linux-kernel - [PATCH v4 25/28] objtool: Improve naming of group alternatives

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Message-ID: <20251113164917.2563486-26-alexandre.chartre@oracle.com>
Date: Thu, 13 Nov 2025 17:49:14 +0100
From: Alexandre Chartre <alexandre.chartre@...cle.com>
To: linux-kernel@...r.kernel.org, mingo@...nel.org, jpoimboe@...nel.org,
        peterz@...radead.org
Cc: alexandre.chartre@...cle.com
Subject: [PATCH v4 25/28] objtool: Improve naming of group alternatives

Improve the naming of group alternatives by showing the feature name and
flags used by the alternative.

Signed-off-by: Alexandre Chartre <alexandre.chartre@...cle.com>
---
 tools/objtool/check.c |  1 -
 tools/objtool/disas.c | 86 ++++++++++++++++++++++++++++---------------
 2 files changed, 57 insertions(+), 30 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 817ac70fed9fa..43593766e80e0 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3661,7 +3661,6 @@ static int validate_insn(struct objtool_file *file, struct symbol *func,
 				return ret;
 			}
 		}
-
 		TRACE_ALT_INFO_NOADDR(insn, "/ ", "DEFAULT");
 	}
 
diff --git a/tools/objtool/disas.c b/tools/objtool/disas.c
index e506fdda4d897..2a6c3708ec315 100644
--- a/tools/objtool/disas.c
+++ b/tools/objtool/disas.c
@@ -9,6 +9,7 @@
 #include <objtool/arch.h>
 #include <objtool/check.h>
 #include <objtool/disas.h>
+#include <objtool/special.h>
 #include <objtool/warn.h>
 
 #include <bfd.h>
@@ -60,6 +61,21 @@ struct disas_alt {
 #define DALT_GROUP(dalt)	(DALT_INSN(dalt)->alt_group)
 #define DALT_ALTID(dalt)	((dalt)->orig_insn->offset)
 
+#define ALT_FLAGS_SHIFT		16
+#define ALT_FLAG_NOT		(1 << 0)
+#define ALT_FLAG_DIRECT_CALL	(1 << 1)
+#define ALT_FEATURE_MASK	((1 << ALT_FLAGS_SHIFT) - 1)
+
+static int alt_feature(unsigned ft_flags)
+{
+	return (ft_flags & ALT_FEATURE_MASK);
+}
+
+static int alt_flags(unsigned ft_flags)
+{
+	return (ft_flags >> ALT_FLAGS_SHIFT);
+}
+
 /*
  * Wrapper around asprintf() to allocate and format a string.
  * Return the allocated string or NULL on error.
@@ -593,7 +609,12 @@ static struct instruction *next_insn_same_alt(struct objtool_file *file,
  */
 char *disas_alt_name(struct alternative *alt)
 {
+	char pfx[4] = { 0 };
 	char *str = NULL;
+	const char *name;
+	int feature;
+	int flags;
+	int num;
 
 	switch (alt->type) {
 
@@ -607,13 +628,37 @@ char *disas_alt_name(struct alternative *alt)
 
 	case ALT_TYPE_INSTRUCTIONS:
 		/*
-		 * This is a non-default group alternative. Create a unique
-		 * name using the offset of the first original and alternative
-		 * instructions.
+		 * This is a non-default group alternative. Create a name
+		 * based on the feature and flags associated with this
+		 * alternative. Use either the feature name (it is available)
+		 * or the feature number. And add a prefix to show the flags
+		 * used.
+		 *
+		 * Prefix flags characters:
+		 *
+		 *   '!'  alternative used when feature not enabled
+		 *   '+'  direct call alternative
+		 *   '?'  unknown flag
 		 */
-		asprintf(&str, "ALTERNATIVE %lx.%lx",
-			 alt->insn->alt_group->orig_group->first_insn->offset,
-			 alt->insn->alt_group->first_insn->offset);
+
+		feature = alt->insn->alt_group->feature;
+		num = alt_feature(feature);
+		flags = alt_flags(feature);
+		str = pfx;
+
+		if (flags & ~(ALT_FLAG_NOT | ALT_FLAG_DIRECT_CALL))
+			*str++ = '?';
+		if (flags & ALT_FLAG_DIRECT_CALL)
+			*str++ = '+';
+		if (flags & ALT_FLAG_NOT)
+			*str++ = '!';
+
+		name = arch_cpu_feature_name(num);
+		if (!name)
+			str = strfmt("%sFEATURE 0x%X", pfx, num);
+		else
+			str = strfmt("%s%s", pfx, name);
+
 		break;
 	}
 
@@ -626,32 +671,15 @@ char *disas_alt_name(struct alternative *alt)
  */
 static int disas_alt_init(struct disas_alt *dalt,
 			  struct instruction *orig_insn,
-			  struct alternative *alt,
-			  int alt_num)
+			  struct alternative *alt)
 {
-	char *str;
-
 	dalt->orig_insn = orig_insn;
 	dalt->alt = alt;
 	dalt->insn_idx = 0;
-	if (!alt) {
-		str = strfmt("<alternative.%lx>", orig_insn->offset);
-	} else {
-		switch (alt->type) {
-		case ALT_TYPE_EX_TABLE:
-			str = strdup("EXCEPTION");
-			break;
-		case ALT_TYPE_JUMP_TABLE:
-			str = strdup("JUMP");
-			break;
-		default:
-			str = strfmt("ALTERNATIVE %d", alt_num);
-			break;
-		}
-	}
-	if (!str)
+	dalt->name = alt ? disas_alt_name(alt) :
+		strfmt("<alternative.%lx>", orig_insn->offset);
+	if (!dalt->name)
 		return -1;
-	dalt->name = str;
 	dalt->width = strlen(dalt->name);
 
 	return 0;
@@ -864,7 +892,7 @@ static void *disas_alt(struct disas_context *dctx,
 	/*
 	 * Initialize and disassemble the default alternative.
 	 */
-	err = disas_alt_init(&alts[0], orig_insn, NULL, 0);
+	err = disas_alt_init(&alts[0], orig_insn, NULL);
 	if (err)
 		goto error;
 
@@ -883,7 +911,7 @@ static void *disas_alt(struct disas_context *dctx,
 			break;
 		}
 		dalt = &alts[i];
-		err = disas_alt_init(dalt, orig_insn, alt, i);
+		err = disas_alt_init(dalt, orig_insn, alt);
 		if (err)
 			goto error;
 
-- 
2.43.5