x86/speculation: Rework SMT state change

arch_smt_update() is only called when the sysfs SMT control knob is
changed. This means that when SMT is enabled in the sysfs control knob the
system is considered to have SMT active even if all siblings are offline.

To allow finegrained control of the speculation mitigations, the actual SMT
state is more interesting than the fact that siblings could be enabled.

Rework the code, so arch_smt_update() is invoked from each individual CPU
hotplug function, and simplify the update function while at it.

Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Ingo Molnar <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Jiri Kosina <[e-mail]>
Cc: Tom Lendacky <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Tim Chen <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Casey Schaufler <[e-mail]>
Cc: Asit Mallick <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Cc: Jon Masters <[e-mail]>
Cc: Waiman Long <[e-mail]>
Cc: Greg KH <[e-mail]>
Cc: Dave Stewart <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: [e-mail]
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/kernel/cpu/bugs.c

···
 #include <linux/module.h>
 #include <linux/nospec.h>
 #include <linux/prctl.h>
+#include <linux/sched/smt.h>
 
 #include <asm/spec-ctrl.h>
 #include <asm/cmdline.h>
···
 		return;
 
 	mutex_lock(&spec_ctrl_mutex);
-	mask = x86_spec_ctrl_base;
-	if (cpu_smt_control == CPU_SMT_ENABLED)
+
+	mask = x86_spec_ctrl_base & ~SPEC_CTRL_STIBP;
+	if (sched_smt_active())
 		mask |= SPEC_CTRL_STIBP;
-	else
-		mask &= ~SPEC_CTRL_STIBP;
 
 	if (mask != x86_spec_ctrl_base) {
 		pr_info("Spectre v2 cross-process SMT mitigation: %s STIBP\n",
-				cpu_smt_control == CPU_SMT_ENABLED ?
-				"Enabling" : "Disabling");
+			mask & SPEC_CTRL_STIBP ? "Enabling" : "Disabling");
 		x86_spec_ctrl_base = mask;
 		on_each_cpu(update_stibp_msr, NULL, 1);
 	}

include/linux/sched/smt.h

···
 static inline bool sched_smt_active(void) { return false; }
 #endif
 
+void arch_smt_update(void);
+
 #endif

kernel/cpu.c

···
 #include <linux/sched/signal.h>
 #include <linux/sched/hotplug.h>
 #include <linux/sched/task.h>
+#include <linux/sched/smt.h>
 #include <linux/unistd.h>
 #include <linux/cpu.h>
 #include <linux/oom.h>
···
 }
 
 #endif	/* CONFIG_HOTPLUG_CPU */
+
+/*
+ * Architectures that need SMT-specific errata handling during SMT hotplug
+ * should override this.
+ */
+void __weak arch_smt_update(void) { }
 
 #ifdef CONFIG_HOTPLUG_SMT
 enum cpuhp_smt_control cpu_smt_control __read_mostly = CPU_SMT_ENABLED;
···
 	 * concurrent CPU hotplug via cpu_add_remove_lock.
 	 */
 	lockup_detector_cleanup();
+	arch_smt_update();
 	return ret;
 }
 
···
 	ret = cpuhp_up_callbacks(cpu, st, target);
 out:
 	cpus_write_unlock();
+	arch_smt_update();
 	return ret;
 }
 
···
 	/* Tell user space about the state change */
 	kobject_uevent(&dev->kobj, KOBJ_ONLINE);
 }
-
-/*
- * Architectures that need SMT-specific errata handling during SMT hotplug
- * should override this.
- */
-void __weak arch_smt_update(void) { };
 
 static int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
 {