linux-kernel - [PATCH v3 2/3] x86/hweight: Use ASM_CALL

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Message-ID: <20250312123905.149298-2-ubizjak@gmail.com>
Date: Wed, 12 Mar 2025 13:38:44 +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 v3 2/3] x86/hweight: Use ASM_CALL_CONSTRAINT in inline asm()

Use ASM_CALL_CONSTRAINT to prevent inline asm that includes call
instruction from being scheduled before the frame pointer gets set
up by the containing function. This unconstrained scheduling might
cause objtool to print a "call without frame pointer save/setup"
warning. Current versions of compilers don't seem to trigger this
condition, but without this constraint there's nothing to prevent
the compiler from scheduling the insn in front of frame creation.

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>
---
v2: Expand ASM_CALL_CONSTRANT commit message to mention that
    current compilers don't schedule insn before freame creation.
v3: Split patch into three separate patches.
---
 arch/x86/include/asm/arch_hweight.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/arch_hweight.h b/arch/x86/include/asm/arch_hweight.h
index a11bb841c434..f233eb00f41f 100644
--- a/arch/x86/include/asm/arch_hweight.h
+++ b/arch/x86/include/asm/arch_hweight.h
@@ -17,7 +17,7 @@ static __always_inline unsigned int __arch_hweight32(unsigned int w)
 	unsigned int res;
 
 	asm (ALTERNATIVE("call __sw_hweight32", "popcntl %[val], %[cnt]", X86_FEATURE_POPCNT)
-			 : [cnt] "=" REG_OUT (res)
+			 : [cnt] "=" REG_OUT (res), ASM_CALL_CONSTRAINT
 			 : [val] REG_IN (w));
 
 	return res;
@@ -45,7 +45,7 @@ static __always_inline unsigned long __arch_hweight64(__u64 w)
 	unsigned long res;
 
 	asm (ALTERNATIVE("call __sw_hweight64", "popcntq %[val], %[cnt]", X86_FEATURE_POPCNT)
-			 : [cnt] "=" REG_OUT (res)
+			 : [cnt] "=" REG_OUT (res), ASM_CALL_CONSTRAINT
 			 : [val] REG_IN (w));
 
 	return res;
-- 
2.48.1