_unlock();
> + return qemu_main();
> + } else {
> + qemu_default_main();
I think you need 'return qemu_default_main()' here but I'm a bit confused
by all this wrapping of qemu_default_main in call_qemu_default_main. I see
that may be needed because qemu_thread_create takes a different function
but now that qemu_default main is static and not replaced externally could
that be changed to the right type and avoid this confusion and simplify
this a bit?
Regards,
BALATON Zoltan
Alright, I've gone ahead and worked through that, and it does indeed make things simpler. This is what I have for include/qemu-main.h and system/main.c now:
diff --git a/include/qemu-main.h b/include/qemu-main.h
index 940960a7dbc..e1041fe99b4 100644
--- a/include/qemu-main.h
+++ b/include/qemu-main.h
@@ -5,7 +5,29 @@
#ifndef QEMU_MAIN_H
#define QEMU_MAIN_H
-int qemu_default_main(void);
+/*
+ * The function to run on the main (initial) thread of the process.
+ * NULL means QEMU's main event loop.
+ * When non-NULL, QEMU's main event loop will run on a purposely created
+ * thread, after which the provided function pointer will be invoked on
+ * the initial thread.
+ * This is useful on platforms which treat the main thread as special
+ * (macOS/Darwin) and/or require all UI API calls to occur from a
+ * specific thread.
+ * In practice, this means that on macOS, it is initialised to a function
+ * that runs the Core Foundation runloop. The Cocoa UI "upgrades" this
+ * to the NSApplication-specific event processing variant. Other platforms
+ * currently leave it at NULL; the SDL and GTK UIs reset it to NULL even
+ * on macOS as they directly poll the runloop.
+ * Implementing this via a global function pointer variable is a bit
+ * ugly, but it's probably worth investigating the existing UI thread rule
+ * violations in the SDL (e.g. #2537) and GTK+ back-ends. Fixing those
+ * issues might precipitate requirements similar but not identical to those
+ * of the Cocoa UI; hopefully we'll see some kind of pattern emerge, which
+ * can then be used as a basis for an overhaul. (In fact, it may turn
+ * out to be simplest to split the UI/native platform event thread from the
+ * QEMU main event loop on all platforms, with any UI or even none at all.)
+ */
extern int (*qemu_main)(void);
#endif /* QEMU_MAIN_H */
diff --git a/system/main.c b/system/main.c
index 9b91d21ea8c..2d9d144ed46 100644
--- a/system/main.c
+++ b/system/main.c
@@ -24,26 +24,48 @@
#include "qemu/osdep.h"
#include "qemu-main.h"
+#include "qemu/main-loop.h"
#include "sysemu/sysemu.h"
-#ifdef CONFIG_SDL
-#include <SDL.h>
+#ifdef CONFIG_DARWIN
+#include <CoreFoundation/CoreFoundation.h>
#endif
-int qemu_default_main(void)
+static void *qemu_default_main(void *opaque)
{
int status;
+ bql_lock();
status = qemu_main_loop();
qemu_cleanup(status);
+ bql_unlock();
- return status;
+ exit(status);
}
-int (*qemu_main)(void) = qemu_default_main;
+#ifdef CONFIG_DARWIN
+static int os_darwin_cfrunloop_main(void)
+{
+ CFRunLoopRun();
+ g_assert_not_reached();
+}
+
+int (*qemu_main)(void) = os_darwin_cfrunloop_main;
+#else
+int (*qemu_main)(void);
+#endif
int main(int argc, char **argv)
{
+ QemuThread main_loop_thread;
+
qemu_init(argc, argv);
- return qemu_main();
+ bql_unlock();
+ if (qemu_main) {
+ qemu_thread_create(&main_loop_thread, "qemu_main",
+ qemu_default_main, NULL, QEMU_THREAD_DETACHED);
+ return qemu_main();
+ } else {
+ qemu_default_main(NULL);
+ }
}