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

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Message-ID: <Z_2w7XJ0LI65qo0i@google.com>
Date: Mon, 14 Apr 2025 18:05:49 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org, 
	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: Re: [PATCH 2/2] KVM: VMX: Use LEAVE in vmx_do_interrupt_irqoff()

On Mon, Apr 14, 2025, Uros Bizjak wrote:
> 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:

Out of curisoity, is LEAVE actually a performance win, or is the benefit essentially
just the few code bytes saves?

> 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
>