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

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Date:	Mon, 11 Aug 2014 16:08:11 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Andy Lutomirski" <luto@...capital.net>,
	"Kees Cook" <keescook@...omium.org>,
	"Will Drewry" <wad@...omium.org>,
	"Frederic Weisbecker" <fweisbec@...il.com>,
	"Denys Vlasenko" <vda.linux@...glemail.com>,
	"X86 ML" <x86@...nel.org>, "Alexei Starovoitov" <ast@...mgrid.com>,
	"Denys Vlasenko" <dvlasenk@...hat.com>,
	"Oleg Nesterov" <oleg@...hat.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] x86: entry_64.S: always allocate complete
 "struct pt_regs"

>>> On 11.08.14 at 16:53, <hpa@...or.com> wrote:
> On 08/11/2014 07:17 AM, Jan Beulich wrote:
>>>
>>> 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.)
>> 
> 
> No, in *human language*.  What does the DW_CFA_def_cfa_expression
> actually aim to accomplish?  If you don't know the innards of the DWARF
> spec, the whole thing might as well be Hungarian.

Just like the other DW_CFA_def_cfa_* ones it sets the current
frame address (CFA), just not via one of the pre-canned shortcuts,
but via an expression (in the case here de-referencing the stack
pointer to read the top of stack, and then adding the necessary
offset). So it indeed is similar enough to other .cfi_* annotations we
use without further comments.

And btw., Hungarian isn't _that_ bad.

Jan

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