From: Jan Beulich <jbeulich@suse.com>
Date: Thu, 4 Aug 2016 08:08:00 +0000 (+0200)
Subject: x86/time: relax barriers
X-Git-Tag: archive/raspbian/4.8.0-1+rpi1~1^2~619
X-Git-Url: https://dgit.raspbian.org/%22http://www.example.com/cgi/%22/%22http:/www.example.com/cgi/%22?a=commitdiff_plain;h=2fd44a3edc1918c5e0eeab2d695c9a0dd2d3f506;p=xen.git

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>
---

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index ca4e53b966..e64644031d 100644
--- a/xen/arch/x86/time.c
+++ b/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);