Re: [PATCH 1/2] system/main: transfer replay mutex ownership from main thread to main loop thread

[Pierrick Bouvier]

On 4/11/25 22:30, Nicholas Piggin wrote:
On Fri Apr 11, 2025 at 8:55 AM AEST, Pierrick Bouvier wrote:
On MacOS, UI event loop has to be ran in the main thread of a process.
Because of that restriction, on this platform, qemu main event loop is
ran on another thread [1].
This breaks record/replay feature, which expects thread running qemu_init
to initialize hold this lock, breaking associated functional tests on
MacOS.

Thus, as a generalization, and similar to how BQL is handled, we release
it after init, and reacquire the lock before entering main event loop,
avoiding a special case if a separate thread is used.

Tested on MacOS with:
$ meson test -C build --setup thorough --print-errorlogs \
func-x86_64-x86_64_replay func-arm-arm_replay func-aarch64-aarch64_replay
$ ./build/qemu-system-x86_64 -nographic -icount 
shift=auto,rr=record,rrfile=replay.log
$ ./build/qemu-system-x86_64 -nographic -icount 
shift=auto,rr=replay,rrfile=replay.log

[1] 
https://gitlab.com/qemu-project/qemu/-/commit/f5ab12caba4f1656479c1feb5248beac1c833243

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/2907
Signed-off-by: Pierrick Bouvier <pierrick.bouv...@linaro.org>
---
  system/main.c | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/system/main.c b/system/main.c
index ecb12fd397c..1c022067349 100644
--- a/system/main.c
+++ b/system/main.c
@@ -25,6 +25,7 @@
  #include "qemu/osdep.h"
  #include "qemu-main.h"
  #include "qemu/main-loop.h"
+#include "system/replay.h"
  #include "system/system.h"
  
  #ifdef CONFIG_SDL
@@ -44,10 +45,12 @@ static void *qemu_default_main(void *opaque)
  {
      int status;
  
+    replay_mutex_lock();
      bql_lock();
      status = qemu_main_loop();
      qemu_cleanup(status);
      bql_unlock();
+    replay_mutex_unlock();
  
      exit(status);
  }
@@ -67,6 +70,7 @@ int main(int argc, char **argv)
  {
      qemu_init(argc, argv);
      bql_unlock();
+    replay_mutex_unlock();
      if (qemu_main) {
          QemuThread main_loop_thread;
          qemu_thread_create(&main_loop_thread, "qemu_main",

Do we actually need to hold replay mutex (or even bql) over qemu_init()?
Both should get dropped before we return here. But as a simple fix, I
guess this is okay.


For the bql, I don't know the exact reason.
For replay lock, we need to hold it as clock gets saved as soon as the 
devices are initialized, which happens before end of qemu_init.

Reviewed-by: Nicholas Piggin <npig...@gmail.com>