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Date:   Wed, 29 Nov 2017 22:23:41 +0100
From:   Radim Krčmář <>
Cc:     Paolo Bonzini <>,
        Borislav Petkov <>, Alexander Graf <>,
        "Michael S. Tsirkin" <>,
        Jan H. Schönherr <>,
        KarimAllah Ahmed <>,
        Jim Mattson <>
Subject: [PATCH v2 1/3] KVM: x86: prevent MWAIT in guest with buggy MONITOR

The bug prevents MWAIT from waking up after a write to the monitored
cache line.
KVM might emulate a CPU model that shouldn't have the bug, so the guest
would not employ a workaround and possibly miss wakeups.
Better to avoid the situation.

Reviewed-by: Alexander Graf <>
Signed-off-by: Radim Krčmář <>
 arch/x86/kvm/x86.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index d0b95b7a90b4..81f5f50794f6 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -281,6 +281,9 @@ static inline bool kvm_mwait_in_guest(void)
 		return false;
+	if (boot_cpu_has_bug(X86_BUG_MONITOR))
+		return false;
 	 * Intel CPUs without CPUID5_ECX_INTERRUPT_BREAK are problematic as
 	 * they would allow guest to stop the CPU completely by disabling

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