x86/speculation: Rework SMT state change

commit a74cfffb03b73d41e08f84c2e5c87dec0ce3db9f upstream.

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 <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Casey Schaufler <casey.schaufler@intel.com>
Cc: Asit Mallick <asit.k.mallick@intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Jon Masters <jcm@redhat.com>
Cc: Waiman Long <longman9394@gmail.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Dave Stewart <david.c.stewart@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20181125185004.521974984@linutronix.de
[bwh: Backported to 4.4: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

3 files changed, 16 insertions, 7 deletions

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index ef98bf0b269c..3ae095584577 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -13,6 +13,7 @@
 #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>
@@ -340,16 +341,14 @@ void arch_smt_update(void)
 		return;
 
 	mutex_lock(&spec_ctrl_mutex);
-	mask = x86_spec_ctrl_base;
-	if (IS_ENABLED(CONFIG_SMP))
+
+	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",
-				IS_ENABLED(CONFIG_SMP) ?
-				"Enabling" : "Disabling");
+			mask & SPEC_CTRL_STIBP ? "Enabling" : "Disabling");
 		x86_spec_ctrl_base = mask;
 		on_each_cpu(update_stibp_msr, NULL, 1);
 	}
diff --git a/include/linux/sched/smt.h b/include/linux/sched/smt.h
index 5209c268c6fd..559ac4590593 100644
--- a/include/linux/sched/smt.h
+++ b/include/linux/sched/smt.h
@@ -15,4 +15,6 @@ static __always_inline bool sched_smt_active(void)
 static inline bool sched_smt_active(void) { return false; }
 #endif
 
+void arch_smt_update(void);
+
 #endif
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 42ce0b0ae5c5..c2bfeed74be3 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -8,6 +8,7 @@
 #include <linux/init.h>
 #include <linux/notifier.h>
 #include <linux/sched.h>
+#include <linux/sched/smt.h>
 #include <linux/unistd.h>
 #include <linux/cpu.h>
 #include <linux/oom.h>
@@ -199,6 +200,12 @@ void cpu_hotplug_enable(void)
 EXPORT_SYMBOL_GPL(cpu_hotplug_enable);
 #endif	/* CONFIG_HOTPLUG_CPU */
 
+/*
+ * Architectures that need SMT-specific errata handling during SMT hotplug
+ * should override this.
+ */
+void __weak arch_smt_update(void) { }
+
 /* Need to know about CPUs going up/down? */
 int register_cpu_notifier(struct notifier_block *nb)
 {
@@ -434,6 +441,7 @@ out_release:
 	cpu_hotplug_done();
 	if (!err)
 		cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
+	arch_smt_update();
 	return err;
 }
 
@@ -537,7 +545,7 @@ out_notify:
 		__cpu_notify(CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL);
 out:
 	cpu_hotplug_done();
-
+	arch_smt_update();
 	return ret;
 }