linux-kernel - Re: [PATCH] x86-64: fix unwind annotations in entry

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Date:	Thu, 12 Mar 2009 11:48:34 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <jbeulich@...ell.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Alexander van Heukelum <heukelum@...tmail.fm>
Cc:	tglx@...utronix.de, hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86-64: fix unwind annotations in entry_64.S


* Jan Beulich <jbeulich@...ell.com> wrote:

>  	.macro XCPT_FRAME start=1 offset=0
> -	INTR_FRAME \start, RIP+\offset-ORIG_RAX
> -	/*CFI_REL_OFFSET orig_rax, ORIG_RAX-ORIG_RAX*/
> +	INTR_FRAME \start, __stringify(RIP+\offset-ORIG_RAX)
>  	.endm
>  
>  /*
>   * frame that enables calling into C.
>   */
>  	.macro PARTIAL_FRAME start=1 offset=0
> -	XCPT_FRAME \start, ORIG_RAX+\offset-ARGOFFSET
> +	.if \start >= 0
> +	XCPT_FRAME \start, __stringify(ORIG_RAX+\offset-ARGOFFSET)
> +	.endif

So we pass in a stringified parameter from PARTIAL_FRAME to 
XCPT_FRAME and then we stringify it again?

> -	movq_cfi rdi, RDI+16-ARGOFFSET
> -	movq_cfi rsi, RSI+16-ARGOFFSET
> -	movq_cfi rdx, RDX+16-ARGOFFSET
> -	movq_cfi rcx, RCX+16-ARGOFFSET
> -	movq_cfi rax, RAX+16-ARGOFFSET
> -	movq_cfi  r8,  R8+16-ARGOFFSET
> -	movq_cfi  r9,  R9+16-ARGOFFSET
> -	movq_cfi r10, R10+16-ARGOFFSET
> -	movq_cfi r11, R11+16-ARGOFFSET
> +	movq %rdi, RDI+16-ARGOFFSET(%rsp)
> +	movq %rsi, RSI+16-ARGOFFSET(%rsp)
> +	movq %rdx, RDX+16-ARGOFFSET(%rsp)
> +	movq %rcx, RCX+16-ARGOFFSET(%rsp)
> +	movq_cfi rax, __stringify(RAX+16-ARGOFFSET)
> +	movq  %r8,  R8+16-ARGOFFSET(%rsp)
> +	movq  %r9,  R9+16-ARGOFFSET(%rsp)
> +	movq %r10, R10+16-ARGOFFSET(%rsp)
> +	movq_cfi r11, __stringify(R11+16-ARGOFFSET)

Hm, we used to have annotations for those arguments too, now 
they are gone.

>  	leaq 8(%rsp), %rbp		/* mov %rsp, %ebp */
> +	CFI_DEF_CFA_REGISTER rbp
> +	CFI_ADJUST_CFA_OFFSET -8

This open-codes CFI annotations again - please introduce a 
helper instead.

> -	popq_cfi %rax			/* move return address... */
> +	popq %rax			/* move return address... */

No annotation needed?

>  	mov %gs:pda_irqstackptr,%rsp
> -	EMPTY_FRAME 0
> -	pushq_cfi %rbp			/* backlink for unwinder */
> -	pushq_cfi %rax			/* ... to the new stack */
> +	pushq %rbp			/* backlink for unwinder */
> +	pushq %rax			/* ... to the new stack */

Ditto?

>  ENTRY(save_rest)
> -	PARTIAL_FRAME 1 REST_SKIP+8
> +	CFI_STARTPROC
>  	movq 5*8+16(%rsp), %r11	/* save return address */
> -	movq_cfi rbx, RBX+16
> -	movq_cfi rbp, RBP+16
> -	movq_cfi r12, R12+16
> -	movq_cfi r13, R13+16
> -	movq_cfi r14, R14+16
> -	movq_cfi r15, R15+16
> +	movq %rbx, RBX+16(%rsp)
> +	movq %rbp, RBP+16(%rsp)
> +	movq %r12, R12+16(%rsp)
> +	movq %r13, R13+16(%rsp)
> +	movq %r14, R14+16(%rsp)
> +	movq %r15, R15+16(%rsp)

Same here?

>  	CFI_ADJUST_CFA_OFFSET REST_SKIP
>  	call save_rest
> -	DEFAULT_FRAME 0 8		/* offset 8: return address */
> +	DEFAULT_FRAME -2 8		/* offset 8: return address */
>  	leaq 8(%rsp), \arg	/* pt_regs pointer */

Open-coded CFI annotation again.

I havent checked the rest of the patch but it shows similar 
patters.

The thing is, this patch is completely unacceptable - it just 
reintroduces the same ugly crufty CFI code we used to have 
there.

The current annotations might be completely broken but at least 
they _look_ structured and attempt to bring some order into the 
CFI annotations chaos.

There's really just two options here:

- You either submit clean, gradual patches that fix the problems 
  and explain what is done and why, and documents the annotation
  rules and makes CFI annotations maintainable,

- Or we'll remove all CFI annotations from all x86 assembly
  files and wait for future, clean patches.

	Ingo
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