KVM: x86: add module parameter to disable periodic kvmclock sync

[Marcelo Tosatti <mtosatti@redhat.com>]

The periodic kvmclock sync can be an undesired source of latencies.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0033df3..be56fd3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -98,6 +98,9 @@ module_param(ignore_msrs, bool, S_IRUGO | S_IWUSR);
 unsigned int min_timer_period_us = 500;
 module_param(min_timer_period_us, uint, S_IRUGO | S_IWUSR);
 
+static bool kvmclock_periodic_sync = 1;
+module_param(kvmclock_periodic_sync, bool, S_IRUGO | S_IWUSR);
+
 bool kvm_has_tsc_control;
 EXPORT_SYMBOL_GPL(kvm_has_tsc_control);
 u32  kvm_max_guest_tsc_khz;
@@ -1718,7 +1721,8 @@ static void kvmclock_sync_fn(struct work_struct *work)
 	struct kvm *kvm = container_of(ka, struct kvm, arch);
 
 	schedule_delayed_work(&kvm->arch.kvmclock_update_work, 0);
-	schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
+	if (kvmclock_periodic_sync)
+		schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
 					KVMCLOCK_SYNC_PERIOD);
 }
 
@@ -6971,7 +6975,8 @@ int kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
 	kvm_write_tsc(vcpu, &msr);
 	vcpu_put(vcpu);
 
-	schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
+	if (kvmclock_periodic_sync)
+		schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
 					KVMCLOCK_SYNC_PERIOD);
 
 	return r;