linux-kernel - [PATCH 2/2] objtool: Optimize/fix retpoline alternative generation

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Date:   Thu, 07 Oct 2021 23:22:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     jpoimboe@...hat.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org, mbenes@...e.cz
Subject: [PATCH 2/2] objtool: Optimize/fix retpoline alternative generation

When re-running objtool it will generate alterantives for all
retpoline hunks, even if they are already present.

Discard the retpoline alternatives later so we can mark the
instructions as already having alternatives and subsequently skip
generating them. Use ->ignore_alts for this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 tools/objtool/arch/x86/decode.c |    3 +++
 tools/objtool/check.c           |    8 ++++++++
 tools/objtool/special.c         |    8 --------
 3 files changed, 11 insertions(+), 8 deletions(-)

--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -806,6 +806,9 @@ int arch_rewrite_retpolines(struct objto
 		if (!strcmp(insn->sec->name, ".text.__x86.indirect_thunk"))
 			continue;
 
+		if (insn->ignore_alts)
+			continue;
+
 		reloc = insn->reloc;
 
 		sprintf(name, "__x86_indirect_alt_%s_%s",
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1468,6 +1468,14 @@ static int add_special_section_alts(stru
 				ret = -1;
 				goto out;
 			}
+			/*
+			 * Skip (but mark) the retpoline alternatives so that we
+			 * don't generate them again.
+			 */
+			if (new_insn->func && arch_is_retpoline(new_insn->func)) {
+				orig_insn->ignore_alts = true;
+				continue;
+			}
 		}
 
 		if (special_alt->group) {
--- a/tools/objtool/special.c
+++ b/tools/objtool/special.c
@@ -109,14 +109,6 @@ static int get_alt_entry(struct elf *elf
 			return -1;
 		}
 
-		/*
-		 * Skip retpoline .altinstr_replacement... we already rewrite the
-		 * instructions for retpolines anyway, see arch_is_retpoline()
-		 * usage in add_{call,jump}_destinations().
-		 */
-		if (arch_is_retpoline(new_reloc->sym))
-			return 1;
-
 		reloc_to_sec_off(new_reloc, &alt->new_sec, &alt->new_off);
 
 		/* _ASM_EXTABLE_EX hack */