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

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Message-ID: <20250314112110.740723-1-ubizjak@gmail.com>
Date: Fri, 14 Mar 2025 12:20:54 +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] x86/asm:  Use asm_inline() instead of asm() in rdtsc_ordered()

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 minimal code size increase
(x86_64 defconfig, gcc-14.2.1):

  add/remove: 0/0 grow/shrink: 1/0 up/down: 23/0 (23)

	Function                         old     new   delta
	----------------------------------------------------
	delay_tsc                        144     167     +23

  Total: Before=21423610, After=21423633, chg +0.00%

The code size increase is due to the compiler inlining rdtsc_ordered()
several times in delay_tsc().

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/msr.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 001853541f1e..67c9573a803e 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -214,7 +214,7 @@ static __always_inline unsigned long long rdtsc_ordered(void)
 	 * Thus, use the preferred barrier on the respective CPU, aiming for
 	 * RDTSCP as the default.
 	 */
-	asm volatile(ALTERNATIVE_2("rdtsc",
+	asm_inline volatile(ALTERNATIVE_2("rdtsc",
 				   "lfence; rdtsc", X86_FEATURE_LFENCE_RDTSC,
 				   "rdtscp", X86_FEATURE_RDTSCP)
 			: EAX_EDX_RET(val, low, high)
-- 
2.48.1