x86: allow sched clock to be overridden by paravirt

This patch turns the sched_clock into native_sched_clock.
sched clock becomes a weak symbol, which can then give its
place to a paravirt definition.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/tsc_64.c

@@ -63,7 +63,7 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
 	local_irq_restore(flags);
 }
 
-unsigned long long sched_clock(void)
+unsigned long long native_sched_clock(void)
 {
 	unsigned long a = 0;
 
@@ -77,6 +77,19 @@ unsigned long long sched_clock(void)
 	return cycles_2_ns(a);
 }
 
+/* We need to define a real function for sched_clock, to override the
+   weak default version */
+#ifdef CONFIG_PARAVIRT
+unsigned long long sched_clock(void)
+{
+	return paravirt_sched_clock();
+}
+#else
+unsigned long long
+sched_clock(void) __attribute__((alias("native_sched_clock")));
+#endif
+
+
 static int tsc_unstable;
 
 inline int check_tsc_unstable(void)