x86/time: tsc_check_writability() may need to be run a second time

While we shouldn't remove its current invocation, we need to re-run it
for the case that the X86_FEATURE_TSC_RELIABLE feature flag has been
cleared, in order to avoid using the TSC rendezvous function in case
the TSC can't be written.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1643,6 +1643,14 @@ static int __init verify_tsc_reliability
     }
 
     /*
+     * Re-run the TSC writability check if it didn't run to completion, as
+     * X86_FEATURE_TSC_RELIABLE may have been cleared by now. This is needed
+     * for determining which rendezvous function to use (below).
+     */
+    if ( !disable_tsc_sync )
+        tsc_check_writability();
+
+    /*
      * While with constant-rate TSCs the scale factor can be shared, when TSCs
      * are not marked as 'reliable', re-sync during rendezvous.
      */