x86, vmi: TSC going backwards check in vmi clocksource

Impact: fix time warps under vmware

Similar to the check for TSC going backwards in the TSC clocksource,
we also need this check for VMI clocksource.

Signed-off-by: Alok N Kataria <[e-mail]>
Cc: Zachary Amsden <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Cc: [e-mail]

arch/x86/kernel/vmiclock_32.c

···
 #endif
 
 /** vmi clocksource */
+static struct clocksource clocksource_vmi;
 
 static cycle_t read_real_cycles(void)
 {
-	return vmi_timer_ops.get_cycle_counter(VMI_CYCLES_REAL);
+	cycle_t ret = (cycle_t)vmi_timer_ops.get_cycle_counter(VMI_CYCLES_REAL);
+	return ret >= clocksource_vmi.cycle_last ?
+		ret : clocksource_vmi.cycle_last;
 }
 
 static struct clocksource clocksource_vmi = {