linux-kernel - [PATCH v2] noinstr: Use asm_inline() in instrumentation

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Message-ID: <165d2a0b882050f3f6cc0315af66cd2d16e5925b.1744676674.git.jpoimboe@kernel.org>
Date: Mon, 14 Apr 2025 17:27:11 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Uros Bizjak <ubizjak@...il.com>
Subject: [PATCH v2] noinstr: Use asm_inline() in instrumentation_{begin,end}()

Use asm_inline() in the instrumentation begin/end macros to prevent the
compiler from making poor inlining decisions based on the length of the
objtool annotations.

Without the objtool annotations, each macro resolves to a single NOP.
Using inline_asm() seems obviously correct here as it accurately
communicates the actual code size to the compiler.

These macros are used by WARN() and lockdep, so this change can affect a
lot of functions.

For a defconfig kernel built with GCC 14.2.1, bloat-o-meter reports a
0.17% increase in text size:

  add/remove: 74/352 grow/shrink: 914/353 up/down: 80747/-47120 (33627)
  Total: Before=19460272, After=19493899, chg +0.17%

The text growth is presumably due to increased inlining.  A net total of
278 functions were removed (+74 / -352).  Each of the removed functions
is likely inlined at multiple sites which explains the somewhat
significant code growth.

One example from Uros:

    $ grep "<encode_string>"  objdump.old
   
    00000000004506e0 <encode_string>:
     45113c:       e8 9f f5 ff ff          call   4506e0 <encode_string>
     452bcb:       e9 10 db ff ff          jmp    4506e0 <encode_string>
     453d33:       e8 a8 c9 ff ff          call   4506e0 <encode_string>
     453ef7:       e8 e4 c7 ff ff          call   4506e0 <encode_string>
     45549f:       e8 3c b2 ff ff          call   4506e0 <encode_string>
     455843:       e8 98 ae ff ff          call   4506e0 <encode_string>
     455b37:       e8 a4 ab ff ff          call   4506e0 <encode_string>
     455b47:       e8 94 ab ff ff          call   4506e0 <encode_string>
     4564fa:       e8 e1 a1 ff ff          call   4506e0 <encode_string>
     456669:       e8 72 a0 ff ff          call   4506e0 <encode_string>
     456691:       e8 4a a0 ff ff          call   4506e0 <encode_string>
     4566a0:       e8 3b a0 ff ff          call   4506e0 <encode_string>
     4569aa:       e8 31 9d ff ff          call   4506e0 <encode_string>
     456e79:       e9 62 98 ff ff          jmp    4506e0 <encode_string>
     456efe:       e9 dd 97 ff ff          jmp    4506e0 <encode_string>
   
    All these are calls now inline:
   
    encode_string                                 58       -     -58
   
    ... where for example encode_putfh() grows by:
   
    encode_putfh                                  70     118     +48

Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
v2:
- improve patch description
- extract from https://lore.kernel.org/cover.1744098446.git.jpoimboe@kernel.org

 include/linux/instrumentation.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/instrumentation.h b/include/linux/instrumentation.h
index bf675a8aef8a..b1007407d272 100644
--- a/include/linux/instrumentation.h
+++ b/include/linux/instrumentation.h
@@ -9,7 +9,7 @@
 
 /* Begin/end of an instrumentation safe region */
 #define __instrumentation_begin(c) ({					\
-	asm volatile(__stringify(c) ": nop\n\t"				\
+	asm_inline volatile(__stringify(c) ": nop\n\t"			\
 		     ANNOTATE_INSTR_BEGIN(__ASM_BREF(c))		\
 		     : : "i" (c));					\
 })
@@ -47,7 +47,7 @@
  * part of the condition block and does not escape.
  */
 #define __instrumentation_end(c) ({					\
-	asm volatile(__stringify(c) ": nop\n\t"				\
+	asm_inline volatile(__stringify(c) ": nop\n\t"			\
 		     ANNOTATE_INSTR_END(__ASM_BREF(c))			\
 		     : : "i" (c));					\
 })
-- 
2.49.0