UBUNTU: SAUCE: x86/speculation: Introduce arch_smt_update()

Introduce an empty arch_smt_update() stub to be used in subsequent x86
speculation code.

It's basically commit 53c613fe ("x86/speculation: Enable
cross-hyperthread spectre v2 STIBP mitigation") without the STIBP-specific
pieces.

CVE-2018-12126
CVE-2018-12127
CVE-2018-12130
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/kernel/cpu/bugs.c

@@ -54,12 +54,10 @@ static void __init spectre_v2_select_mitigation(void);
 static void __init ssb_select_mitigation(void);
 static void __init l1tf_select_mitigation(void);
 
-/*
- * Our boot-time value of the SPEC_CTRL MSR. We read it once so that any
- * writes to SPEC_CTRL contain whatever reserved bits have been set.
- */
-u64 __ro_after_init x86_spec_ctrl_base;
+/* The base value of the SPEC_CTRL MSR that always has to be preserved. */
+u64 x86_spec_ctrl_base;
 EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
+static DEFINE_MUTEX(spec_ctrl_mutex);
 
 /*
  * The vendor and possibly platform specific bits which can be modified in
@@ -354,6 +352,12 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 	return cmd;
 }
 
+void arch_smt_update(void)
+{
+	mutex_lock(&spec_ctrl_mutex);
+	mutex_unlock(&spec_ctrl_mutex);
+}
+
 static void __init spectre_v2_select_mitigation(void)
 {
 	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
@@ -462,6 +466,8 @@ static void __init spectre_v2_select_mitigation(void)
 		    mode != SPECTRE_V2_RETPOLINE_AMD)
 			set_ibrs_enabled(1);   /* Enable IBRS */
 	}
+
+	arch_smt_update();
 }
 
 #undef pr_fmt

kernel/cpu.c

@@ -855,6 +855,12 @@ static void cpuhp_online_cpu_device(unsigned int cpu)
 	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)
 {
 	int cpu, ret = 0;
@@ -881,8 +887,10 @@ static int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
 		 */
 		cpuhp_offline_cpu_device(cpu);
 	}
-	if (!ret)
+	if (!ret) {
 		cpu_smt_control = ctrlval;
+		arch_smt_update();
+	}
 	cpu_maps_update_done();
 	return ret;
 }
@@ -893,6 +901,7 @@ static int cpuhp_smt_enable(void)
 
 	cpu_maps_update_begin();
 	cpu_smt_control = CPU_SMT_ENABLED;
+	arch_smt_update();
 	for_each_present_cpu(cpu) {
 		/* Skip online CPUs and CPUs on offline nodes */
 		if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))