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

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Message-ID: <aKSRbjgtp7Nk8-sb@google.com>
Date: Tue, 19 Aug 2025 07:59:58 -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>, 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] KVM: VMX: Micro-optimize SPEC_CTRL handling in __vmx_vcpu_run()

On Thu, Aug 07, 2025, Uros Bizjak wrote:
> 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

Huh.  There's a pre-existing bug lurking here, and in the SVM code.  SPEC_CTRL
is an MSR, i.e. a 64-bit value, but the assembly code assumes bits 63:32 are always
zero.

So, while this patch looks good, I'm leaning toward skipping it and going straight
to a fix.

>  	je .Lspec_ctrl_done
>  	mov $MSR_IA32_SPEC_CTRL, %ecx
>  	xor %edx, %edx
> -	mov %edi, %eax
>  	wrmsr
>  
>  .Lspec_ctrl_done:
> -- 
> 2.50.1
>