[PATCH v5 4/6] tsc: implement the extended tsc reading function

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

By doing that, add tsc clocksource to a list of KVM clocksources
providing valid cycle values, meaning that KVM can use its masterclock.

This is a part of the work aiming to move to a more simple scheme of
masterclock related values calculation in KVM.

Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
---
 arch/x86/kernel/tsc.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 796d96b..8786454 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1036,6 +1036,15 @@ static void tsc_cs_tick_stable(struct clocksource *cs)
 		sched_clock_tick_stable();
 }
 
+static bool tsc_read_with_stamp(struct clocksource *cs,
+				u64 *cycles, u64 *cycles_stamp)
+{
+	u64 tsc = read_tsc(cs);
+	*cycles = tsc;
+	*cycles_stamp = tsc;
+	return true;
+}
+
 /*
  * .mask MUST be CLOCKSOURCE_MASK(64). See comment above read_tsc()
  */
@@ -1043,6 +1052,7 @@ static struct clocksource clocksource_tsc = {
 	.name                   = "tsc",
 	.rating                 = 300,
 	.read                   = read_tsc,
+	.read_with_stamp	= tsc_read_with_stamp,
 	.mask                   = CLOCKSOURCE_MASK(64),
 	.flags                  = CLOCK_SOURCE_IS_CONTINUOUS |
 				  CLOCK_SOURCE_MUST_VERIFY,
-- 
2.7.4