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    Subject[PATCH 5.0 141/146] x86/smp: Enforce CONFIG_HOTPLUG_CPU when SMP=y
    Date
    5.0-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Thomas Gleixner <tglx@linutronix.de>

    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
    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
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -2221,14 +2221,8 @@ 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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