[Xen-devel] [PATCH] x86/tsc: limit the usage of synchronization rendezvous

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

If Xen detects the TSC is unreliable it will set a rendezvous helper
that tries to synchronize the different CPUs TSC value by propagating
the one from CPU#0 and writing it into the IA32_TSC MSR on each CPU.

When the system has a single thread and there are no hotplugable CPUs
doing the above just results in reading the TSC from CPU#0 and writing
it into the IA32_TSC MSR of CPU#0, which is pointless, so limit the
usage of the synchronization rendezvous to systems that have more than
one CPU, even if those CPUs are yet to be hotplugged.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
I'm not sure whether this is suitable for 4.13, being a performance
improvement but not fixing a functional bug.
---
 xen/arch/x86/time.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index d8242295ef..dddbb60f58 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1821,7 +1821,8 @@ static int __init verify_tsc_reliability(void)
      * are not marked as 'reliable', re-sync during rendezvous.
      */
     if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
-         !boot_cpu_has(X86_FEATURE_TSC_RELIABLE) )
+         !boot_cpu_has(X86_FEATURE_TSC_RELIABLE) &&
+         num_present_cpus() > 1 )
         time_calibration_rendezvous_fn = time_calibration_tsc_rendezvous;
 
     return 0;
-- 
2.23.0


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