[RFC PATCH V1 09/10] powerpc: use jump label for mmu_has_feature

[Aneesh Kumar K.V]

From: Kevin Hao <haokexin at gmail.com>

The mmu features are fixed once the probe of mmu features are done.
And the function mmu_has_feature() does be used in some hot path.
The checking of the mmu features for each time of invoking of
mmu_has_feature() seems suboptimal. This tries to reduce this
overhead of this check by using jump label.

The generated assemble code of the following c program:
	if (mmu_has_feature(MMU_FTR_XXX))
		xxx()
Before:
	lis     r9,-16230
	lwz     r9,12324(r9)
	lwz     r9,24(r9)
	andi.   r10,r9,16
	beqlr+

After:
	nop	if MMU_FTR_XXX is enabled
	b xxx	if MMU_FTR_XXX is not enabled

Signed-off-by: Kevin Hao <haokexin at gmail.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/mmu.h | 36 ++++++++++++++++++++++++++++++++++++
 arch/powerpc/kernel/cputable.c | 17 +++++++++++++++++
 arch/powerpc/kernel/setup_32.c |  1 +
 arch/powerpc/kernel/setup_64.c |  1 +
 4 files changed, 55 insertions(+)

diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
index c488db09f7a0..e453e095579e 100644
--- a/arch/powerpc/include/asm/mmu.h
+++ b/arch/powerpc/include/asm/mmu.h
@@ -143,6 +143,41 @@ static inline bool __mmu_has_feature(unsigned long feature)
 	return !!(MMU_FTRS_POSSIBLE & cur_cpu_spec->mmu_features & feature);
 }
 
+#ifdef CONFIG_JUMP_LABEL
+#include <linux/jump_label.h>
+
+#define MAX_MMU_FEATURES	(8 * sizeof(((struct cpu_spec *)0)->mmu_features))
+
+extern struct static_key_true mmu_feat_keys[MAX_MMU_FEATURES];
+
+extern void mmu_feat_keys_init(void);
+
+static __always_inline bool mmu_has_feature(unsigned long feature)
+{
+	int i;
+
+	if (!(MMU_FTRS_POSSIBLE & feature))
+		return false;
+
+	i = __builtin_ctzl(feature);
+	return static_branch_likely(&mmu_feat_keys[i]);
+}
+
+static inline void mmu_clear_feature(unsigned long feature)
+{
+	int i;
+
+	i = __builtin_ctzl(feature);
+	cur_cpu_spec->mmu_features &= ~feature;
+	static_branch_disable(&mmu_feat_keys[i]);
+}
+#else
+
+static inline void mmu_feat_keys_init(void)
+{
+
+}
+
 static inline bool mmu_has_feature(unsigned long feature)
 {
 	return __mmu_has_feature(feature);
@@ -152,6 +187,7 @@ static inline void mmu_clear_feature(unsigned long feature)
 {
 	cur_cpu_spec->mmu_features &= ~feature;
 }
+#endif /* CONFIG_JUMP_LABEL */
 
 extern unsigned int __start___mmu_ftr_fixup, __stop___mmu_ftr_fixup;
 
diff --git a/arch/powerpc/kernel/cputable.c b/arch/powerpc/kernel/cputable.c
index b0a24b425441..a98630183e92 100644
--- a/arch/powerpc/kernel/cputable.c
+++ b/arch/powerpc/kernel/cputable.c
@@ -2243,4 +2243,21 @@ void __init cpu_feat_keys_init(void)
 			static_branch_disable(&cpu_feat_keys[i]);
 	}
 }
+
+struct static_key_true mmu_feat_keys[MAX_MMU_FEATURES] = {
+			[0 ... MAX_MMU_FEATURES - 1] = STATIC_KEY_TRUE_INIT
+};
+EXPORT_SYMBOL_GPL(mmu_feat_keys);
+
+void __init mmu_feat_keys_init(void)
+{
+	int i;
+
+	for (i = 0; i < MAX_MMU_FEATURES; i++) {
+		unsigned long f = 1ul << i;
+
+		if (!(cur_cpu_spec->mmu_features & f))
+			static_branch_disable(&mmu_feat_keys[i]);
+	}
+}
 #endif
diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
index ac5b41ad94ed..cd0d8814bd9b 100644
--- a/arch/powerpc/kernel/setup_32.c
+++ b/arch/powerpc/kernel/setup_32.c
@@ -107,6 +107,7 @@ notrace unsigned long __init early_init(unsigned long dt_ptr)
 	 */
 	jump_label_init();
 	cpu_feat_keys_init();
+	mmu_feat_keys_init();
 
 	return KERNELBASE + offset;
 }
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index 1bd0d19afe1d..8222950f820f 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -487,6 +487,7 @@ void __init setup_system(void)
 	 */
 	jump_label_init();
 	cpu_feat_keys_init();
+	mmu_feat_keys_init();
 
 	/*
 	 * Unflatten the device-tree passed by prom_init or kexec
-- 
2.7.4