[patch 18/24] x86/speculation: Avoid __switch_to_xtra() calls

[Thomas Gleixner]

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.

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 <t...@linutronix.de>
---
 arch/x86/include/asm/spec-ctrl.h   |    5 +++++
 arch/x86/include/asm/thread_info.h |   13 +++++++++++--
 arch/x86/kernel/process_32.c       |    9 +++++++++
 arch/x86/kernel/process_64.c       |    9 +++++++++
 4 files changed, 34 insertions(+), 2 deletions(-)

--- a/arch/x86/include/asm/spec-ctrl.h
+++ b/arch/x86/include/asm/spec-ctrl.h
@@ -115,8 +115,13 @@ static inline void switch_to_ibpb(struct
 }
 
 #ifdef CONFIG_SMP
+static __always_inline bool switch_to_xtra_needs_spec_ib(void)
+{
+       return static_branch_likely(&switch_to_cond_stibp);
+}
 extern void speculative_store_bypass_ht_init(void);
 #else
+static inline bool switch_to_xtra_needs_spec_ib(void) { return false; }
 static inline void speculative_store_bypass_ht_init(void) { }
 #endif
 
--- a/arch/x86/include/asm/thread_info.h
+++ b/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)
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -272,6 +272,15 @@ EXPORT_SYMBOL_GPL(start_thread);
        switch_to_ibpb(next_p, prev_tif, next_tif);
 
        /*
+        * Avoid __switch_to_xtra() invocation when conditional stpib is
+        * disabled.
+        */
+       if (!switch_to_xtra_needs_spec_ib()) {
+               prev_tif &= ~_TIF_SPEC_IB;
+               next_tif &= ~_TIF_SPEC_IB;
+       }
+
+       /*
         * Now maybe handle debug registers and/or IO bitmaps
         */
        if (unlikely(next_tif & _TIF_WORK_CTXSW_NEXT ||
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -625,6 +625,15 @@ void compat_start_thread(struct pt_regs
        switch_to_ibpb(next_p, prev_tif, next_tif);
 
        /*
+        * Avoid __switch_to_xtra() invocation when conditional stpib is
+        * disabled.
+        */
+       if (!switch_to_xtra_needs_spec_ib()) {
+               prev_tif &= ~_TIF_SPEC_IB;
+               next_tif &= ~_TIF_SPEC_IB;
+       }
+
+       /*
         * Now maybe reload the debug registers and handle I/O bitmaps
         */
        if (unlikely(next_tif & _TIF_WORK_CTXSW_NEXT ||