linux-kernel - [RFC][PATCH 8/9] jump_label,objtool: Generate possible statistics

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Date:   Mon, 07 Oct 2019 10:44:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, mhiramat@...nel.org, bristot@...hat.com,
        jbaron@...mai.com, torvalds@...ux-foundation.org,
        tglx@...utronix.de, mingo@...nel.org, namit@...are.com,
        hpa@...or.com, luto@...nel.org, ard.biesheuvel@...aro.org,
        jpoimboe@...hat.com, hjl.tools@...il.com
Subject: [RFC][PATCH 8/9] jump_label,objtool: Generate possible statistics

	x86_64-defconfig	x86_64-allmodconfig-UBSAN-KASAN

NOP2	1641			21796
JMP8	48			114

NOP5	1010			31042
JMP32	29			91

Which results in a possible 3*(1641+48) ~ 5k saving for defconfig
and 3*(21796+114) ~ 64k saving for allmodconfig.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: "H.J. Lu" <hjl.tools@...il.com>
---
 tools/objtool/check.c |   43 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 2 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -817,6 +817,8 @@ static int handle_jump_alt(struct objtoo
 			   struct instruction *orig_insn,
 			   struct instruction **new_insn)
 {
+	long disp;
+
 	if ((orig_insn->len != 2) && (orig_insn->len != 5)) {
 		WARN_FUNC("jump_label: unsupported INSN length: %d",
 				orig_insn->sec, orig_insn->offset, orig_insn->len);
@@ -824,9 +826,12 @@ static int handle_jump_alt(struct objtoo
 	}
 
 	if (orig_insn->type == INSN_NOP) {
-		long disp;
 
-		if (orig_insn->len == 2) {
+		switch (orig_insn->len) {
+		case 2:
+			WARN_FUNC("jump_label: NOP2 present",
+					orig_insn->sec, orig_insn->offset);
+
 			if (special_alt->orig_sec != special_alt->new_sec) {
 				WARN_FUNC("jump_label: JMP8 crossing sections",
 						orig_insn->sec, orig_insn->offset);
@@ -839,6 +844,20 @@ static int handle_jump_alt(struct objtoo
 						orig_insn->sec, orig_insn->offset);
 				return -1;
 			}
+			break;
+
+		case 5:
+			if (special_alt->orig_sec == special_alt->new_sec) {
+				disp = special_alt->new_off - (special_alt->orig_off + 2);
+				if ((disp>>31) == (disp>>7)) {
+					WARN_FUNC("jump_label: NOP2 possible",
+							orig_insn->sec, orig_insn->offset);
+					break;
+				}
+			}
+			WARN_FUNC("jump_label: NOP5 present",
+					orig_insn->sec, orig_insn->offset);
+			break;
 		}
 
 		return 0;
@@ -850,6 +869,26 @@ static int handle_jump_alt(struct objtoo
 		return -1;
 	}
 
+	switch (orig_insn->len) {
+	case 2:
+		WARN_FUNC("jump_label: JMP8 present",
+				orig_insn->sec, orig_insn->offset);
+		break;
+
+	case 5:
+		if (special_alt->orig_sec == special_alt->new_sec) {
+			disp = special_alt->new_off - (special_alt->orig_off + 2);
+			if ((disp>>31) == (disp>>7)) {
+				WARN_FUNC("jump_label: JMP8 possible",
+						orig_insn->sec, orig_insn->offset);
+				break;
+			}
+		}
+		WARN_FUNC("jump_label: JMP32 present",
+				orig_insn->sec, orig_insn->offset);
+		break;
+	}
+
 	*new_insn = list_next_entry(orig_insn, list);
 	return 0;
 }