arch/sim: add simulated time ratio support

Add CONFIG_SIM_WALLTIME_RATIO Kconfig option and --sim-rt-ratio=
command-line argument to control the ratio of simulated time to
real time in percent. 100 means real-time (default), 200 means
simulated time advances twice as fast, 50 means half speed.

The implementation applies the ratio in host_gettime(), host_sleepuntil()
and host_settimer() so both SIM_WALLTIME_SLEEP and SIM_WALLTIME_SIGNAL
modes are supported.

This is inspired by the --rt-ratio feature in Zephyr's native_sim board.

Tested on sim:nsh with the following sleep test:

  $ echo -e "sleep 2\nexit" | time ./nuttx
  real    0m2.0xxs

  $ echo -e "sleep 2\nexit" | time ./nuttx --sim-rt-ratio=200
  real    0m1.0xxs

  $ echo -e "sleep 2\nexit" | time ./nuttx --sim-rt-ratio=50
  real    0m4.0xxs

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>

arch/sim/Kconfig

@@ -170,6 +170,16 @@ config SIM_WALLTIME_SIGNAL
 
 endchoice
 
+config SIM_WALLTIME_RATIO
+	int "Simulated time ratio in percent relative to real time"
+	default 100
+	range 1 100000
+	---help---
+		Set the ratio of simulated time to real time in percent.
+		100 means real-time (default).  200 means simulated time advances
+		twice as fast as real time.  50 means half speed.
+		This can also be overridden at runtime with --sim-rt-ratio=<percent>.
+
 config SIM_LOOP_INTERVAL
 	int "loop interval in ms"
 	default 10

arch/sim/src/sim/posix/sim_hosttime.c

@@ -41,6 +41,13 @@
  ****************************************************************************/
 
 static uint64_t g_start;
+
+/* Ratio of simulated time to real time in percent.  100 means real-time
+ * (default).  Values > 100 speed up simulated time; values < 100 slow it
+ * down.  Overridable at runtime via --sim-rt-ratio=<percent>.
+ */
+
+static int g_time_ratio = CONFIG_SIM_WALLTIME_RATIO;
 #ifdef __APPLE__
 static dispatch_source_t g_timer;
 
@@ -105,7 +112,14 @@ uint64_t host_gettime(bool rtc)
   clock_gettime(rtc ? CLOCK_REALTIME : CLOCK_MONOTONIC, &tp);
   current = 1000000000ull * tp.tv_sec + tp.tv_nsec;
 
-  return rtc ? current : current - g_start;
+  if (rtc)
+    {
+      return current;
+    }
+
+  /* Apply time ratio: simulated_time = real_elapsed * ratio / 100 */
+
+  return ((current - g_start) * g_time_ratio) / 100;
 }
 
 /****************************************************************************
@@ -128,7 +142,31 @@ void host_sleepuntil(uint64_t nsec)
   now = host_gettime(false);
   if (nsec > now + 1000)
     {
-      usleep((nsec - now) / 1000);
+      /* nsec is in simulated time; convert back to real duration to sleep */
+
+      usleep((((nsec - now) * 100) / g_time_ratio) / 1000);
+    }
+}
+
+/****************************************************************************
+ * Name: host_set_timeratio
+ *
+ * Description:
+ *   Set the ratio of simulated time to real time in percent.  100 (default)
+ *   means simulated time advances at the same rate as real time.  Values
+ *   greater than 100 speed up simulated time; values less than 100 slow it
+ *   down.
+ *
+ * Input Parameters:
+ *   ratio - The new time ratio in percent (must be > 0)
+ *
+ ****************************************************************************/
+
+void host_set_timeratio(int ratio)
+{
+  if (ratio > 0)
+    {
+      g_time_ratio = ratio;
     }
 }
 
@@ -148,9 +186,9 @@ void host_sleepuntil(uint64_t nsec)
 
 int host_settimer(uint64_t nsec)
 {
-  /* Convert to microseconds and set minimum timer to 1 microsecond. */
+  /* nsec is in simulated time; convert back to real absolute time. */
 
-  nsec += g_start;
+  nsec = ((nsec * 100) / g_time_ratio) + g_start;
 
 #ifdef __APPLE__
   dispatch_time_t start;

arch/sim/src/sim/sim_head.c

@@ -31,6 +31,7 @@
 #include <setjmp.h>
 #include <syslog.h>
 #include <assert.h>
+#include <string.h>
 
 #include <nuttx/init.h>
 #include <nuttx/arch.h>
@@ -164,9 +165,24 @@ noprofile_function const char *__ubsan_default_options(void)
 
 int main(int argc, char **argv, char **envp)
 {
+  int i;
+
   g_argc = argc;
   g_argv = argv;
 
+  /* Parse simulator-specific options before handing control to NuttX.
+   * --sim-rt-ratio=<percent>  Set simulated-to-real time ratio in percent
+   *   (default 100).  Values > 100 speed up simulated time; < 100 slow down.
+   */
+
+  for (i = 1; i < argc; i++)
+    {
+      if (strncmp(argv[i], "--sim-rt-ratio=", 15) == 0)
+        {
+          host_set_timeratio(atoi(argv[i] + 15));
+        }
+    }
+
 #ifdef CONFIG_ALLSYMS
   allsyms_relocate();
 #endif

arch/sim/src/sim/sim_internal.h

@@ -245,6 +245,7 @@ int host_inittimer(void);
 uint64_t host_gettime(bool rtc);
 void host_sleep(uint64_t nsec);
 void host_sleepuntil(uint64_t nsec);
+void host_set_timeratio(int ratio);
 int host_timerirq(void);
 int host_settimer(uint64_t nsec);