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Subject[PATCH 4.19 122/134] x86/smp: Enforce CONFIG_HOTPLUG_CPU when SMP=y
4.19-stable review patch.  If anyone has any objections, please let me know.


From: Thomas Gleixner <>

commit bebd024e4815b1a170fcd21ead9c2222b23ce9e6 upstream.

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 <>
Acked-by: Greg Kroah-Hartman <>
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: Linus Torvalds <>
Cc: Borislav Petkov <>
Cc: K. Y. Srinivasan <>
Signed-off-by: Greg Kroah-Hartman <>

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

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2199,14 +2199,8 @@ 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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