[PATCH v2 3/7] runtime constants: add x86 architecture support

[Jimmy Tran <jtoantran@google.com>]

From: Linus Torvalds <torvalds@linux-foundation.org>

commit e3c92e81711d14b46c3121d36bc8e152cb843923 upstream.

This implements the runtime constant infrastructure for x86, allowing
the dcache d_hash() function to be generated using as a constant for
hash table address followed by shift by a constant of the hash index.

Cc: <stable@vger.kernel.org> # 6.10.x: e60cc61: vfs: dcache: move hashlen_hash
Cc: <stable@vger.kernel.org> # 6.10.x: e782985: runtime constants: add default
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jimmy Tran <jtoantran@google.com>
---
 arch/x86/include/asm/runtime-const.h | 61 ++++++++++++++++++++++++++++
 arch/x86/kernel/vmlinux.lds.S        |  3 ++
 2 files changed, 64 insertions(+)
 create mode 100644 arch/x86/include/asm/runtime-const.h

diff --git a/arch/x86/include/asm/runtime-const.h b/arch/x86/include/asm/runtime-const.h
new file mode 100644
index 0000000000000..76fdeaa0faa3f
--- /dev/null
+++ b/arch/x86/include/asm/runtime-const.h
@@ -0,0 +1,61 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_RUNTIME_CONST_H
+#define _ASM_RUNTIME_CONST_H
+
+#define runtime_const_ptr(sym) ({				\
+	typeof(sym) __ret;					\
+	asm_inline("mov %1,%0\n1:\n"				\
+		".pushsection runtime_ptr_" #sym ",\"a\"\n\t"	\
+		".long 1b - %c2 - .\n\t"			\
+		".popsection"					\
+		 : "=r" (__ret)					\
+		 : "i" (0x0123456789abcdefULL),	\
+		 "i" (sizeof(long)));				\
+	__ret; })
+
+// The 'typeof' will create at _least_ a 32-bit type, but
+// will happily also take a bigger type and the 'shrl' will
+// clear the upper bits
+#define runtime_const_shift_right_32(val, sym) ({		\
+	typeof(0u+(val)) __ret = (val);				\
+	asm_inline("shrl $12,%k0\n1:\n"				\
+		".pushsection runtime_shift_" #sym ",\"a\"\n\t"	\
+		".long 1b - 1 - .\n\t"				\
+		".popsection"					\
+		 : "+r" (__ret));					\
+	__ret; })
+
+#define runtime_const_init(type, sym) do {		\
+	extern s32 __start_runtime_##type##_##sym[];	\
+	extern s32 __stop_runtime_##type##_##sym[];	\
+	runtime_const_fixup(__runtime_fixup_##type,	\
+		(unsigned long)(sym),				\
+		__start_runtime_##type##_##sym,		\
+		__stop_runtime_##type##_##sym);		\
+} while (0)
+
+/*
+ * The text patching is trivial - you can only do this at init time,
+ * when the text section hasn't been marked RO, and before the text
+ * has ever been executed.
+ */
+static inline void __runtime_fixup_ptr(void *where, unsigned long val)
+{
+	*(unsigned long *)where = val;
+}
+
+static inline void __runtime_fixup_shift(void *where, unsigned long val)
+{
+	*(unsigned char *)where = val;
+}
+
+static inline void runtime_const_fixup(void (*fn)(void *, unsigned long),
+	unsigned long val, s32 *start, s32 *end)
+{
+	while (start < end) {
+		fn(*start + (void *)start, val);
+		start++;
+	}
+}
+
+#endif
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index c57d5df1abc60..cb5b41480a848 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -371,6 +371,9 @@ SECTIONS
 	PERCPU_SECTION(INTERNODE_CACHE_BYTES)
 #endif
 
+	RUNTIME_CONST(shift, d_hash_shift)
+	RUNTIME_CONST(ptr, dentry_hashtable)
+
 	. = ALIGN(PAGE_SIZE);
 
 	/* freed after init ends here */
-- 
2.50.1.470.g6ba607880d-goog