Manage ticks to suppress RCU CPU stall warning

Makefile

@@ -132,6 +132,16 @@ DTC ?= dtc
 E :=
 S := $E $E
 
+# During boot process, he emulator manually manages the growth of ticks to 
+# suppress RCU CPU stall warnings. Thus, we need an target time to set the 
+# increment of ticks. According to Using RCU’s CPU Stall Detector[1], the
+# grace period for RCU CPU stalls is typically set to 21 seconds. 
+# By dividing this value by two as the expected completion time, we can 
+# provide a sufficient buffer to reduce the impact of errors and avoid 
+# RCU CPU stall warnings.
+# [1] docs.kernel.org/RCU/stallwarn.html#config-rcu-cpu-stall-timeout
+CFLAGS += -D SEMU_BOOT_TARGET_TIME=10
+
 SMP ?= 1
 .PHONY: riscv-harts.dtsi
 riscv-harts.dtsi:

main.c

@@ -678,7 +678,7 @@ static int semu_init(emu_state_t *emu, int argc, char **argv)
     virtio_rng_init();
 #endif
     /* Set up ACLINT */
-    semu_timer_init(&emu->mtimer.mtime, CLOCK_FREQ);
+    semu_timer_init(&emu->mtimer.mtime, CLOCK_FREQ, hart_count);
     emu->mtimer.mtimecmp = calloc(vm->n_hart, sizeof(uint64_t));
     emu->mswi.msip = calloc(vm->n_hart, sizeof(uint32_t));
     emu->sswi.ssip = calloc(vm->n_hart, sizeof(uint32_t));

riscv.c

@@ -382,6 +382,14 @@ static void op_sret(hart_t *vm)
     vm->s_mode = vm->sstatus_spp;
     vm->sstatus_sie = vm->sstatus_spie;
 
+    /* After the booting process is complete, initrd will be loaded. At this
+     * point, the sytstem will switch to U mode for the first time. Therefore,
+     * by checking whether the switch to U mode has already occurred, we can
+     * determine if the boot process has been completed.
+     */
+    if (!boot_complete && !vm->s_mode)
+        boot_complete = true;
+
     /* Reset stack */
     vm->sstatus_spp = false;
     vm->sstatus_spie = true;

utils.c

@@ -1,3 +1,4 @@
+#include <stdbool.h>
 #include <time.h>
 
 #include "utils.h"
@@ -19,6 +20,10 @@
 #endif
 #endif
 
+bool boot_complete = false;
+static double ticks_increment;
+static double boot_ticks;
+
 /* Calculate "x * n / d" without unnecessary overflow or loss of precision.
  *
  * Reference:
@@ -32,35 +37,93 @@ static inline uint64_t mult_frac(uint64_t x, uint64_t n, uint64_t d)
     return q * n + r * n / d;
 }
 
-void semu_timer_init(semu_timer_t *timer, uint64_t freq)
-{
-    timer->freq = freq;
-    semu_timer_rebase(timer, 0);
-}
-
-static uint64_t semu_timer_clocksource(uint64_t freq)
+/* High-precision time measurement:
+ * - POSIX systems: clock_gettime() for nanosecond precision
+ * - macOS: mach_absolute_time() with timebase conversion
+ * - Other platforms: time(0) with conversion to nanoseconds as fallback
+ *
+ * The platform-specific timing logic is now clearly separated: POSIX and macOS
+ * implementations provide high-precision measurements, while the fallback path
+ * uses time(0) for a coarser but portable approach.
+ */
+static inline uint64_t host_time_ns()
 {
 #if defined(HAVE_POSIX_TIMER)
-    struct timespec t;
-    clock_gettime(CLOCKID, &t);
-    return t.tv_sec * freq + mult_frac(t.tv_nsec, freq, 1e9);
+    struct timespec ts;
+    clock_gettime(CLOCKID, &ts);
+    return (uint64_t) ts.tv_sec * 1e9 + (uint64_t) ts.tv_nsec;
+
 #elif defined(HAVE_MACH_TIMER)
-    static mach_timebase_info_data_t t;
-    if (t.denom == 0)
-        (void) mach_timebase_info(&t);
-    return mult_frac(mult_frac(mach_absolute_time(), t.numer, t.denom), freq,
-                     1e9);
+    static mach_timebase_info_data_t ts = {0};
+    if (ts.denom == 0)
+        (void) mach_timebase_info(&ts);
+
+    uint64_t now = mach_absolute_time();
+    /* convert to nanoseconds: (now * t.numer / t.denom) */
+    return mult_frac(now, ts.numer, (uint64_t) ts.denom);
+
 #else
-    return time(0) * freq;
+    /* Fallback to non-HRT calls time(0) in seconds => convert to ns. */
+    time_t now_sec = time(0);
+    return (uint64_t) now_sec * 1e9;
 #endif
 }
 
+/* The function that returns the "emulator time" in ticks.
+ *
+ * Before the boot process is completed, the emulator manually manages the
+ * growth of ticks to suppress RCU CPU stall warnings. After the boot process is
+ * completed, the emulator switches back to the real-time timer, using an offset
+ * bridging to ensure that the ticks of both timers remain consistent.
+ */
+static uint64_t semu_timer_clocksource(semu_timer_t *timer)
+{
+    /* After boot process complete, the timer will switch to real time. Thus,
+     * there is an offset between the real time and the emulator time.
+     *
+     * After switching to real time, the correct way to update time is to
+     * calculate the increment of time. Then add it to the emulator time.
+     */
+    static int64_t offset = 0;
+    static bool first_switch = true;
+
+    if (!boot_complete) {
+        boot_ticks += ticks_increment;
+        return (uint64_t) boot_ticks;
+    }
+
+    uint64_t real_ticks = mult_frac(host_time_ns(), timer->freq, 1e9);
+    if (first_switch) {
+        first_switch = false;
+
+        /* Calculate the offset between the real time and the emulator time */
+        offset = (int64_t) (real_ticks - boot_ticks);
+    }
+    return (uint64_t) ((int64_t) real_ticks - offset);
+}
+
+void semu_timer_init(semu_timer_t *timer, uint64_t freq, int n_harts)
+{
+    timer->freq = freq;
+    timer->begin = mult_frac(host_time_ns(), timer->freq, 1e9);
+    boot_ticks = timer->begin; /* Initialize the fake ticks for boot process */
+
+    /* According to statistics, the number of times 'semu_timer_clocksource'
+     * called is approximately 'SMP count * 2.15 * 1e8'. By the time the boot
+     * process is completed, the emulator will have a total of 'boot seconds *
+     * frequency' ticks. Therefore, each time, '(boot seconds * frequency) /
+     * (2.15 * 1e8 * SMP count)' ticks need to be added.
+     */
+    ticks_increment =
+        (SEMU_BOOT_TARGET_TIME * CLOCK_FREQ) / (2.15 * 1e8 * n_harts);
+}
+
 uint64_t semu_timer_get(semu_timer_t *timer)
 {
-    return semu_timer_clocksource(timer->freq) - timer->begin;
+    return semu_timer_clocksource(timer) - timer->begin;
 }
 
 void semu_timer_rebase(semu_timer_t *timer, uint64_t time)
 {
-    timer->begin = semu_timer_clocksource(timer->freq) - time;
+    timer->begin = semu_timer_clocksource(timer) - time;
 }

utils.h

@@ -3,13 +3,23 @@
 #include <stddef.h>
 #include <stdint.h>
 
+/* To suppress RCU CPU stall warnings, the emulator provides a fake timer to
+ * the Guest OS during the boot process. After the boot process is complete, the
+ * emulator will switch to real-time timer.
+ *
+ * Since the Guest OS transitions to U mode for the first time when it loads the
+ * initial user-mode process, we use this transition to determine whether the
+ * boot process has completed.
+ */
+extern bool boot_complete;
+
 /* TIMER */
 typedef struct {
     uint64_t begin;
     uint64_t freq;
 } semu_timer_t;
 
-void semu_timer_init(semu_timer_t *timer, uint64_t freq);
+void semu_timer_init(semu_timer_t *timer, uint64_t freq, int n_harts);
 uint64_t semu_timer_get(semu_timer_t *timer);
 void semu_timer_rebase(semu_timer_t *timer, uint64_t time);