x86/time: correctly honor late clearing of TSC related feature flags

As such clearing of flags may have an impact on the selected rendezvous
function, defer the establishing of a rendezvous function other than
the initial default one (std) until after all APs have been brought up.

But don't allow such feature flags to be cleared during CPU hotplug:
Platform and local system times may have diverged significantly by
then, potentially causing noticeably (even if only temporary) strange
behavior. As we're anyway expecting only sufficiently similar CPUs to
appear during hotplug, this shouldn't be introducing new limitations.

Reported-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v3: Drop original approach entirely - defer everything to
    verify_tsc_reliability(), making for quite a bit smaller a patch.

Note: Considering that tsc_check_writability() checks
      TSC_RELIABLE, it being run before that feature flag has obtained
      its final value seems problematic too. Should we defer that call
      too?

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1638,9 +1638,18 @@ static int __init verify_tsc_reliability
 
             printk("Switched to Platform timer %s TSC\n",
                    freq_string(plt_src.frequency));
+            return 0;
         }
     }
 
+    /*
+     * While with constant-rate TSCs the scale factor can be shared, when TSCs
+     * are not marked as 'reliable', re-sync during rendezvous.
+     */
+    if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
+         !boot_cpu_has(X86_FEATURE_TSC_RELIABLE) )
+        time_calibration_rendezvous_fn = time_calibration_tsc_rendezvous;
+
     return 0;
 }
 __initcall(verify_tsc_reliability);
@@ -1650,14 +1659,6 @@ int __init init_xen_time(void)
 {
     tsc_check_writability();
 
-    /* If we have constant-rate TSCs then scale factor can be shared. */
-    if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
-    {
-        /* If TSCs are not marked as 'reliable', re-sync during rendezvous. */
-        if ( !boot_cpu_has(X86_FEATURE_TSC_RELIABLE) )
-            time_calibration_rendezvous_fn = time_calibration_tsc_rendezvous;
-    }
-
     open_softirq(TIME_CALIBRATE_SOFTIRQ, local_time_calibration);
 
     /* NB. get_cmos_time() can take over one second to execute. */