[RFC PATCH v2 1/7] x86/runtime-const: Introduce runtime_const_mask_32()

[K Prateek Nayak]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Futex hash computation requires a mask operation with read-only after
init data that will be converted to a runtime constant in the subsequent
commit.

Introduce runtime_const_mask_32 to further optimize the mask operation
in the futex hash computation hot path.

  [ prateek: Broke off the x86 chunk, commit message. ]

Link: https://patch.msgid.link/20260227161841.GH606826@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Not-yet-signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
 arch/x86/include/asm/runtime-const.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/x86/include/asm/runtime-const.h b/arch/x86/include/asm/runtime-const.h
index 4cd94fdcb45e..b13f7036c1c9 100644
--- a/arch/x86/include/asm/runtime-const.h
+++ b/arch/x86/include/asm/runtime-const.h
@@ -41,6 +41,15 @@
 		:"+r" (__ret));					\
 	__ret; })
 
+#define runtime_const_mask_32(val, sym) ({			\
+	typeof(0u+(val)) __ret = (val);				\
+	asm_inline("and $0x12345678, %k0\n1:\n"				\
+		   ".pushsection runtime_mask_" #sym ",\"a\"\n\t"\
+		   ".long 1b - 4 - .\n"				\
+		   ".popsection"				\
+		   : "+r" (__ret));				\
+	__ret; })
+
 #define runtime_const_init(type, sym) do {		\
 	extern s32 __start_runtime_##type##_##sym[];	\
 	extern s32 __stop_runtime_##type##_##sym[];	\
@@ -65,6 +74,11 @@ static inline void __runtime_fixup_shift(void *where, unsigned long val)
 	*(unsigned char *)where = val;
 }
 
+static inline void __runtime_fixup_mask(void *where, unsigned long val)
+{
+	*(unsigned int *)where = val;
+}
+
 static inline void runtime_const_fixup(void (*fn)(void *, unsigned long),
 	unsigned long val, s32 *start, s32 *end)
 {
-- 
2.43.0