linux-kernel - Re: [PATCH 11/24] x86/entry: Replace the most of asm code of error

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Date:   Thu, 2 Sep 2021 12:16:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
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>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Joerg Roedel <jroedel@...e.de>,
        Youquan Song <youquan.song@...el.com>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH 11/24] x86/entry: Replace the most of asm code of
 error_entry to C code

On Wed, Sep 01, 2021 at 01:50:12AM +0800, Lai Jiangshan wrote:
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 42d2918f5646..bc9e2f5ad370 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -972,83 +972,14 @@ SYM_CODE_START_LOCAL(error_entry)
>  	cld
>  	PUSH_AND_CLEAR_REGS save_ret=1
>  	ENCODE_FRAME_POINTER 8
>  
>  	popq	%r12				/* save return addr in %12 */
>  	movq	%rsp, %rdi			/* arg0 = pt_regs pointer */
> +	call	do_error_entry
>  	movq	%rax, %rsp			/* switch stack */
>  	ENCODE_FRAME_POINTER
>  	pushq	%r12
>  	ret

There's only a single error_entry callsite, which is idtentry_body. One
of the things I wanted to do is change this lot so we change to the
task_stack in 'C', using an adaptation of call_on_irqstack() and
basically don't return frrom C until we're done with \cfunc.

That is, once we call C, stay there, and don't do this back and forth
between C and asm.

As is, the resulting asm in error_entry is somewhat confusing given that
we sometimes don't actually switch stacks.