linux-kernel - [PATCH] KVM: VMX: Micro-optimize SPEC_CTRL handling in __vmx_vcpu

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Message-ID: <20250807063733.6943-1-ubizjak@gmail.com>
Date: Thu,  7 Aug 2025 08:36:51 +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>,
	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] KVM: VMX: Micro-optimize SPEC_CTRL handling in __vmx_vcpu_run()

Use memory operand in CMP instruction to avoid usage of a
temporary register. Use %eax register to hold VMX_spec_ctrl
and use it directly in the follow-up WRMSR.

The new code saves a few bytes by removing two MOV insns, from:

  2d:	48 8b 7c 24 10       	mov    0x10(%rsp),%rdi
  32:	8b bf 48 18 00 00    	mov    0x1848(%rdi),%edi
  38:	65 8b 35 00 00 00 00 	mov    %gs:0x0(%rip),%esi
  3f:	39 fe                	cmp    %edi,%esi
  41:	74 0b                	je     4e <...>
  43:	b9 48 00 00 00       	mov    $0x48,%ecx
  48:	31 d2                	xor    %edx,%edx
  4a:	89 f8                	mov    %edi,%eax
  4c:	0f 30                	wrmsr

to:

  2d:	48 8b 7c 24 10       	mov    0x10(%rsp),%rdi
  32:	8b 87 48 18 00 00    	mov    0x1848(%rdi),%eax
  38:	65 3b 05 00 00 00 00 	cmp    %gs:0x0(%rip),%eax
  3f:	74 09                	je     4a <...>
  41:	b9 48 00 00 00       	mov    $0x48,%ecx
  46:	31 d2                	xor    %edx,%edx
  48:	0f 30                	wrmsr

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: 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 | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 0a6cf5bff2aa..c65de5de92ab 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -118,13 +118,11 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	 * and vmentry.
 	 */
 	mov 2*WORD_SIZE(%_ASM_SP), %_ASM_DI
-	movl VMX_spec_ctrl(%_ASM_DI), %edi
-	movl PER_CPU_VAR(x86_spec_ctrl_current), %esi
-	cmp %edi, %esi
+	movl VMX_spec_ctrl(%_ASM_DI), %eax
+	cmp PER_CPU_VAR(x86_spec_ctrl_current), %eax
 	je .Lspec_ctrl_done
 	mov $MSR_IA32_SPEC_CTRL, %ecx
 	xor %edx, %edx
-	mov %edi, %eax
 	wrmsr
 
 .Lspec_ctrl_done:
-- 
2.50.1