[PATCH v4 3/3] x86/time: avoid reading the platform timer in rendezvous functions

[Jan Beulich <jbeulich@suse.com>]

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@suse.com>
---
v4: New.
---
I realize there's some risk associated with potential new uses of the
field down the road. What would people think about compiling time.c a
2nd time into a dummy object file, with a conditional enabled to force
assuming CONSTANT_TSC, and with that conditional used to suppress
presence of the field as well as all audited used of it (i.e. in
particular that large part of local_time_calibration())? Unexpected new
users of the field would then cause build time errors.

--- 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_rendezv
                  * 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_rendezv
     {
         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