x86/speculation: Avoid __switch_to_xtra() calls

The TIF_SPEC_IB bit does not need to be evaluated in the decision to invoke
__switch_to_xtra() when:

 - CONFIG_SMP is disabled

 - The conditional STIPB mode is disabled

The TIF_SPEC_IB bit still controls IBPB in both cases so the TIF work mask
checks might invoke __switch_to_xtra() for nothing if TIF_SPEC_IB is the
only set bit in the work masks.

Optimize it out by masking the bit at compile time for CONFIG_SMP=n and at
run time when the static key controlling the conditional STIBP mode is
disabled.

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>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20181125185005.374062201@linutronix.de

arch/x86/include/asm/thread_info.h

@@ -147,9 +147,18 @@ struct thread_info {
 	 _TIF_FSCHECK)
 
 /* flags to check in __switch_to() */
-#define _TIF_WORK_CTXSW							\
+#define _TIF_WORK_CTXSW_BASE						\
 	(_TIF_IO_BITMAP|_TIF_NOCPUID|_TIF_NOTSC|_TIF_BLOCKSTEP|		\
-	 _TIF_SSBD|_TIF_SPEC_IB)
+	 _TIF_SSBD)
+
+/*
+ * Avoid calls to __switch_to_xtra() on UP as STIBP is not evaluated.
+ */
+#ifdef CONFIG_SMP
+# define _TIF_WORK_CTXSW	(_TIF_WORK_CTXSW_BASE | _TIF_SPEC_IB)
+#else
+# define _TIF_WORK_CTXSW	(_TIF_WORK_CTXSW_BASE)
+#endif
 
 #define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW|_TIF_USER_RETURN_NOTIFY)
 #define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW)

arch/x86/kernel/process.h

@@ -2,6 +2,8 @@
 //
 // Code shared between 32 and 64 bit
 
+#include <asm/spec-ctrl.h>
+
 void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p);
 
 /*
@@ -14,6 +16,19 @@ static inline void switch_to_extra(struct task_struct *prev,
 	unsigned long next_tif = task_thread_info(next)->flags;
 	unsigned long prev_tif = task_thread_info(prev)->flags;
 
+	if (IS_ENABLED(CONFIG_SMP)) {
+		/*
+		 * Avoid __switch_to_xtra() invocation when conditional
+		 * STIPB is disabled and the only different bit is
+		 * TIF_SPEC_IB. For CONFIG_SMP=n TIF_SPEC_IB is not
+		 * in the TIF_WORK_CTXSW masks.
+		 */
+		if (!static_branch_likely(&switch_to_cond_stibp)) {
+			prev_tif &= ~_TIF_SPEC_IB;
+			next_tif &= ~_TIF_SPEC_IB;
+		}
+	}
+
 	/*
 	 * __switch_to_xtra() handles debug registers, i/o bitmaps,
 	 * speculation mitigations etc.