linux-kernel - Re: [PATCH] x86/entry/64: Use TEST %reg,%reg instead of CMP $0,%reg

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Date:   Thu, 29 Oct 2020 11:55:49 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/entry/64: Use TEST %reg,%reg instead of CMP $0,%reg

On Thu, Oct 29, 2020 at 03:29:15PM +0100, Uros Bizjak wrote:
> Use TEST %reg,%reg which sets the zero flag in the same way
> as CMP $0,%reg, but the encoding uses one byte less.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>

Please use x86/boot/64 or x86/boot/compressed/64 for the commit subject.
x86/entry is used for syscall/exception handler entries in the main
kernel.

> ---
>  arch/x86/boot/compressed/head_64.S | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 017de6cc87dc..e94874f4bbc1 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -241,12 +241,12 @@ SYM_FUNC_START(startup_32)
>  	leal	rva(startup_64)(%ebp), %eax
>  #ifdef CONFIG_EFI_MIXED
>  	movl	rva(efi32_boot_args)(%ebp), %edi
> -	cmp	$0, %edi
> +	testl	%edi, %edi
>  	jz	1f
>  	leal	rva(efi64_stub_entry)(%ebp), %eax
>  	movl	rva(efi32_boot_args+4)(%ebp), %esi
>  	movl	rva(efi32_boot_args+8)(%ebp), %edx	// saved bootparams pointer
> -	cmpl	$0, %edx
> +	testl	%edx, %edx
>  	jnz	1f
>  	/*
>  	 * efi_pe_entry uses MS calling convention, which requires 32 bytes of
> @@ -592,7 +592,7 @@ SYM_CODE_START(trampoline_32bit_src)
>  	movl	%eax, %cr0
>  
>  	/* Check what paging mode we want to be in after the trampoline */
> -	cmpl	$0, %edx
> +	testl	%edx, %edx
>  	jz	1f
>  
>  	/* We want 5-level paging: don't touch CR3 if it already points to 5-level page tables */
> @@ -622,7 +622,7 @@ SYM_CODE_START(trampoline_32bit_src)
>  
>  	/* Enable PAE and LA57 (if required) paging modes */
>  	movl	$X86_CR4_PAE, %eax
> -	cmpl	$0, %edx
> +	testl	%edx, %edx
>  	jz	1f
>  	orl	$X86_CR4_LA57, %eax
>  1:
> -- 
> 2.26.2
>