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

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Date:	Mon, 11 Aug 2014 07:53:57 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...e.com>,
	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>
Subject: Re: [PATCH 4/5] x86: entry_64.S: always allocate complete "struct
 pt_regs"

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.

	-hpa

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