jump_label: s390: avoid pointless initial NOP patching

Patching NOPs into other NOPs at boot time serves no purpose, so let's
use the same NOP encodings at compile time and runtime.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220615154142.1574619-2-ardb@kernel.org

Author: ardbiesheuvel

arch/s390/include/asm/jump_label.h

@@ -10,7 +10,6 @@
 #include <linux/stringify.h>
 
 #define JUMP_LABEL_NOP_SIZE 6
-#define JUMP_LABEL_NOP_OFFSET 2
 
 #ifdef CONFIG_CC_IS_CLANG
 #define JUMP_LABEL_STATIC_KEY_CONSTRAINT "i"
@@ -21,12 +20,12 @@
 #endif
 
 /*
- * We use a brcl 0,2 instruction for jump labels at compile time so it
+ * We use a brcl 0,<offset> instruction for jump labels so it
  * can be easily distinguished from a hotpatch generated instruction.
  */
 static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
 {
-	asm_volatile_goto("0:	brcl	0,"__stringify(JUMP_LABEL_NOP_OFFSET)"\n"
+	asm_volatile_goto("0:	brcl 0,%l[label]\n"
 			  ".pushsection __jump_table,\"aw\"\n"
 			  ".balign	8\n"
 			  ".long	0b-.,%l[label]-.\n"

arch/s390/kernel/jump_label.c

@@ -44,14 +44,8 @@ static void jump_label_bug(struct jump_entry *entry, struct insn *expected,
 	panic("Corrupted kernel text");
 }
 
-static struct insn orignop = {
-	.opcode = 0xc004,
-	.offset = JUMP_LABEL_NOP_OFFSET >> 1,
-};
-
 static void jump_label_transform(struct jump_entry *entry,
-				 enum jump_label_type type,
-				 int init)
+				 enum jump_label_type type)
 {
 	void *code = (void *)jump_entry_code(entry);
 	struct insn old, new;
@@ -63,27 +57,22 @@ static void jump_label_transform(struct jump_entry *entry,
 		jump_label_make_branch(entry, &old);
 		jump_label_make_nop(entry, &new);
 	}
-	if (init) {
-		if (memcmp(code, &orignop, sizeof(orignop)))
-			jump_label_bug(entry, &orignop, &new);
-	} else {
-		if (memcmp(code, &old, sizeof(old)))
-			jump_label_bug(entry, &old, &new);
-	}
+	if (memcmp(code, &old, sizeof(old)))
+		jump_label_bug(entry, &old, &new);
 	s390_kernel_write(code, &new, sizeof(new));
 }
 
 void arch_jump_label_transform(struct jump_entry *entry,
 			       enum jump_label_type type)
 {
-	jump_label_transform(entry, type, 0);
+	jump_label_transform(entry, type);
 	text_poke_sync();
 }
 
 bool arch_jump_label_transform_queue(struct jump_entry *entry,
 				     enum jump_label_type type)
 {
-	jump_label_transform(entry, type, 0);
+	jump_label_transform(entry, type);
 	return true;
 }
 
@@ -95,6 +84,4 @@ void arch_jump_label_transform_apply(void)
 void __init_or_module arch_jump_label_transform_static(struct jump_entry *entry,
 						       enum jump_label_type type)
 {
-	jump_label_transform(entry, type, 1);
-	text_poke_sync();
 }