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

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Message-Id: <53E8EC9F020000780002B383@mail.emea.novell.com>
Date:	Mon, 11 Aug 2014 15:17:35 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Denys Vlasenko" <vda.linux@...glemail.com>,
	"Denys Vlasenko" <dvlasenk@...hat.com>
Cc:	"Andy Lutomirski" <luto@...capital.net>,
	"Kees Cook" <keescook@...omium.org>,
	"Will Drewry" <wad@...omium.org>,
	"Frederic Weisbecker" <fweisbec@...il.com>,
	"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 11.08.14 at 15:26, <dvlasenk@...hat.com> wrote:
> 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?

Short answer: DW_CFA_def_cfa_expression.

Longer response: Just like I said before, what you're asking for is
identical to ask for each other CFI annotation to get a comment
associated to tell you what it's doing, which I don't think you
really mean to ask for. (Our main problem here is that we can't
specify expressions with the .cfi_* gas directives, and hence have
to resort to .cfi_escape.)

Jan

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