[xen staging] x86/time: avoid reading the platform timer in rendezvous functions

[patchbot@xxxxxxx]

commit 48bb237fca65e2bcec7dbe7f4b178cf33c5e15f0
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue May 4 10:49:59 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue May 4 10:49:59 2021 +0200

    x86/time: avoid reading the platform timer in rendezvous functions
    
    Reading the platform timer isn't cheap, so we'd better avoid it when the
    resulting value is of no interest to anyone.
    
    The consumer of master_stime, obtained by
    time_calibration_{std,tsc}_rendezvous() and propagated through
    this_cpu(cpu_calibration), is local_time_calibration(). With
    CONSTANT_TSC the latter function uses an early exit path, which doesn't
    explicitly use the field. While this_cpu(cpu_calibration) (including the
    master_stime field) gets propagated to this_cpu(cpu_time).stamp on that
    path, both structures' fields get consumed only by the !CONSTANT_TSC
    logic of the function.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/time.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 5578b7b3a2..10418cdc54 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -52,6 +52,7 @@ unsigned long pit0_ticks;
 struct cpu_time_stamp {
     u64 local_tsc;
     s_time_t local_stime;
+    /* Next field unconditionally valid only when !CONSTANT_TSC. */
     s_time_t master_stime;
 };
 
@@ -1702,7 +1703,7 @@ static void time_calibration_tsc_rendezvous(void *_r)
                  * iteration.
                  */
                 r->master_tsc_stamp = r->max_tsc_stamp;
-            else if ( i == 0 )
+            else if ( !boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && i == 0 )
                 r->master_stime = read_platform_stime(NULL);
 
             atomic_inc(&r->semaphore);
@@ -1776,8 +1777,11 @@ static void time_calibration_std_rendezvous(void *_r)
     {
         while ( atomic_read(&r->semaphore) != (total_cpus - 1) )
             cpu_relax();
-        r->master_stime = read_platform_stime(NULL);
-        smp_wmb(); /* write r->master_stime /then/ signal */
+        if ( !boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
+        {
+            r->master_stime = read_platform_stime(NULL);
+            smp_wmb(); /* write r->master_stime /then/ signal */
+        }
         atomic_inc(&r->semaphore);
     }
     else
--
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