[tip:smp/urgent] x86/smp: Enforce CONFIG_HOTPLUG_CPU when SMP=y

[tip-bot for Thomas Gleixner <tipbot@zytor.com>]

Commit-ID:  bebd024e4815b1a170fcd21ead9c2222b23ce9e6
Gitweb:     https://git.kernel.org/tip/bebd024e4815b1a170fcd21ead9c2222b23ce9e6
Author:     Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Tue, 26 Mar 2019 17:36:06 +0100
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Thu, 28 Mar 2019 13:34:58 +0100

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@microsoft.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Konrad Wilk <konrad.wilk@oracle.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Mukesh Ojha <mojha@codeaurora.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Rik van Riel <riel@surriel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Micheal Kelley <michael.h.kelley@microsoft.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20190326163811.598166056@linutronix.de

---
 arch/x86/Kconfig | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c1f9b3cf437c..5ad92419be19 100644
--- 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"