[Xen-changelog] [xen master] x86/time: relax barriers

[patchbot@xxxxxxx]

commit 2fd44a3edc1918c5e0eeab2d695c9a0dd2d3f506
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Aug 4 10:08:00 2016 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Aug 4 10:08:00 2016 +0200

    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@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/time.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index ca4e53b..e646440 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);
--
generated by git-patchbot for /home/xen/git/xen.git#master

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