x86: Add new static_cpu_has() function using alternatives

For CPU-feature-specific code that touches performance-critical paths,
introduce a static patching version of [boot_]cpu_has().  This is run
at alternatives time and is therefore not appropriate for most
initialization code, but on the other hand initialization code is
generally not performance critical.

On gcc 4.5+ this uses the new "asm goto" feature.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <1273135546-29690-2-git-send-email-avi@redhat.com>

arch/x86/include/asm/cpufeature.h

@@ -175,6 +175,7 @@
 
 #if defined(__KERNEL__) && !defined(__ASSEMBLY__)
 
+#include <asm/asm.h>
 #include <linux/bitops.h>
 
 extern const char * const x86_cap_flags[NCAPINTS*32];
@@ -283,6 +284,62 @@ extern const char * const x86_power_flags[32];
 
 #endif /* CONFIG_X86_64 */
 
+/*
+ * 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(u8 bit)
+{
+#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
+		asm goto("1: jmp %l[t_no]\n"
+			 "2:\n"
+			 ".section .altinstructions,\"a\"\n"
+			 _ASM_ALIGN "\n"
+			 _ASM_PTR "1b\n"
+			 _ASM_PTR "0\n" 	/* no replacement */
+			 " .byte %P0\n"		/* feature bit */
+			 " .byte 2b - 1b\n"	/* source len */
+			 " .byte 0\n"		/* replacement len */
+			 " .byte 0xff + 0 - (2b-1b)\n"	/* padding */
+			 ".previous\n"
+			 : : "i" (bit) : : t_no);
+		return true;
+	t_no:
+		return false;
+#else
+		u8 flag;
+		/* Open-coded due to __stringify() in ALTERNATIVE() */
+		asm volatile("1: movb $0,%0\n"
+			     "2:\n"
+			     ".section .altinstructions,\"a\"\n"
+			     _ASM_ALIGN "\n"
+			     _ASM_PTR "1b\n"
+			     _ASM_PTR "3f\n"
+			     " .byte %P1\n"		/* feature bit */
+			     " .byte 2b - 1b\n"		/* source len */
+			     " .byte 4f - 3f\n"		/* replacement len */
+			     " .byte 0xff + (4f-3f) - (2b-1b)\n" /* padding */
+			     ".previous\n"
+			     ".section .altinstr_replacement,\"ax\"\n"
+			     "3: movb $1,%0\n"
+			     "4:\n"
+			     ".previous\n"
+			     : "=qm" (flag) : "i" (bit));
+		return flag;
+#endif
+}
+
+#define static_cpu_has(bit)					\
+(								\
+	__builtin_constant_p(boot_cpu_has(bit)) ?		\
+		boot_cpu_has(bit) :				\
+	(__builtin_constant_p(bit) && !((bit) & ~0xff)) ?	\
+		__static_cpu_has(bit) :				\
+		boot_cpu_has(bit)				\
+)
+
 #endif /* defined(__KERNEL__) && !defined(__ASSEMBLY__) */
 
 #endif /* _ASM_X86_CPUFEATURE_H */