linux-kernel - [PATCH] clocksource: hyper-v: make sure Invariant-TSC is used if it is available

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Message-Id: <20230408210339.15085-1-decui@microsoft.com>
Date:   Sat,  8 Apr 2023 14:03:39 -0700
From:   Dexuan Cui <decui@...rosoft.com>
To:     kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, daniel.lezcano@...aro.org, tglx@...utronix.de,
        linux-hyperv@...r.kernel.org, mikelley@...rosoft.com
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] clocksource: hyper-v: make sure Invariant-TSC is used if it is available

If Hyper-V TSC page is unavailable and Invariant-TSC is available,
currently hyperv_cs_msr (rather than Invariant-TSC) is used by default.

Use Invariant-TSC by default by downgrading hyperv_cs_msr.rating in
hv_init_tsc_clocksource(), if Invariant-TSC is available.

Signed-off-by: Dexuan Cui <decui@...rosoft.com>
---
 drivers/clocksource/hyperv_timer.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
index c0cef92b12b82..7901a88ad9188 100644
--- a/drivers/clocksource/hyperv_timer.c
+++ b/drivers/clocksource/hyperv_timer.c
@@ -506,9 +506,6 @@ static bool __init hv_init_tsc_clocksource(void)
 {
 	union hv_reference_tsc_msr tsc_msr;
 
-	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
-		return false;
-
 	/*
 	 * If Hyper-V offers TSC_INVARIANT, then the virtualized TSC correctly
 	 * handles frequency and offset changes due to live migration,
@@ -525,6 +522,9 @@ static bool __init hv_init_tsc_clocksource(void)
 		hyperv_cs_msr.rating = 250;
 	}
 
+	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
+		return false;
+
 	hv_read_reference_counter = read_hv_clock_tsc;
 
 	/*
-- 
2.25.1