diff -r 246ecf354c85 xen/arch/x86/acpi/cpu_idle.c
--- a/xen/arch/x86/acpi/cpu_idle.c	Mon Feb 16 12:21:52 2009 +0800
+++ b/xen/arch/x86/acpi/cpu_idle.c	Mon Feb 16 12:57:08 2009 +0800
@@ -737,6 +737,15 @@ long set_cx_pminfo(uint32_t cpu, struct 
 
     if ( cpu_id == 0 && pm_idle_save == NULL )
     {
+        int deepest_cx = acpi_power->states[acpi_power->count - 1].type;
+        if ( max_cstate >= 3 && deepest_cx >= ACPI_STATE_C3 )
+            tsc_may_stop = 1;
+        else if ( max_cstate >= 2 && deepest_cx >= ACPI_STATE_C2
+                  && !local_apic_timer_c2_ok )
+            tsc_may_stop = 1;
+        else
+            tsc_may_stop = 0;
+
         pm_idle_save = pm_idle;
         pm_idle = acpi_processor_idle;
     }
diff -r 246ecf354c85 xen/arch/x86/time.c
--- a/xen/arch/x86/time.c	Mon Feb 16 12:21:52 2009 +0800
+++ b/xen/arch/x86/time.c	Mon Feb 16 13:10:24 2009 +0800
@@ -1091,6 +1091,8 @@ struct calibration_rendezvous {
     u64 master_tsc_stamp;
 };
 
+int tsc_may_stop __read_mostly = 0;
+
 static void time_calibration_rendezvous(void *_r)
 {
     struct cpu_calibration *c = &this_cpu(cpu_calibration);
@@ -1102,7 +1104,9 @@ static void time_calibration_rendezvous(
         while ( atomic_read(&r->nr_cpus) != (total_cpus - 1) )
             cpu_relax();
         r->master_stime = read_platform_stime();
-        if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
+        if ( !boot_cpu_has(X86_FEATURE_NOSTOP_TSC)
+             && boot_cpu_has(X86_FEATURE_CONSTANT_TSC)
+             && tsc_may_stop )
             rdtscll(r->master_tsc_stamp);
         mb(); /* write r->master_* /then/ signal */
         atomic_inc(&r->nr_cpus);
@@ -1114,7 +1118,7 @@ static void time_calibration_rendezvous(
             mb(); /* receive signal /then/ read r->master_* */
     }
 
-    if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
+    if ( r->master_tsc_stamp )
         wrmsrl(MSR_IA32_TSC, r->master_tsc_stamp);
     rdtscll(c->local_tsc_stamp);
     c->stime_local_stamp = get_s_time();
@@ -1127,7 +1131,8 @@ static void time_calibration(void *unuse
 {
     struct calibration_rendezvous r = {
         .cpu_calibration_map = cpu_online_map,
-        .nr_cpus = ATOMIC_INIT(0)
+        .nr_cpus = ATOMIC_INIT(0),
+        .master_tsc_stamp = 0
     };
 
     /* @wait=1 because we must wait for all cpus before freeing @r. */
diff -r 246ecf354c85 xen/include/asm-x86/time.h
--- a/xen/include/asm-x86/time.h	Mon Feb 16 12:21:52 2009 +0800
+++ b/xen/include/asm-x86/time.h	Mon Feb 16 12:57:08 2009 +0800
@@ -41,4 +41,6 @@ uint64_t acpi_pm_tick_to_ns(uint64_t tic
 uint64_t acpi_pm_tick_to_ns(uint64_t ticks);
 uint64_t ns_to_acpi_pm_tick(uint64_t ns);
 
+extern int tsc_may_stop;
+
 #endif /* __X86_TIME_H__ */