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From:   Ani Sinha <ani@anisinha.ca>
To:     linux-kernel@vger.kernel.org
Cc:     anirban.sinha@nokia.com, mikelley@microsoft.com,
        Ani Sinha <ani@anisinha.ca>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        linux-hyperv@vger.kernel.org
Subject: [PATCH] Hyper-V: fix for unwanted manipulation of sched_clock when TSC marked unstable
Date:   Tue, 13 Jul 2021 08:35:21 +0530
Message-Id: <20210713030522.1714803-1-ani@anisinha.ca>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Marking TSC as unstable has a side effect of marking sched_clock as
unstable when TSC is still being used as the sched_clock. This is not
desirable. Hyper-V ultimately uses a paravirtualized clock source that
provides a stable scheduler clock even on systems without TscInvariant
CPU capability. Hence, mark_tsc_unstable() call should be called _after_
scheduler clock has been changed to the paravirtualized clocksource. This
will prevent any unwanted manipulation of the sched_clock. Only TSC will
be correctly marked as unstable.

Signed-off-by: Ani Sinha <ani@anisinha.ca>
---
 arch/x86/kernel/cpu/mshyperv.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 22f13343b5da..715458b7729a 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -370,8 +370,6 @@ static void __init ms_hyperv_init_platform(void)
 	if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
 		wrmsrl(HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x1);
 		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
-	} else {
-		mark_tsc_unstable("running on Hyper-V");
 	}
 
 	/*
@@ -432,6 +430,12 @@ static void __init ms_hyperv_init_platform(void)
 	/* Register Hyper-V specific clocksource */
 	hv_init_clocksource();
 #endif
+	/* TSC should be marked as unstable only after Hyper-V
+	 * clocksource has been initialized. This ensures that the
+	 * stability of the sched_clock is not altered.
+	 */
+	if (!(ms_hyperv.features & HV_ACCESS_TSC_INVARIANT))
+		mark_tsc_unstable("running on Hyper-V");
 }
 
 static bool __init ms_hyperv_x2apic_available(void)
-- 
2.25.1