linux-kernel - [PATCH 09/13] jump_label,x86: Emit short JMP

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Message-ID: <20210506194157.967034497@infradead.org>
Date:   Thu, 06 May 2021 21:34:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, jpoimboe@...hat.com, jbaron@...mai.com,
        rostedt@...dmis.org, ardb@...nel.org
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org
Subject: [PATCH 09/13] jump_label,x86: Emit short JMP

Now that we can patch short JMP/NOP, allow the compiler/assembler to
emit short JMP instructions.

There is no way to have the assembler emit short NOPs based on the
potential displacement, so leave those long for now.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/jump_label.h |    3 +--
 arch/x86/kernel/jump_label.c      |    8 +++++++-
 2 files changed, 8 insertions(+), 3 deletions(-)

--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -35,8 +35,7 @@ static __always_inline bool arch_static_
 static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
-		".byte 0xe9 \n\t"
-		".long %l[l_yes] - (. + 4) \n\t"
+		"jmp %l[l_yes]\n\t"
 		JUMP_TABLE_ENTRY
 		: :  "i" (key), "i" (branch) : : l_yes);
 
--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -15,12 +15,18 @@
 #include <asm/kprobes.h>
 #include <asm/alternative.h>
 #include <asm/text-patching.h>
+#include <asm/insn.h>
 
 #define JUMP_LABEL_NOP_SIZE	JMP32_INSN_SIZE
 
 int arch_jump_entry_size(struct jump_entry *entry)
 {
-	return JMP32_INSN_SIZE;
+	struct insn insn = {};
+
+	insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
+	BUG_ON(insn.length != 2 && insn.length != 5);
+
+	return insn.length;
 }
 
 struct jump_label_patch {