x86/cpu: Move arch_smt_update() to a neutral place

arch_smt_update() will be used to control IPI/NMI broadcasting via the
shorthand mechanism. Keeping it in the bugs file and calling the apic
function from there is possible, but not really intuitive.

Move it to a neutral place and invoke the bugs function from there.

No functional change.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190722105219.910317273@linutronix.de

arch/x86/include/asm/bugs.h

@@ -18,4 +18,6 @@ int ppro_with_ram_bug(void);
 static inline int ppro_with_ram_bug(void) { return 0; }
 #endif
 
+extern void cpu_bugs_smt_update(void);
+
 #endif /* _ASM_X86_BUGS_H */

arch/x86/kernel/cpu/bugs.c

@@ -700,7 +700,7 @@ static void update_mds_branch_idle(void)
 
 #define MDS_MSG_SMT "MDS CPU bug present and SMT on, data leak possible. See https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/mds.html for more details.\n"
 
-void arch_smt_update(void)
+void cpu_bugs_smt_update(void)
 {
 	/* Enhanced IBRS implies STIBP. No update required. */
 	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)

arch/x86/kernel/cpu/common.c

@@ -1945,3 +1945,12 @@ void microcode_check(void)
 	pr_warn("x86/CPU: CPU features have changed after loading microcode, but might not take effect.\n");
 	pr_warn("x86/CPU: Please consider either early loading through initrd/built-in or a potential BIOS update.\n");
 }
+
+/*
+ * Invoked from core CPU hotplug code after hotplug operations
+ */
+void arch_smt_update(void)
+{
+	/* Handle the speculative execution misfeatures */
+	cpu_bugs_smt_update();
+}