ui: Mimic QEMU thread and set thread-local tcg_ctx to tcg_init_ctx

Necessary to handle callbacks (e.g. rom_reset) from UI thread which will
flush TBs and reference thread-local TCG context, which is initialized
for main QEMU thread early on as tcg_init_ctx.

include/tcg/tcg.h

@@ -851,6 +851,10 @@ static inline void *tcg_malloc(int size)
     }
 }
 
+#ifdef XBOX
+void tcg_register_init_ctx(void);
+#endif
+
 void tcg_context_init(TCGContext *s);
 void tcg_register_thread(void);
 void tcg_prologue_init(TCGContext *s);

tcg/tcg.c

@@ -775,6 +775,21 @@ void tcg_register_thread(void)
     tcg_ctx = &tcg_init_ctx;
 }
 #else
+
+#ifdef XBOX
+void tcg_register_init_ctx(void)
+{
+    /*
+     * For xemu we may exercise functions on the UI thread that would otherwise
+     * run on the main thread following initialization. We retain the BQL when
+     * running such commands, which should make this safe, but there are some
+     * data stored in TLS which get initialized early on and may be required
+     * later.
+     */
+    tcg_ctx = &tcg_init_ctx;
+}
+#endif
+
 void tcg_register_thread(void)
 {
     MachineState *ms = MACHINE(qdev_get_machine());

ui/xemu.c

@@ -31,6 +31,7 @@
 #include "qemu/module.h"
 #include "qemu/thread.h"
 #include "qemu/main-loop.h"
+#include "qemu/rcu.h"
 #include "qemu-version.h"
 #include "qemu-common.h"
 #include "qapi/error.h"
@@ -49,6 +50,8 @@
 #include "hw/xbox/smbus.h" // For eject, drive tray
 #include "hw/xbox/nv2a/nv2a.h"
 
+void tcg_register_init_ctx(void); // tcg.c
+
 // #define DEBUG_XEMU_C
 
 #ifdef DEBUG_XEMU_C
@@ -1451,11 +1454,20 @@ int main(int argc, char **argv)
     DPRINTF("Main thread: waiting for display_init_sem\n");
     qemu_sem_wait(&display_init_sem);
 
+    /*
+     * FIXME: May want to create a callback mechanism for main QEMU thread
+     * to just run functions to avoid TLS bugs and locking issues.
+     */
+    tcg_register_init_ctx();
+    rcu_register_thread();
+
     DPRINTF("Main thread: initializing app\n");
 
     while (1) {
         sdl2_gl_refresh(&sdl2_console[0].dcl);
     }
+
+    rcu_unregister_thread();
 }
 
 void xemu_eject_disc(void)