Trap guestOS to run SDL-oriented applications

To trap the guest OS SDL applications, virtual memory translation should
be handled when accessing the bidirectional event queues in
syscall_sdl.c. Additionally, when using the guest OS, the SDL
application might be launched and terminated multiple times. To enhance
the user experience, it is heuristic to properly handle three main types
of exits:
1, SDL application built-in exit: The source code of the SDL application
   should be modified to call the syscall_exit(syscall number: 93)
   somewhere when executing cleanup routine before exit().
2. Ctrl-c (SIGINT) exit: Detect the ctrl-c keycode.
3. SDL window Quit event: The source code of the SDL application
   should be modified to call the syscall_exit(syscall number: 93) when
   receiving SDL window Quit event.

The main reason for handling these three types of exits is that
SDL2_Mixer may malfunction if not properly initialized and destroyed, as
it runs on the host side not on the guest side. Additionally, when
terminating an SDL application in the guest OS, the previous SDL window
should be closed.

Moreover, the default audio backend of SDL2_Mixer(downloadable from brew
or Linux distro pkg managers), FluidSynth, occasionally generates the
warning: "fluidsynth: warning: Ringbuffer full, try
increasing synth.polyphony!" This issue causes the audio to hang,
leading to an unstable playback experience. Thus, dropping FluidSynth in
favor of the Timidity backend, which provides a stable audio playback
experience.

Known issue: Calling SDL_DestroyWindow() on macOS does not close the
previous SDL window.

Close: #510

Author: ChinYikMing

src/devices/uart.c

@@ -12,7 +12,6 @@
 #include <unistd.h>
 
 #include "uart.h"
-
 /* Emulate 8250 (plain, without loopback mode support) */
 
 #define U8250_INTR_THRE 1
@@ -66,10 +65,22 @@ static uint8_t u8250_handle_in(u8250_state_t *uart)
     if (value == 1) {           /* start of heading (Ctrl-a) */
         if (getchar() == 120) { /* keyboard x */
             printf("\n");       /* end emulator with newline */
-            exit(0);
+            exit(EXIT_SUCCESS);
         }
     }
 
+#if RV32_HAS(SDL) && RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
+    /*
+     * The guestOS may repeatedly open and close the SDL window,
+     * and the user could close the application by pressing the ctrl-c key.
+     * Need to trap the ctrl-c key and ensure the SDL window and
+     * SDL mixer are destroyed properly.
+     */
+    extern void sdl_video_audio_cleanup();
+    if (value == 3) /* ctrl-c */
+        sdl_video_audio_cleanup();
+#endif
+
     return value;
 }
 

src/emulate.c

@@ -1299,7 +1299,22 @@ void ecall_handler(riscv_t *rv)
     syscall_handler(rv);
 #elif RV32_HAS(SYSTEM)
     if (rv->priv_mode == RV_PRIV_U_MODE) {
-        SET_CAUSE_AND_TVAL_THEN_TRAP(rv, ECALL_U, 0);
+        switch (rv_get_reg(
+            rv,
+            rv_reg_a7)) { /* trap guestOS's SDL-oriented application syscall */
+        case 0xBEEF:
+        case 0xC0DE:
+        case 0xFEED:
+        case 0xBABE:
+        case 0xD00D:
+        case 93:
+            syscall_handler(rv);
+            rv->PC += 4;
+            break;
+        default:
+            SET_CAUSE_AND_TVAL_THEN_TRAP(rv, ECALL_U, 0);
+            break;
+        }
     } else if (rv->priv_mode ==
                RV_PRIV_S_MODE) { /* trap to SBI syscall handler */
         rv->PC += 4;

src/syscall.c

@@ -132,9 +132,20 @@ static void syscall_write(riscv_t *rv)
     rv_set_reg(rv, rv_reg_a0, -1);
 }
 
-
 static void syscall_exit(riscv_t *rv)
 {
+#if RV32_HAS(SDL) && RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
+    /*
+     * The guestOS may repeatedly open and close the SDL window,
+     * and the user could close the application using the application’s
+     * built-in exit function. Need to trap the built-in exit and
+     * ensure the SDL window and SDL mixer are destroyed properly.
+     */
+    extern void sdl_video_audio_cleanup();
+    sdl_video_audio_cleanup();
+    return;
+#endif
+
     /* simply halt cpu and save exit code.
      * the application decides the usage of exit code
      */