linux-kernel - [tip: x86/asm] x86/asm: Use asm_inline() instead of asm() in clwb()

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Message-ID: <174186374489.14745.3324838262167564569.tip-bot2@tip-bot2>
Date: Thu, 13 Mar 2025 11:02:24 -0000
From: "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>,
 Andy Lutomirski <luto@...nel.org>, Brian Gerst <brgerst@...il.com>,
 "H. Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/asm] x86/asm: Use asm_inline() instead of asm() in clwb()

The following commit has been merged into the x86/asm branch of tip:

Commit-ID:     6e9e4961767b76ec67268c991ae90116ce76cc70
Gitweb:        https://git.kernel.org/tip/6e9e4961767b76ec67268c991ae90116ce76cc70
Author:        Uros Bizjak <ubizjak@...il.com>
AuthorDate:    Thu, 13 Mar 2025 11:26:56 +01:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Thu, 13 Mar 2025 11:50:14 +01:00

x86/asm: Use asm_inline() instead of asm() in clwb()

Use asm_inline() to instruct the compiler that the size of asm()
is the minimum size of one instruction, ignoring how many instructions
the compiler thinks it is. ALTERNATIVE macro that expands to several
pseudo directives causes instruction length estimate to count
more than 20 instructions.

bloat-o-meter reports slight increase of the code size
for x86_64 defconfig object file, compiled with gcc-14.2:

  add/remove: 0/2 grow/shrink: 3/0 up/down: 190/-59 (131)

  Function                                     old     new   delta
  __copy_user_flushcache                       166     247     +81
  __memcpy_flushcache                          369     437     +68
  arch_wb_cache_pmem                             6      47     +41
  __pfx_clean_cache_range                       16       -     -16
  clean_cache_range                             43       -     -43

  Total: Before=22807167, After=22807298, chg +0.00%

The compiler now inlines and removes the clean_cache_range() function.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Brian Gerst <brgerst@...il.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: https://lore.kernel.org/r/20250313102715.333142-2-ubizjak@gmail.com
---
 arch/x86/include/asm/special_insns.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 856d4ad..f5a8ff4 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -185,7 +185,7 @@ static inline void clwb(volatile void *__p)
 {
 	volatile struct { char x[64]; } *p = __p;

-	asm volatile(ALTERNATIVE_2(
+	asm_inline volatile(ALTERNATIVE_2(
 		"ds clflush %0",
 		"clflushopt %0", X86_FEATURE_CLFLUSHOPT,
 		"clwb %0", X86_FEATURE_CLWB)