3 files changed, 46 insertions, 2 deletions

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index c198b7e13e7b..c6acdf752715 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -304,4 +304,14 @@ config DEBUG_NMI_SELFTEST
 
           If unsure, say N.
 
+config X86_DEBUG_STATIC_CPU_HAS
+        bool "Debug alternatives"
+        depends on DEBUG_KERNEL
+        ---help---
+          This option causes additional code to be generated which
+          fails if static_cpu_has() is used before alternatives have
+          run.
+
+          If unsure, say N.
+
 endmenu
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 043d61e66efc..252b28f1176a 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -356,15 +356,35 @@ extern const char * const x86_power_flags[32];
 #endif /* CONFIG_X86_64 */
 
 #if __GNUC__ >= 4
+extern void warn_pre_alternatives(void);
+
 /*
  * Static testing of CPU features.  Used the same as boot_cpu_has().
  * These are only valid after alternatives have run, but will statically
  * patch the target code for additional performance.
- *
  */
 static __always_inline __pure bool __static_cpu_has(u16 bit)
 {
 #if __GNUC__ > 4 || __GNUC_MINOR__ >= 5
+
+#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS
+                /*
+                 * Catch too early usage of this before alternatives
+                 * have run.
+                 */
+                asm goto("1: jmp %l[t_warn]\n"
+                         "2:\n"
+                         ".section .altinstructions,\"a\"\n"
+                         " .long 1b - .\n"
+                         " .long 0\n"           /* no replacement */
+                         " .word %P0\n"         /* 1: do replace */
+                         " .byte 2b - 1b\n"     /* source len */
+                         " .byte 0\n"           /* replacement len */
+                         ".previous\n"
+                         /* skipping size check since replacement size = 0 */
+                         : : "i" (X86_FEATURE_ALWAYS) : : t_warn);
+#endif
+
                 asm goto("1: jmp %l[t_no]\n"
                          "2:\n"
                          ".section .altinstructions,\"a\"\n"
@@ -379,7 +399,13 @@ static __always_inline __pure bool __static_cpu_has(u16 bit)
                 return true;
         t_no:
                 return false;
-#else
+
+#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS
+        t_warn:
+                warn_pre_alternatives();
+                return false;
+#endif
+#else /* GCC_VERSION >= 40500 */
                 u8 flag;
                 /* Open-coded due to __stringify() in ALTERNATIVE() */
                 asm volatile("1: movb $0,%0\n"
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index d388ce1f3e1b..59adaa17e220 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1364,3 +1364,11 @@ void __cpuinit cpu_init(void)
         fpu_init();
 }
 #endif
+
+#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS
+void warn_pre_alternatives(void)
+{
+        WARN(1, "You're using static_cpu_has before alternatives have run!\n");
+}
+EXPORT_SYMBOL_GPL(warn_pre_alternatives);
+#endif