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From:   Michael Kelley <mikelley@microsoft.com>
To:     kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com,
        wei.liu@kernel.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org,
        hpa@zytor.com, linux-kernel@vger.kernel.org,
        linux-hyperv@vger.kernel.org
Cc:     mikelley@microsoft.com
Subject: [PATCH 1/1] x86/hyperv: Disable hardlockup detector by default in Hyper-V guests
Date:   Mon,  9 May 2022 08:44:23 -0700
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In newer versions of Hyper-V, the x86/x64 PMU can be virtualized
into guest VMs by explicitly enabling it. Linux kernels are typically
built to automatically enable the hardlockup detector if the PMU is
found. To prevent the possibility of false positives due to the
vagaries of VM scheduling, disable the PMU-based hardlockup detector
by default in a VM on Hyper-V.  The hardlockup detector can still be
enabled by overriding the default with the nmi_watchdog=1 option on
the kernel boot line or via sysctl at runtime.

This change mimics the approach taken with KVM guests in
commit 692297d8f968 ("watchdog: introduce the hardlockup_detector_disable()
function").

Linux on ARM64 does not provide a PMU-based hardlockup detector, so
there's no corresponding disable in the Hyper-V init code on ARM64.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
---
 arch/x86/kernel/cpu/mshyperv.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 5b8f2c3..8316139 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -457,6 +457,8 @@ static void __init ms_hyperv_init_platform(void)
 	 */
 	if (!(ms_hyperv.features & HV_ACCESS_TSC_INVARIANT))
 		mark_tsc_unstable("running on Hyper-V");
+
+	hardlockup_detector_disable();
 }
 
 static bool __init ms_hyperv_x2apic_available(void)
-- 
1.8.3.1