diff -r 0b0e7c2b4eef xen/arch/x86/time.c
--- a/xen/arch/x86/time.c	Tue Jan 20 21:21:16 2009 +0800
+++ b/xen/arch/x86/time.c	Mon Feb 09 02:21:50 2009 +0800
@@ -1095,22 +1095,21 @@ static void time_calibration_rendezvous(
         while ( atomic_read(&r->nr_cpus) != (total_cpus - 1) )
             cpu_relax();
         r->master_stime = read_platform_stime();
-        rdtscll(r->master_tsc_stamp);
+        if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
+            rdtscll(r->master_tsc_stamp);
         mb(); /* write r->master_* /then/ signal */
         atomic_inc(&r->nr_cpus);
-        c->local_tsc_stamp = r->master_tsc_stamp;
     }
     else
     {
         atomic_inc(&r->nr_cpus);
         while ( atomic_read(&r->nr_cpus) != total_cpus )
-            cpu_relax();
-        mb(); /* receive signal /then/ read r->master_* */
-        if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
-            wrmsrl(MSR_IA32_TSC, r->master_tsc_stamp);
-        rdtscll(c->local_tsc_stamp);
-    }
-
+            mb(); /* receive signal /then/ read r->master_* */
+    }
+
+    if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
+        wrmsrl(MSR_IA32_TSC, r->master_tsc_stamp);
+    rdtscll(c->local_tsc_stamp);
     c->stime_local_stamp = get_s_time();
     c->stime_master_stamp = r->master_stime;