[PATCH 2/2] jump labels/x86: Use etiher 5 byte or 2 byte jumps

[Steven Rostedt]

From: Steven Rostedt <srostedt@xxxxxxxxxx>

Have the jump labels add a "jmp" in the assembly instead
of a default nop. This will cause the assembler to put in
either a 2 byte or 5 byte jmp depending on where the target
lable is.

Then at compile time, the update_jump_label code will replace
the jmps with either 2 or 5 byte nops.

On boot up, the code can be examined to see if the jump label
uses either a 2 or 5 byte nop and replace it.

By allowing the jump labels to be 2 bytes, it speeds up the
nops, not only 2 byte nops are faster than 5 byte nops, but also
because it saves on cache foot print.

   text    data     bss     dec     hex filename
13403667 3666856 2998272 20068795 13239bb ../nobackup/mxtest/vmlinux-old
13398536 3666856 2998272 20063664 13225b0 ../nobackup/mxtest/vmlinux-new

Converting the current v3.2 trace points saved 5,131 bytes.
As more places use jump labels, this will have a bigger savings.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 arch/x86/Kconfig                  |    1 +
 arch/x86/include/asm/jump_label.h |    2 +-
 arch/x86/kernel/jump_label.c      |   86 ++++++++++++++++++++++++++++++++++---
 3 files changed, 81 insertions(+), 8 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index efb4294..b5004c1 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -61,6 +61,7 @@ config X86
 	select HAVE_ARCH_KMEMCHECK
 	select HAVE_USER_RETURN_NOTIFIER
 	select HAVE_ARCH_JUMP_LABEL
+	select HAVE_BUILD_TIME_JUMP_LABEL
 	select HAVE_TEXT_POKE_SMP
 	select HAVE_GENERIC_HARDIRQS
 	select HAVE_SPARSE_IRQ
diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index a32b18c..872b3e1 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -14,7 +14,7 @@
 static __always_inline bool arch_static_branch(struct jump_label_key *key)
 {
 	asm goto("1:"
-		JUMP_LABEL_INITIAL_NOP
+		"jmp %l[l_yes]\n"
 		".pushsection __jump_table,  \"aw\" \n\t"
 		_ASM_ALIGN "\n\t"
 		_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
index ea9d5f2f..d5b84de 100644
--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -16,12 +16,27 @@
 
 #ifdef HAVE_JUMP_LABEL
 
+static unsigned char nop_short[] = { P6_NOP2 };
+
+/* These are the nops added at compile time */
+#ifdef CONFIG_X86_32
+static unsigned char default_nop[5] = { 0x3e, 0x8d, 0x74, 0x26, 0x00 };
+#else
+static unsigned char default_nop[5] = { 0x0f, 0x1f, 0x44, 0x00, 0x00 };
+#endif
+
+static int update_nops;
+
 union jump_code_union {
 	char code[JUMP_LABEL_NOP_SIZE];
 	struct {
 		char jump;
 		int offset;
-	} __attribute__((packed));
+	} __packed;
+	struct {
+		char jump_short;
+		char offset_short;
+	} __packed;
 };
 
 static void __jump_label_transform(struct jump_entry *entry,
@@ -29,20 +44,70 @@ static void __jump_label_transform(struct jump_entry *entry,
 				   void *(*poker)(void *, const void *, size_t))
 {
 	union jump_code_union code;
+	unsigned char nop;
+	unsigned char op;
+	unsigned size;
+	void *ip = (void *)entry->code;
+	void *ideal = (void *)ideal_nops[NOP_ATOMIC5];
+
+	/* Use probe_kernel_read()? */
+	op = *(unsigned char *)ip;
+	nop = ideal_nops[NOP_ATOMIC5][0];
 
 	if (type == JUMP_LABEL_ENABLE) {
-		code.jump = 0xe9;
-		code.offset = entry->target -
-				(entry->code + JUMP_LABEL_NOP_SIZE);
-	} else
-		memcpy(&code, ideal_nops[NOP_ATOMIC5], JUMP_LABEL_NOP_SIZE);
+		if (op == 0xe9 || op == 0xeb)
+			/* Already enabled. Warn? */
+			return;
+
+		if (memcmp(ip, nop_short, 2) == 0) {
+			size = 2;
+			code.jump_short = 0xeb;
+			code.offset = entry->target -
+				(entry->code + 2);
+			/* Check for overflow ? */
+		} else if (memcmp(ip, ideal, 5) == 0 ||
+			   memcmp(ip, default_nop, 5)) {
+			size = JUMP_LABEL_NOP_SIZE;
+			code.jump = 0xe9;
+			code.offset = entry->target - (entry->code + size);
+		} else
+			BUG();
 
-	(*poker)((void *)entry->code, &code, JUMP_LABEL_NOP_SIZE);
+	} else {
+		/* Check if already disabled */
+		if (memcmp(ip, nop_short, 2) == 0)
+			return;
+
+		if (memcmp(ip, ideal, 5) == 0)
+			return;
+
+		/* This may need to update from default to ideal */
+		if (update_nops && memcmp(ip, default_nop, 5) == 0) {
+			/* Set to the ideal nop */
+			size = JUMP_LABEL_NOP_SIZE;
+			memcpy(&code, ideal_nops[NOP_ATOMIC5], size);
+
+		} else if (op == 0xe9) {
+			/* Replace a 5 byte jmp */
+			size = JUMP_LABEL_NOP_SIZE;
+			memcpy(&code, ideal_nops[NOP_ATOMIC5], size);
+		} else if (op == 0xeb) {
+			/* Replace a 2 byte jmp */
+			size = 2;
+			memcpy(&code, nop_short, size);
+		} else
+			BUG();
+	}
+
+	(*poker)((void *)entry->code, &code, size);
 }
 
 void arch_jump_label_transform(struct jump_entry *entry,
 			       enum jump_label_type type)
 {
+	/* All nops should be updated to the ideal nop by now */
+	update_nops = 0;
+
 	get_online_cpus();
 	mutex_lock(&text_mutex);
 	__jump_label_transform(entry, type, text_poke_smp);
@@ -53,6 +118,13 @@ void arch_jump_label_transform(struct jump_entry *entry,
 void arch_jump_label_transform_static(struct jump_entry *entry,
 				      enum jump_label_type type)
 {
+	/*
+	 * If the default nop does not equal the ideal nop, then
+	 * update them.
+	 */
+	if (memcmp(default_nop, ideal_nops[NOP_ATOMIC5], 5) != 0)
+		update_nops = 1;
+
 	__jump_label_transform(entry, type, text_poke_early);
 }
 
-- 
1.7.8.3


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