linux-kernel - [PATCH -tip v3] x86/idle: Use MONITOR and MWAIT mnemonics in <asm/mwait.h>

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Message-ID: <20250403125111.429805-1-ubizjak@gmail.com>
Date: Thu,  3 Apr 2025 14:50:45 +0200
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 -tip v3] x86/idle: Use MONITOR and MWAIT mnemonics in <asm/mwait.h>

Current minimum required version of binutils is 2.25,
which supports MONITOR and MWAIT instruction mnemonics.

Replace the byte-wise specification of MONITOR and
MWAIT with these proper mnemonics.

No functional change intended.

Note: LLVM assembler is not able to assemble correct forms of MONITOR
and MWAIT instructions with explicit operands and reports [1]:

  error: invalid operand for instruction
          monitor %rax,%ecx,%edx
                       ^~~~

Use instruction mnemonics with implicit operands to
work around this issue.

[1] https://lore.kernel.org/oe-kbuild-all/202504030802.2lEVBSpN-lkp@intel.com/

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>
---
v3: Use instruction mnemonics with implicit operands.
---
 arch/x86/include/asm/mwait.h | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index 6a2ec2083043..26b68ee5b6a5 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -27,9 +27,9 @@
 
 static __always_inline void __monitor(const void *eax, u32 ecx, u32 edx)
 {
-	/* "monitor %eax, %ecx, %edx;" */
-	asm volatile(".byte 0x0f, 0x01, 0xc8;"
-		     :: "a" (eax), "c" (ecx), "d"(edx));
+	/* Use the instruction mnemonic with implicit operands, as the LLVM
+	   assembler fails to assemble the mnemonic with explicit operands. */
+	asm volatile("monitor" :: "a" (eax), "c" (ecx), "d" (edx));
 }
 
 static __always_inline void __monitorx(const void *eax, u32 ecx, u32 edx)
@@ -43,9 +43,9 @@ static __always_inline void __mwait(u32 eax, u32 ecx)
 {
 	mds_idle_clear_cpu_buffers();
 
-	/* "mwait %eax, %ecx;" */
-	asm volatile(".byte 0x0f, 0x01, 0xc9;"
-		     :: "a" (eax), "c" (ecx));
+	/* Use the instruction mnemonic with implicit operands, as the LLVM
+	   assembler fails to assemble the mnemonic with explicit operands. */
+	asm volatile("mwait" :: "a" (eax), "c" (ecx));
 }
 
 /*
@@ -95,9 +95,8 @@ static __always_inline void __mwaitx(u32 eax, u32 ebx, u32 ecx)
 static __always_inline void __sti_mwait(u32 eax, u32 ecx)
 {
 	mds_idle_clear_cpu_buffers();
-	/* "mwait %eax, %ecx;" */
-	asm volatile("sti; .byte 0x0f, 0x01, 0xc9;"
-		     :: "a" (eax), "c" (ecx));
+
+	asm volatile("sti; mwait" :: "a" (eax), "c" (ecx));
 }
 
 /*
-- 
2.49.0