linux-kernel - [PATCH 2/2] x86/asm: Use asm_inline instead of asm in clwb()

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Message-ID: <20250313102715.333142-2-ubizjak@gmail.com>
Date: Thu, 13 Mar 2025 11:26:56 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 2/2] 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 clean_cache_range() function.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.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 9b10bd102d3d..6266d6b9e0b8 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)
-- 
2.48.1