cpu/hotplug: detect SMT disabled by BIOS

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 <[email protected]> Tested-by: Jiri Kosina <[email protected]> Fixes: f048c39 ("x86/topology: Provide topology_smt_supported()") Signed-off-by: Josh Poimboeuf <[email protected]> Signed-off-by: Peter Zijlstra <[email protected]>
OpenDingux · Jul 24, 2018 · 73d5e2b · 73d5e2b
1 parent 1949f9f
commit 73d5e2b
Showing 1 changed file with 9 additions and 0 deletions.
diff --git a/kernel/cpu.c b/kernel/cpu.c
@@ -2125,6 +2125,15 @@ static const struct attribute_group cpuhp_smt_attr_group = {
 
 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);
 }