linux-kernel - [PATCH 1/2] KVM: x86: Use asm_inline() instead of asm() in kvm

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Message-ID: <20250414081131.97374-1-ubizjak@gmail.com>
Date: Mon, 14 Apr 2025 10:10:50 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: kvm@...r.kernel.org,
	x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Vitaly Kuznetsov <vkuznets@...hat.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 1/2] KVM: x86: Use asm_inline() instead of asm() in kvm_hypercall[0-4]()

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: 10/0 (10)

	Function                          old     new   delta
	-----------------------------------------------------
	__send_ipi_mask                   525     535     +10

  Total: Before=23751224, After=23751234, chg +0.00%

due to different compiler decisions with more precise size
estimations.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Vitaly Kuznetsov <vkuznets@...hat.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/kvm_para.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index 57bc74e112f2..519ab5aee250 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -38,7 +38,7 @@ static inline long kvm_hypercall0(unsigned int nr)
 	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
 		return tdx_kvm_hypercall(nr, 0, 0, 0, 0);
 
-	asm volatile(KVM_HYPERCALL
+	asm_inline volatile(KVM_HYPERCALL
 		     : "=a"(ret)
 		     : "a"(nr)
 		     : "memory");
@@ -52,7 +52,7 @@ static inline long kvm_hypercall1(unsigned int nr, unsigned long p1)
 	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
 		return tdx_kvm_hypercall(nr, p1, 0, 0, 0);
 
-	asm volatile(KVM_HYPERCALL
+	asm_inline volatile(KVM_HYPERCALL
 		     : "=a"(ret)
 		     : "a"(nr), "b"(p1)
 		     : "memory");
@@ -67,7 +67,7 @@ static inline long kvm_hypercall2(unsigned int nr, unsigned long p1,
 	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
 		return tdx_kvm_hypercall(nr, p1, p2, 0, 0);
 
-	asm volatile(KVM_HYPERCALL
+	asm_inline volatile(KVM_HYPERCALL
 		     : "=a"(ret)
 		     : "a"(nr), "b"(p1), "c"(p2)
 		     : "memory");
@@ -82,7 +82,7 @@ static inline long kvm_hypercall3(unsigned int nr, unsigned long p1,
 	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
 		return tdx_kvm_hypercall(nr, p1, p2, p3, 0);
 
-	asm volatile(KVM_HYPERCALL
+	asm_inline volatile(KVM_HYPERCALL
 		     : "=a"(ret)
 		     : "a"(nr), "b"(p1), "c"(p2), "d"(p3)
 		     : "memory");
@@ -98,7 +98,7 @@ static inline long kvm_hypercall4(unsigned int nr, unsigned long p1,
 	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
 		return tdx_kvm_hypercall(nr, p1, p2, p3, p4);
 
-	asm volatile(KVM_HYPERCALL
+	asm_inline volatile(KVM_HYPERCALL
 		     : "=a"(ret)
 		     : "a"(nr), "b"(p1), "c"(p2), "d"(p3), "S"(p4)
 		     : "memory");
-- 
2.49.0