feat: Allow icount mode import checkpoints created w/o icount

accel/tcg/icount-common.c

@@ -125,7 +125,7 @@ static int64_t icount_get_locked(void)
 {
     int64_t icount = icount_get_raw_locked();
     return qatomic_read_i64(&timers_state.qemu_icount_bias) +
-        icount_to_ns(icount);
+        icount_to_ns(icount) + timers_state.virtual_clock_snapshot;
 }
 
 int64_t icount_get_raw(void)

include/sysemu/cpu-timers-internal.h

@@ -52,6 +52,9 @@ typedef struct TimersState {
     int64_t vm_clock_warp_start;
     int64_t cpu_clock_offset;
 
+    /* Add by Cyan. the vm_clock from the recent snapshot */
+    int64_t virtual_clock_snapshot;
+
     /* Only written by TCG thread */
     int64_t qemu_icount;
 

system/cpu-timers.c

@@ -109,6 +109,14 @@ void cpu_enable_ticks(void)
     seqlock_write_lock(&timers_state.vm_clock_seqlock,
                        &timers_state.vm_clock_lock);
     if (!timers_state.cpu_ticks_enabled) {
+        timers_state.virtual_clock_snapshot = timers_state.cpu_clock_offset;
+
+        if (icount_enabled()) {
+            // well, we have adjust the virtual_clock_snapshot time, so we should clean the current icount.
+            // by assigning offset, we cancel the value of the current icount. 
+            timers_state.qemu_icount_bias = - icount_to_ns(timers_state.qemu_icount);
+        }
+
         timers_state.cpu_ticks_offset -= cpu_get_host_ticks();
         timers_state.cpu_clock_offset -= get_clock();
         timers_state.cpu_ticks_enabled = 1;
@@ -128,7 +136,16 @@ void cpu_disable_ticks(void)
                        &timers_state.vm_clock_lock);
     if (timers_state.cpu_ticks_enabled) {
         timers_state.cpu_ticks_offset += cpu_get_host_ticks();
-        timers_state.cpu_clock_offset = cpu_get_clock_locked();
+
+        if (icount_enabled()) {
+            // record the current time calculated with icount.
+            int64_t current_system_time = icount_to_ns(timers_state.qemu_icount) + timers_state.qemu_icount_bias + timers_state.virtual_clock_snapshot;
+            timers_state.cpu_clock_offset = current_system_time; // record it to the snapshot.
+        } else {
+            // record the CPU time.
+            timers_state.cpu_clock_offset = cpu_get_clock_locked();
+        }
+
         timers_state.cpu_ticks_enabled = 0;
     }
     seqlock_write_unlock(&timers_state.vm_clock_seqlock,
@@ -137,7 +154,8 @@ void cpu_disable_ticks(void)
 
 static bool icount_state_needed(void *opaque)
 {
-    return icount_enabled();
+    // return icount_enabled();
+    return false;
 }
 
 static bool warp_timer_state_needed(void *opaque)

target/arm/helper.c

@@ -934,7 +934,8 @@ static int64_t cycles_ns_per(uint64_t cycles)
 
 static bool instructions_supported(CPUARMState *env)
 {
-    return icount_enabled() == 1; /* Precise instruction counting */
+    // return icount_enabled() == 1; /* Precise instruction counting */
+    return false;
 }
 
 static uint64_t instructions_get_count(CPUARMState *env)