arm64: jump labels: NOP out NOP -> NOP replacement

In the arm64 arch_static_branch implementation we place an A64 NOP into the instruction stream and log relevant details to a jump_entry in a __jump_table section. Later this may be replaced with an immediate branch without link to the code for the unlikely case.

At init time, the core calls arch_jump_label_transform_static to initialise the NOPs. On x86 this involves inserting the optimal NOP for a given microarchitecture, but on arm64 we only use the architectural NOP, and hence replace each NOP with the exact same NOP. This is somewhat pointless.

Additionally, at module load time we don't call jump_label_apply_nops to patch the optimal NOPs in, unlike other architectures, but get away with this because we only use the architectural NOP anyway. A later notifier will patch NOPs with branches as required.

Similarly to x86 commit 11570da1c5b1dee1 (x86/jump-label: Do not bother updating NOPs if they are correct), we can avoid patching NOPs with identical NOPs. Given that we only use a single NOP encoding, this means we can NOP-out the body of arch_jump_label_transform_static entirely. As the default __weak arch_jump_label_transform_static implementation performs a patch, we must use an empty function to achieve this.

Signed-off-by Mark Rutland <[email protected]>

Cc Catalin Marinas <[email protected]>

Cc Jiang Liu <[email protected]>

Cc Laura Abbott <[email protected]>

Cc Will Deacon <[email protected]>

---

 arch/arm64/kernel/jump_label.c | 23 +++++++++--------------
 1 file changed, 9 insertions(+), 14 deletions(-)

arch/arm64/kernel/jump_label.c +9 -14

index 9ac30bb..4f1fec7 100644
@@ -22,9 +22,8 @@
 
 #ifdef HAVE_JUMP_LABEL
 
-static void __arch_jump_label_transform(struct jump_entry *entry,
-					enum jump_label_type type,
-					bool is_static)
+void arch_jump_label_transform(struct jump_entry *entry,
+			       enum jump_label_type type)
 {
 	void *addr = (void *)entry->code;
 	u32 insn;

@@ -37,22 +36,18 @@ static void __arch_jump_label_transform(struct jump_entry *entry,
 		insn = aarch64_insn_gen_nop();
 	}
 
-	if (is_static)
-		__aarch64_insn_patch_text_nosync(addr, insn, true);
-	else
-		aarch64_insn_patch_text(&addr, &insn, 1);
-}
-
-void arch_jump_label_transform(struct jump_entry *entry,
-			       enum jump_label_type type)
-{
-	__arch_jump_label_transform(entry, type, false);
+	aarch64_insn_patch_text(&addr, &insn, 1);
 }
 
 void arch_jump_label_transform_static(struct jump_entry *entry,
 				      enum jump_label_type type)
 {
-	__arch_jump_label_transform(entry, type, true);
+	/*
+	 * We use the architected A64 NOP in arch_static_branch, so there's no
+	 * need to patch an identical A64 NOP over the top of it here. The core
+	 * will call arch_jump_label_transform from a module notifier if the
+	 * NOP needs to be replaced by a branch.
+	 */
 }
 
 #endif	/* HAVE_JUMP_LABEL */

Signature · 1 line

1.9.1

? 2014?11?26????3:04?Mark Rutland <[email protected]> ???

In the arm64 arch_static_branch implementation we place an A64 NOP into the instruction stream and log relevant details to a jump_entry in a __jump_table section. Later this may be replaced with an immediate branch without link to the code for the unlikely case.

At init time, the core calls arch_jump_label_transform_static to initialise the NOPs. On x86 this involves inserting the optimal NOP for a given microarchitecture, but on arm64 we only use the architectural NOP, and hence replace each NOP with the exact same NOP. This is somewhat pointless.

Additionally, at module load time we don't call jump_label_apply_nops to patch the optimal NOPs in, unlike other architectures, but get away with this because we only use the architectural NOP anyway. A later notifier will patch NOPs with branches as required.

Similarly to x86 commit 11570da1c5b1dee1 (x86/jump-label: Do not bother updating NOPs if they are correct), we can avoid patching NOPs with identical NOPs. Given that we only use a single NOP encoding, this means we can NOP-out the body of arch_jump_label_transform_static entirely. As the default __weak arch_jump_label_transform_static implementation performs a patch, we must use an empty function to achieve this.

Signed-off-by Mark Rutland <[email protected]>

Cc Catalin Marinas <[email protected]>

Cc Jiang Liu <[email protected]>

Cc Laura Abbott <[email protected]>

Cc Will Deacon <[email protected]>

---
arch/arm64/kernel/jump_label.c | 23 +++++++++--------------

1 file changed, 9 insertions(+), 14 deletions(-)

arch/arm64/kernel/jump_label.c +9 -14

index 9ac30bb..4f1fec7 100644
@@ -22,9 +22,8 @@
 
 #ifdef HAVE_JUMP_LABEL
 
-static void __arch_jump_label_transform(struct jump_entry *entry,
-                    enum jump_label_type type,
-                    bool is_static)
+void arch_jump_label_transform(struct jump_entry *entry,
+                   enum jump_label_type type)
 {
   void *addr = (void *)entry->code;
   u32 insn;

@@ -37,22 +36,18 @@ static void __arch_jump_label_transform(struct jump_entry *entry,
       insn = aarch64_insn_gen_nop();
   }
 
-    if (is_static)
-        __aarch64_insn_patch_text_nosync(addr, insn, true);
-    else
-        aarch64_insn_patch_text(&addr, &insn, 1);
-}
-
-void arch_jump_label_transform(struct jump_entry *entry,
-                   enum jump_label_type type)
-{
-    __arch_jump_label_transform(entry, type, false);
+    aarch64_insn_patch_text(&addr, &insn, 1);
 }
 
 void arch_jump_label_transform_static(struct jump_entry *entry,
                     enum jump_label_type type)
 {
-    __arch_jump_label_transform(entry, type, true);
+    /*
+     * We use the architected A64 NOP in arch_static_branch, so there's no
+     * need to patch an identical A64 NOP over the top of it here. The core
+     * will call arch_jump_label_transform from a module notifier if the
+     * NOP needs to be replaced by a branch.
+     */
 }
 
 #endif    /* HAVE_JUMP_LABEL */

Signature · 1 line

1.9.1