linux-kernel - [PATCH 2/2] KVM: VMX: Use LEAVE in vmx_do_interrupt

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Message-ID: <20250414081131.97374-2-ubizjak@gmail.com>
Date: Mon, 14 Apr 2025 10:10:51 +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 2/2] KVM: VMX: Use LEAVE in vmx_do_interrupt_irqoff()

Micro-optimize vmx_do_interrupt_irqoff() by substituting
MOV %RBP,%RSP; POP %RBP instruction sequence with equivalent
LEAVE instruction. GCC compiler does this by default for
a generic tuning and for all modern processors:

DEF_TUNE (X86_TUNE_USE_LEAVE, "use_leave",
	  m_386 | m_CORE_ALL | m_K6_GEODE | m_AMD_MULTIPLE | m_ZHAOXIN
	  | m_TREMONT | m_CORE_HYBRID | m_CORE_ATOM | m_GENERIC)

The new code also saves a couple of bytes, from:

  27:	48 89 ec             	mov    %rbp,%rsp
  2a:	5d                   	pop    %rbp

to:

  27:	c9                   	leave

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/kvm/vmx/vmenter.S | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index f6986dee6f8c..0a6cf5bff2aa 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -59,8 +59,7 @@
 	 * without the explicit restore, thinks the stack is getting walloped.
 	 * Using an unwind hint is problematic due to x86-64's dynamic alignment.
 	 */
-	mov %_ASM_BP, %_ASM_SP
-	pop %_ASM_BP
+	leave
 	RET
 .endm
 
-- 
2.49.0