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Date:   Tue, 26 Mar 2019 17:36:06 +0100
From:   Thomas Gleixner <>
To:     LKML <>
Cc:     Tianyu Lan <>,
        Konrad Wilk <>,
        Josh Poimboeuf <>,
        Mukesh Ojha <>,
        Peter Zijlstra <>,
        Jiri Kosina <>, Rik van Riel <>,
        Andy Lutomirski <>,
        Micheal Kelley <>,
        "K. Y. Srinivasan" <>,
        Greg KH <>,
        Linus Torvalds <>,
        Borislav Petkov <>,
Subject: [patch 2/2] x86/smp: Enforce CONFIG_HOTPLUG_CPU when SMP=y

The SMT disable 'nosmt' command line argument is not working properly when
CONFIG_HOTPLUG_CPU is disabled. The teardown of the sibling CPUs which are
required to be brought up due to the MCE issues, cannot work. The CPUs are
then kept in a half dead state.

As the 'nosmt' functionality has become popular due to the speculative
hardware vulnerabilities, the half torn down state is not a proper solution
to the problem.

Enforce CONFIG_HOTPLUG_CPU=y when SMP is enabled so the full operation is

Reported-by: Tianyu Lan <>
Signed-off-by: Thomas Gleixner <>
Cc: Konrad Wilk <>
Cc: Josh Poimboeuf <>
Cc: Mukesh Ojha <>
Cc: Peter Zijlstra <>
Cc: Jiri Kosina <>
Cc: Rik van Riel <>
Cc: Andy Lutomirski <>
Cc: Micheal Kelley <>
Cc: K. Y. Srinivasan <>
Cc: Greg KH <>
Cc: Linus Torvalds <>
Cc: Borislav Petkov <>
 arch/x86/Kconfig |    8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2217,14 +2217,8 @@ config RANDOMIZE_MEMORY_PHYSICAL_PADDING
 	   If unsure, leave at the default value.
-	bool "Support for hot-pluggable CPUs"
+	def_bool y
 	depends on SMP
-	---help---
-	  Say Y here to allow turning CPUs off and on. CPUs can be
-	  controlled through /sys/devices/system/cpu.
-	  ( Note: power management support will enable this option
-	    automatically on SMP systems. )
-	  Say N if you want to disable CPU hotplug.
 	bool "Set default setting of cpu0_hotpluggable"

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