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

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Message-ID: <Z_2wtfAAxb5uNP4g@google.com>
Date: Mon, 14 Apr 2025 18:04:53 -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 1/2] KVM: x86: Use asm_inline() instead of asm() in kvm_hypercall[0-4]()

Nit, this is guest code, i.e. should use "kvm/x86:" for the scope.  No need to
send a new version just for that.

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