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, 20 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/cputable.c b/arch/powerpc/kernel/cputable.c
index d81f826..e3da41f 100644
--- a/arch/powerpc/kernel/cputable.c
+++ b/arch/powerpc/kernel/cputable.c
@@ -15,6 +15,7 @@
 #include <linux/threads.h>
 #include <linux/init.h>
 #include <linux/export.h>
+#include <linux/jump_label.h>
 
 #include <asm/oprofile_impl.h>
 #include <asm/cputable.h>
@@ -2224,3 +2225,22 @@ struct cpu_spec * __init identify_cpu(unsigned long offset, unsigned int pvr)
 
 	return NULL;
 }
+
+#ifdef CONFIG_JUMP_LABEL_FEATURE_CHECKS
+struct static_key_true cpu_feature_keys[NUM_CPU_FTR_KEYS] = {
+			[0 ... NUM_CPU_FTR_KEYS - 1] = STATIC_KEY_TRUE_INIT
+};
+EXPORT_SYMBOL_GPL(cpu_feature_keys);
+
+void __init cpu_feature_keys_init(void)
+{
+	int i;
+
+	for (i = 0; i < NUM_CPU_FTR_KEYS; i++) {
+		unsigned long f = 1ul << i;
+
+		if (!(cur_cpu_spec->cpu_features & f))
+			static_branch_disable(&cpu_feature_keys[i]);
+	}
+}
+#endif