linux-kernel - [PATCH 17/17] clocksource/drivers/hyper-v: Set TSC clocksource as default w/ InvariantTSC

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Date:   Thu, 16 Jan 2020 19:23:04 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org,
        Andrea Parri <parri.andrea@...il.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        linux-hyperv@...r.kernel.org (open list:Hyper-V CORE AND DRIVERS)
Subject: [PATCH 17/17] clocksource/drivers/hyper-v: Set TSC clocksource as default w/ InvariantTSC

From: Andrea Parri <parri.andrea@...il.com>

Change the Hyper-V clocksource ratings to 250, below the TSC clocksource
rating of 300.  In configurations where Hyper-V offers an InvariantTSC,
the TSC is not marked "unstable", so the TSC clocksource is available
and preferred.  With the higher rating, it will be the default.  On
older hardware and Hyper-V versions, the TSC is marked "unstable", so no
TSC clocksource is created and the selected Hyper-V clocksource will be
the default.

Signed-off-by: Andrea Parri <parri.andrea@...il.com>
Reviewed-by: Michael Kelley <mikelley@...rosoft.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
Link: https://lore.kernel.org/r/20200109160650.16150-3-parri.andrea@gmail.com
---
 drivers/clocksource/hyperv_timer.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
index 42748adccc98..9d808d595ca8 100644
--- a/drivers/clocksource/hyperv_timer.c
+++ b/drivers/clocksource/hyperv_timer.c
@@ -302,6 +302,14 @@ EXPORT_SYMBOL_GPL(hv_stimer_global_cleanup);
  * the other that uses the TSC reference page feature as defined in the
  * TLFS.  The MSR version is for compatibility with old versions of
  * Hyper-V and 32-bit x86.  The TSC reference page version is preferred.
+ *
+ * The Hyper-V clocksource ratings of 250 are chosen to be below the
+ * TSC clocksource rating of 300.  In configurations where Hyper-V offers
+ * an InvariantTSC, the TSC is not marked "unstable", so the TSC clocksource
+ * is available and preferred.  With the higher rating, it will be the
+ * default.  On older hardware and Hyper-V versions, the TSC is marked
+ * "unstable", so no TSC clocksource is created and the selected Hyper-V
+ * clocksource will be the default.
  */
 
 u64 (*hv_read_reference_counter)(void);
@@ -363,7 +371,7 @@ static void resume_hv_clock_tsc(struct clocksource *arg)
 
 static struct clocksource hyperv_cs_tsc = {
 	.name	= "hyperv_clocksource_tsc_page",
-	.rating	= 400,
+	.rating	= 250,
 	.read	= read_hv_clock_tsc_cs,
 	.mask	= CLOCKSOURCE_MASK(64),
 	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
@@ -395,7 +403,7 @@ static u64 read_hv_sched_clock_msr(void)
 
 static struct clocksource hyperv_cs_msr = {
 	.name	= "hyperv_clocksource_msr",
-	.rating	= 400,
+	.rating	= 250,
 	.read	= read_hv_clock_msr_cs,
 	.mask	= CLOCKSOURCE_MASK(64),
 	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
-- 
2.17.1