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 <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>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 37eec872042b72f9971e789b968cb0795b65bfee..2bc3d2f5b2a5bb8ca51f9e81ebb9403f73e34522 100644 (file)
--- 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);
 }