[PATCH 4.18 60/79] cpu/hotplug: detect SMT disabled by BIOS

From: Greg Kroah-Hartman
Date: Tue Aug 14 2018 - 13:26:56 EST

4.18-stable review patch. If anyone has any objections, please let me know.


From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

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

Reported-by: Joe Mario <jmario@xxxxxxxxxx>
Tested-by: Jiri Kosina <jkosina@xxxxxxx>
Fixes: f048c399e0f7 ("x86/topology: Provide topology_smt_supported()")
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
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,