KVM guest: prevent tracing recursion with kvmclock

Prevent tracing of preempt_disable() in get_cpu_var() in
kvm_clock_read(). When CONFIG_DEBUG_PREEMPT is enabled,
preempt_disable/enable() are traced and this causes the function_graph
tracer to go into an infinite recursion. By open coding the
preempt_disable() around the get_cpu_var(), we can use the notrace
version which prevents preempt_disable/enable() from being traced and
prevents the recursion.

Based on a similar patch for Xen from Jeremy Fitzhardinge.

Tested-by: Gleb Natapov <gleb@redhat.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kernel/kvmclock.c

@@ -74,9 +74,10 @@ static cycle_t kvm_clock_read(void)
 	struct pvclock_vcpu_time_info *src;
 	cycle_t ret;
 
-	src = &get_cpu_var(hv_clock);
+	preempt_disable_notrace();
+	src = &__get_cpu_var(hv_clock);
 	ret = pvclock_clocksource_read(src);
-	put_cpu_var(hv_clock);
+	preempt_enable_notrace();
 	return ret;
 }