powerpc: Add option to use jump label for cpu_has_feature()

We do binary patching of asm code using CPU features, which is a
one-time operation, done during early boot. However checks of CPU
features in C code are currently done at run time, even though the set
of CPU features can never change after boot.

We can optimise this by using jump labels to implement cpu_has_feature(),
meaning checks in C code are binary patched into a single nop or branch.

For a C sequence along the lines of:

    if (cpu_has_feature(FOO))
         return 2;

The generated code before is roughly:

    ld      r9,-27640(r2)
    ld      r9,0(r9)
    lwz     r9,32(r9)
    cmpwi   cr7,r9,0
    bge     cr7, 1f
    li      r3,2
    blr
1:  ...

After (true):
    nop
    li      r3,2
    blr

After (false):
    b	1f
    li      r3,2
    blr
1:  ...

mpe: Rename MAX_CPU_FEATURES as we already have a #define with that
name, and define it simply as a constant, rather than doing tricks with
sizeof and NULL pointers. Rename the array to cpu_feature_keys. Use the
kconfig we added to guard it. Add BUILD_BUG_ON() if the feature is not a
compile time constant. Rewrite the change log.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 24 insertions, 0 deletions

diff --git a/arch/powerpc/include/asm/cpu_has_feature.h b/arch/powerpc/include/asm/cpu_has_feature.h
index c935c5854fed..b9ce6425fdea 100644
--- a/arch/powerpc/include/asm/cpu_has_feature.h
+++ b/arch/powerpc/include/asm/cpu_has_feature.h
@@ -11,10 +11,34 @@ static inline bool early_cpu_has_feature(unsigned long feature)
 		  (CPU_FTRS_POSSIBLE & cur_cpu_spec->cpu_features & feature));
 }
 
+#ifdef CONFIG_JUMP_LABEL_FEATURE_CHECKS
+#include <linux/jump_label.h>
+
+#define NUM_CPU_FTR_KEYS	64
+
+extern struct static_key_true cpu_feature_keys[NUM_CPU_FTR_KEYS];
+
+static __always_inline bool cpu_has_feature(unsigned long feature)
+{
+	int i;
+
+	BUILD_BUG_ON(!__builtin_constant_p(feature));
+
+	if (CPU_FTRS_ALWAYS & feature)
+		return true;
+
+	if (!(CPU_FTRS_POSSIBLE & feature))
+		return false;
+
+	i = __builtin_ctzl(feature);
+	return static_branch_likely(&cpu_feature_keys[i]);
+}
+#else
 static inline bool cpu_has_feature(unsigned long feature)
 {
 	return early_cpu_has_feature(feature);
 }
+#endif
 
 #endif /* __ASSEMBLY__ */
 #endif /* __ASM_POWERPC_CPUFEATURE_H */