x86/time: use correct (local) time stamp in constant-TSC calibration fast path

This looks like a copy and paste mistake in commit 1b6a99892d ("x86:
Simpler time handling when TSC is constant across all power saving
states"), responsible for occasional many-microsecond cross-CPU skew of
what NOW() returns.

Also improve the correlation between local TSC and stime stamps
obtained at the end of the two calibration handlers: Compute the stime
one from the TSC one, instead of doing another rdtsc() for that
compuation.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/time.c

@@ -998,7 +998,7 @@ static void local_time_calibration(void)
         /* Atomically read cpu_calibration struct and write cpu_time struct. */
         local_irq_disable();
         t->local_tsc_stamp    = c->local_tsc_stamp;
-        t->stime_local_stamp  = c->stime_master_stamp;
+        t->stime_local_stamp  = c->stime_local_stamp;
         t->stime_master_stamp = c->stime_master_stamp;
         local_irq_enable();
         update_vcpu_system_time(current);
@@ -1275,7 +1275,7 @@ static void time_calibration_tsc_rendezvous(void *_r)
     }
 
     c->local_tsc_stamp = rdtsc();
-    c->stime_local_stamp = get_s_time();
+    c->stime_local_stamp = get_s_time_fixed(c->local_tsc_stamp);
     c->stime_master_stamp = r->master_stime;
 
     raise_softirq(TIME_CALIBRATE_SOFTIRQ);
@@ -1305,7 +1305,7 @@ static void time_calibration_std_rendezvous(void *_r)
     }
 
     c->local_tsc_stamp = rdtsc();
-    c->stime_local_stamp = get_s_time();
+    c->stime_local_stamp = get_s_time_fixed(c->local_tsc_stamp);
     c->stime_master_stamp = r->master_stime;
 
     raise_softirq(TIME_CALIBRATE_SOFTIRQ);