x86/time: relax barriers

On x86 there's no need for full barriers in loops waiting for some
memory location to change. Nor do we need full barriers between two
reads and two writes - SMP ones fully suffice.

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

xen/arch/x86/time.c

@@ -1211,7 +1211,7 @@ static void tsc_check_slave(void *unused)
     unsigned int cpu = smp_processor_id();
     local_irq_disable();
     while ( !cpumask_test_cpu(cpu, &tsc_check_cpumask) )
-        mb();
+        cpu_relax();
     check_tsc_warp(cpu_khz, &tsc_max_warp);
     cpumask_clear_cpu(cpu, &tsc_check_cpumask);
     local_irq_enable();
@@ -1276,7 +1276,7 @@ static void time_calibration_tsc_rendezvous(void *_r)
         if ( smp_processor_id() == 0 )
         {
             while ( atomic_read(&r->semaphore) != (total_cpus - 1) )
-                mb();
+                cpu_relax();
 
             if ( r->master_stime == 0 )
             {
@@ -1289,21 +1289,21 @@ static void time_calibration_tsc_rendezvous(void *_r)
                 write_tsc(r->master_tsc_stamp);
 
             while ( atomic_read(&r->semaphore) != (2*total_cpus - 1) )
-                mb();
+                cpu_relax();
             atomic_set(&r->semaphore, 0);
         }
         else
         {
             atomic_inc(&r->semaphore);
             while ( atomic_read(&r->semaphore) < total_cpus )
-                mb();
+                cpu_relax();
 
             if ( i == 0 )
                 write_tsc(r->master_tsc_stamp);
 
             atomic_inc(&r->semaphore);
             while ( atomic_read(&r->semaphore) > total_cpus )
-                mb();
+                cpu_relax();
         }
     }
 
@@ -1321,7 +1321,7 @@ static void time_calibration_std_rendezvous(void *_r)
         while ( atomic_read(&r->semaphore) != (total_cpus - 1) )
             cpu_relax();
         r->master_stime = read_platform_stime();
-        mb(); /* write r->master_stime /then/ signal */
+        smp_wmb(); /* write r->master_stime /then/ signal */
         atomic_inc(&r->semaphore);
     }
     else
@@ -1329,7 +1329,7 @@ static void time_calibration_std_rendezvous(void *_r)
         atomic_inc(&r->semaphore);
         while ( atomic_read(&r->semaphore) != total_cpus )
             cpu_relax();
-        mb(); /* receive signal /then/ read r->master_stime */
+        smp_rmb(); /* receive signal /then/ read r->master_stime */
     }
 
     time_calibration_rendezvous_tail(r);