linux-kernel - Re: [PATCH 02/10] x86/tdx: Use cmovc to save a label in TDX_MODULE

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Date:   Wed, 12 Jul 2023 12:27:18 -0700
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Kai Huang <kai.huang@...el.com>, peterz@...radead.org,
        kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org
Cc:     dave.hansen@...el.com, tglx@...utronix.de, bp@...en8.de,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org, seanjc@...gle.com,
        pbonzini@...hat.com, kvm@...r.kernel.org, isaku.yamahata@...el.com
Subject: Re: [PATCH 02/10] x86/tdx: Use cmovc to save a label in
 TDX_MODULE_CALL asm



On 7/12/23 1:55 AM, Kai Huang wrote:
> Change 'jnc .Lno_vmfailinvalid' to 'cmovc %rdi, %rax' to save the
> .Lno_vmfailinvalid label in the TDX_MODULE_CALL asm macro.

You are removing the label, right? What use "save"?

> 
> Note %rdi, which is used as the first argument, has been saved to %rax
> as TDCALL leaf ID thus is free to hold the error code for cmovc.

> 
> This is basically based on Peter's code.
> 
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Kai Huang <kai.huang@...el.com>
> ---
>  arch/x86/virt/vmx/tdx/tdxcall.S | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
> index 49a54356ae99..3524915d8bd9 100644
> --- a/arch/x86/virt/vmx/tdx/tdxcall.S
> +++ b/arch/x86/virt/vmx/tdx/tdxcall.S
> @@ -57,10 +57,8 @@
>  	 * This value will never be used as actual SEAMCALL error code as
>  	 * it is from the Reserved status code class.
>  	 */
> -	jnc .Lno_vmfailinvalid
> -	mov $TDX_SEAMCALL_VMFAILINVALID, %rax
> -.Lno_vmfailinvalid:
> -
> +	mov $TDX_SEAMCALL_VMFAILINVALID, %rdi
> +	cmovc %rdi, %rax
>  	.else
>  	tdcall
>  	.endif

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer