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Date:   Tue, 26 Mar 2019 17:36:06 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Konrad Wilk <konrad.wilk@...cle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mukesh Ojha <mojha@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jkosina@...e.cz>, Rik van Riel <riel@...riel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Micheal Kelley <michael.h.kelley@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org
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
possible.

Reported-by: Tianyu Lan <Tianyu.Lan@...rosoft.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Konrad Wilk <konrad.wilk@...cle.com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Mukesh Ojha <mojha@...eaurora.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Jiri Kosina <jkosina@...e.cz>
Cc: Rik van Riel <riel@...riel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Micheal Kelley <michael.h.kelley@...rosoft.com>
Cc: K. Y. Srinivasan <kys@...rosoft.com>
Cc: Greg KH <gregkh@...uxfoundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: stable@...r.kernel.org
---
 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.
 
 config HOTPLUG_CPU
-	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.
 
 config BOOTPARAM_HOTPLUG_CPU0
 	bool "Set default setting of cpu0_hotpluggable"


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