x86, paravirt: don't compute pvclock adjustments if we trust the tsc

If the HV told us we can fully trust the TSC, skip any
correction

Signed-off-by: Glauber Costa <glommer@redhat.com>
Acked-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

2 files changed, 7 insertions, 0 deletions

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 59c740fcb9b3..eb9b76c716c2 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -215,4 +215,7 @@ void __init kvmclock_init(void)
         clocksource_register(&kvm_clock);
         pv_info.paravirt_enabled = 1;
         pv_info.name = "KVM";
+
+        if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
+                pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
 }
diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index f5bc40e1697e..239427ca02af 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -135,6 +135,10 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
                 barrier();
         } while (version != src->version);
 
+        if ((valid_flags & PVCLOCK_TSC_STABLE_BIT) &&
+                (shadow.flags & PVCLOCK_TSC_STABLE_BIT))
+                return ret;
+
         /*
          * Assumption here is that last_value, a global accumulator, always goes
          * forward. If we are less than that, we should not be much smaller.