linux-kernel - Re: [PATCH V3 2/6] x86_32: use percpu instead of offset-calculation to get thread.sp0 when SWITCH_TO_KERNEL

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Message-ID: <20210208192109.GH18227@zn.tnic>
Date:   Mon, 8 Feb 2021 20:21:09 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        Kees Cook <keescook@...omium.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Brian Gerst <brgerst@...il.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Mike Rapoport <rppt@...nel.org>, Mike Hommey <mh@...ndium.org>,
        Mark Gross <mgross@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        Jay Lang <jaytlang@....edu>,
        "Chang S. Bae" <chang.seok.bae@...el.com>
Subject: Re: [PATCH V3 2/6] x86_32: use percpu instead of offset-calculation
 to get thread.sp0 when SWITCH_TO_KERNEL_STACK

On Thu, Jan 28, 2021 at 12:32:18AM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> TSS_entry2task_stack is used to refer to tss.sp1 which is stored the value
> of thread.sp0.

"... which is a copy of thread.sp0."

> 
> At the code where TSS_entry2task_stack is used in SWITCH_TO_KERNEL_STACK,

"TSS_entry2task_stack is used in SWITCH_TO_KERNEL_STACK and CR3 is
already... "

> the CR3 is already kernel CR3 and kernel segments is loaded.
> 
> So we can directly use the percpu to get tss.sp1(thread.sp0) instead of

Who's "we"?

> the complex offset-calculation.
> 
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
>  arch/x86/entry/entry_32.S | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index df8c017e6161..3b4d1a63d1f0 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -465,16 +465,11 @@
>  	cmpl	$SIZEOF_entry_stack, %ecx
>  	jae	.Lend_\@
>  
> -	/* Load stack pointer into %esi and %edi */
> +	/* Load stack pointer into %esi */
>  	movl	%esp, %esi
> -	movl	%esi, %edi
> -
> -	/* Move %edi to the top of the entry stack */
> -	andl	$(MASK_entry_stack), %edi
> -	addl	$(SIZEOF_entry_stack), %edi
>  
>  	/* Load top of task-stack into %edi */
> -	movl	TSS_entry2task_stack(%edi), %edi
> +	movl	PER_CPU_VAR(cpu_tss_rw + TSS_sp1), %edi
>  
>  	/* Special case - entry from kernel mode via entry stack */
>  #ifdef CONFIG_VM86
> -- 

The change itself looks ok.

Thx.

-- 
Regards/Gruss,
    Boris.

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