x86/time: yield to hyperthreads after updating TSC during rendezvous

Since we'd like the updates to be done as synchronously as possible,
make an attempt at yielding immediately after the TSC write.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index e92365f5bda5df4d908b6d8966a3edef5e02f3ce..5578b7b3a2692ec2e1663f351bf031a2ab1bed33 100644 (file)
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1708,7 +1708,14 @@ static void time_calibration_tsc_rendezvous(void *_r)
             atomic_inc(&r->semaphore);
 
             if ( i == 0 )
+            {
                 write_tsc(master_tsc);
+                /*
+                 * Try to give our hyperthread(s), if any, a chance to do
+                 * the same as instantly as possible.
+                 */
+                cpu_relax();
+            }
 
             while ( atomic_read(&r->semaphore) != (2*total_cpus - 1) )
                 cpu_relax();
@@ -1730,7 +1737,14 @@ static void time_calibration_tsc_rendezvous(void *_r)
             }
 
             if ( i == 0 )
+            {
                 write_tsc(master_tsc);
+                /*
+                 * Try to give our hyperthread(s), if any, a chance to do
+                 * the same as instantly as possible.
+                 */
+                cpu_relax();
+            }
 
             atomic_inc(&r->semaphore);
             while ( atomic_read(&r->semaphore) > total_cpus )