[PATCH v2 2/7] runtime constants: add default dummy infrastructure

[Jimmy Tran <jtoantran@google.com>]

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

commit e78298556ee5d881f6679effb2a6743969ea6e2d upstream.

This adds the initial dummy support for 'runtime constants' for when
an architecture doesn't actually support an implementation of fixing
up said runtime constants.

This ends up being the fallback to just using the variables as regular
__ro_after_init variables, and changes the dcache d_hash() function to
use this model.

Cc: <stable@vger.kernel.org> # 6.10.x: e60cc61: vfs: dcache: move hashlen_hash
Fixes: e78298556ee5 ("runtime constants: add default dummy infrastructure")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jimmy Tran <jtoantran@google.com>
---
 fs/dcache.c                         | 11 ++++++++++-
 include/asm-generic/Kbuild          |  1 +
 include/asm-generic/runtime-const.h | 15 +++++++++++++++
 include/asm-generic/vmlinux.lds.h   |  8 ++++++++
 4 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 include/asm-generic/runtime-const.h

diff --git a/fs/dcache.c b/fs/dcache.c
index 82adee104f82c..9e5c92b4b4aaa 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -35,6 +35,8 @@
 #include "internal.h"
 #include "mount.h"
 
+#include <asm/runtime-const.h>
+
 /*
  * Usage:
  * dcache->d_inode->i_lock protects:
@@ -102,7 +104,8 @@ static struct hlist_bl_head *dentry_hashtable __read_mostly;
 
 static inline struct hlist_bl_head *d_hash(unsigned long hashlen)
 {
-	return dentry_hashtable + ((u32)hashlen >> d_hash_shift);
+	return runtime_const_ptr(dentry_hashtable) +
+		runtime_const_shift_right_32(hashlen, d_hash_shift);
 }
 
 #define IN_LOOKUP_SHIFT 10
@@ -3297,6 +3300,9 @@ static void __init dcache_init_early(void)
 					0,
 					0);
 	d_hash_shift = 32 - d_hash_shift;
+
+	runtime_const_init(shift, d_hash_shift);
+	runtime_const_init(ptr, dentry_hashtable);
 }
 
 static void __init dcache_init(void)
@@ -3325,6 +3331,9 @@ static void __init dcache_init(void)
 					0,
 					0);
 	d_hash_shift = 32 - d_hash_shift;
+
+	runtime_const_init(shift, d_hash_shift);
+	runtime_const_init(ptr, dentry_hashtable);
 }
 
 /* SLAB cache for __getname() consumers */
diff --git a/include/asm-generic/Kbuild b/include/asm-generic/Kbuild
index 941be574bbe00..22673ec5defbb 100644
--- a/include/asm-generic/Kbuild
+++ b/include/asm-generic/Kbuild
@@ -46,6 +46,7 @@ mandatory-y += pci.h
 mandatory-y += percpu.h
 mandatory-y += pgalloc.h
 mandatory-y += preempt.h
+mandatory-y += runtime-const.h
 mandatory-y += rwonce.h
 mandatory-y += sections.h
 mandatory-y += serial.h
diff --git a/include/asm-generic/runtime-const.h b/include/asm-generic/runtime-const.h
new file mode 100644
index 0000000000000..3e68a17fbf287
--- /dev/null
+++ b/include/asm-generic/runtime-const.h
@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_RUNTIME_CONST_H
+#define _ASM_RUNTIME_CONST_H
+
+/*
+ * This is the fallback for when the architecture doesn't
+ * support the runtime const operations.
+ *
+ * We just use the actual symbols as-is.
+ */
+#define runtime_const_ptr(sym) (sym)
+#define runtime_const_shift_right_32(val, sym) ((u32)(val)>>(sym))
+#define runtime_const_init(type, sym) do { } while (0)
+
+#endif
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index cf3f8b9bf43f0..66bfd3dc91a33 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -907,6 +907,14 @@
 #define CON_INITCALL							\
 	BOUNDED_SECTION_POST_LABEL(.con_initcall.init, __con_initcall, _start, _end)
 
+#define RUNTIME_NAME(t, x) runtime_##t##_##x
+
+#define RUNTIME_CONST(t, x)						\
+	. = ALIGN(8);							\
+	RUNTIME_NAME(t, x) : AT(ADDR(RUNTIME_NAME(t, x)) - LOAD_OFFSET) {	\
+		*(RUNTIME_NAME(t, x));					\
+	}
+
 /* Alignment must be consistent with (kunit_suite *) in include/kunit/test.h */
 #define KUNIT_TABLE()							\
 		. = ALIGN(8);						\
-- 
2.50.1.470.g6ba607880d-goog