linux-kernel - Re: [PATCH v1 2/8] x86/entry/clearregs: Add infrastructure to clear registers

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Date:   Thu, 11 Jan 2018 14:58:39 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, dwmw@...zon.co.uk, pjt@...gle.com,
        luto@...nel.org, peterz@...radead.org, thomas.lendacky@....com,
        tim.c.chen@...ux.intel.com, gregkh@...ux-foundation.org,
        dave.hansen@...el.com, jikos@...nel.org,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v1 2/8] x86/entry/clearregs: Add infrastructure to clear
 registers

On Tue, Jan 09, 2018 at 05:03:22PM -0800, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> Add 64bit assembler macros to clear registers on kernel entry.
> Used in followon patches.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  arch/x86/entry/calling.h | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 45a63e00a6af..9444e7623185 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -172,6 +172,34 @@ For 32-bit we have the following conventions - kernel is built with
>  	.byte 0xf1
>  	.endm
>  
> +	.macro CLEAR_R11_TO_R15
> +	xorq %r15, %r15
> +	xorq %r14, %r14
> +	xorq %r13, %r13
> +	xorq %r12, %r12
> +	xorq %r11, %r11
> +	.endm
> +
> +	.macro CLEAR_R8_TO_R15
> +	CLEAR_R11_TO_R15
> +	xorq %r10, %r10
> +	xorq %r9, %r9
> +	xorq %r8, %r8
> +	.endm
> +
> +	.macro CLEAR_ALL_REGS
> +	CLEAR_R8_TO_R15
> +	xorl %eax, %eax
> +	xorl %ebx, %ebx

How come you use xorl vs xorq?

> +	xorl %ecx, %ecx
> +	xorl %edx, %edx
> +	xorl %esi, %esi
> +	xorl %edi, %edi
> +#ifndef CONFIG_FRAME_POINTER
> +	xorl %ebp, %ebp
> +#endif
> +	.endm
> +
>  /*
>   * This is a sneaky trick to help the unwinder find pt_regs on the stack.  The
>   * frame pointer is replaced with an encoded pointer to pt_regs.  The encoding
> -- 
> 2.14.3
>