linux-kernel - Re: [PATCH 4/5] x86: entry_64.S: always allocate complete "struct pt

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Date:	Mon, 11 Aug 2014 15:26:14 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Jan Beulich <JBeulich@...e.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Denys Vlasenko <vda.linux@...glemail.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>, X86 ML <x86@...nel.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 4/5] x86: entry_64.S: always allocate complete "struct
 pt_regs"

On 08/11/2014 10:40 AM, Jan Beulich wrote:
>>>>>       CFI_ESCAPE      0x0f /* DW_CFA_def_cfa_expression */, 6, \
>>>>>                       0x77 /* DW_OP_breg7 */, 0, \
>>>>>                       0x06 /* DW_OP_deref */, \
>>>>> -                     0x08 /* DW_OP_const1u */, SS+8-RBP, \
>>>>> +                     0x08 /* DW_OP_const1u */, SS+8, \
>>>>>                       0x22 /* DW_OP_plus */
>>>>>       /* We entered an interrupt context - irqs are off: */
>>>>>       TRACE_IRQS_OFF
...
...
>>> While we are at it, what this  CFI_ESCAPE thing does here?
>>> As usual, it has no comment :/
> 
> Each of its lines has a comment; with other CFI annotations not
> each having comments, I don't see what else is needed here.

The existing comments explain what every byte means.
They are useful if CFI-literate reader wants to check correctness
of the encoding of this annotation.

There is no overall comment what this CFI annotation
*achieves*. In human language, what do we say
to DWARF decoder here?

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