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    Subject[PATCH 4.18 60/79] cpu/hotplug: detect SMT disabled by BIOS
    Date
    4.18-stable review patch.  If anyone has any objections, please let me know.

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

    From: Josh Poimboeuf <jpoimboe@redhat.com>

    If SMT is disabled in BIOS, the CPU code doesn't properly detect it.
    The /sys/devices/system/cpu/smt/control file shows 'on', and the 'l1tf'
    vulnerabilities file shows SMT as vulnerable.

    Fix it by forcing 'cpu_smt_control' to CPU_SMT_NOT_SUPPORTED in such a
    case. Unfortunately the detection can only be done after bringing all
    the CPUs online, so we have to overwrite any previous writes to the
    variable.

    Reported-by: Joe Mario <jmario@redhat.com>
    Tested-by: Jiri Kosina <jkosina@suse.cz>
    Fixes: f048c399e0f7 ("x86/topology: Provide topology_smt_supported()")
    Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
    Signed-off-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/cpu.c | 9 +++++++++
    1 file changed, 9 insertions(+)

    --- a/kernel/cpu.c
    +++ b/kernel/cpu.c
    @@ -2125,6 +2125,15 @@ static const struct attribute_group cpuh

    static int __init cpu_smt_state_init(void)
    {
    + /*
    + * If SMT was disabled by BIOS, detect it here, after the CPUs have
    + * been brought online. This ensures the smt/l1tf sysfs entries are
    + * consistent with reality. Note this may overwrite cpu_smt_control's
    + * previous setting.
    + */
    + if (topology_max_smt_threads() == 1)
    + cpu_smt_control = CPU_SMT_NOT_SUPPORTED;
    +
    return sysfs_create_group(&cpu_subsys.dev_root->kobj,
    &cpuhp_smt_attr_group);
    }

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